restoring an rca avr-11 ‘airport receiver’ – gerry o’hara
TRANSCRIPT
Restoring an RCA AVR-11 ‘Airport Receiver’ – Gerry O’Hara
The RCA AVR-11 receiver was introduced in 1937 by
the RCA Manufacturing Company Inc.1 especially for
airports and airline ground station services. It covered
‘Long Wave’ from 140kHz, through the ‘Standard
Broadcast’ band, ‘Medium Wave’, and the ‘Short
Wave’ bands up to 23MHz. Air navigation, weather
reports and some airport communications at that time
used the longer wavelengths and airports therefore
required receivers that includes these wavelengths.
However, higher frequencies were also starting to
come into general use, so coverage up to 23MHz was
also useful. Very few AVR-11 receivers were produced
and they are therefore considered rare and collectible.
The sales brochure for AVR-11 is reproduced in the
Appendix to this article which provides some useful
technical information as well as the usual sales ‘hype’.
The AVR-11 was supplied in a tall metal cabinet that
also housed the speaker mounted on a separate
panel above the receiver. The speaker sported the
most imposing and magnificent art-deco winged
RCA emblem I have ever seen on a receiver – a true
‘statement’ that you are using a high-end receiver
that RCA was very proud of (image on sales
brochure, above).
In addition to the AVR-11 model supplied in the
single (dual height) metal cabinet, it could be
supplied in two separate single height cabinets
(page 2 of the brochure – see Appendix). Two
other variants were also available: the AVR-11A,
designed for rack mounting, with a grey painted
front panel, the chassis of which did not have a
dust cover, supplied with a matching panel-
mounted speaker, and the AVR-11B, with a black
painted front panel, with a dust cover over the
chassis, and again supplied with a matching panel-
mounted speaker.
1 The RCA Manufacturing Company Inc. was a division of RCA that built communications and broadcast equipment
Restoration of an RCA AVR-11 ‘Airport Receiver’ Gerry O’Hara
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The AVR-11 Receiver
The AVR-11 is a 16-tube single conversion
superheterodyne receiver that included many of
RCA innovations from the mid-1930’s, including
metal envelope tubes, a ‘Magic Eye’ tuning
indicator, a dynamic noise suppressor, automatic
noise limiter, and a separate RF sub-chassis (rather
esoterically marketed as the ‘Magic Brain’) that
included a ‘band-in-use’ dial mask and a slow
motion logging scale that enabled accurate visual
resetting of the dial to a particular frequency. Up
to 8W of push-pull audio was available, fed to an
8” electrodynamic speaker. The receiver covers a
140KHz to 23MHz2 frequency range in four bands,
omitting 420KHz – 530KHz due to the 460KHz IF
frequency:
Band ‘X’ (‘Long Wave’) – 140KHz – 420KHz Band ‘B’ (‘Medium Wave’) – 1.8MHz – 6.4MHz
Band ‘A’ (‘Standard Broadcast’) – 530KHz – 1.8MHz Band ‘C’ (‘Short Wave’) – 6.4MHz – 23MHz
The circuit is reasonably conventional for a high quality receiver of the period, comprising two tuned RF
stages (2 x 6K7), mixer (6L7), local oscillator (6J7), two IF stages (2 x 6K7) operating at 460KHz3,
detector/eye tube rectifier/noise limiter (6H6), two stages of AF amplification (2 x 6C5), push-pull audio
output (2 x 6F6), noise suppressor (6J7), AGC detector/amplifier (6R7), BFO (6J7), eye tube (6E5) and
rectifier (5Z3). The schematic for the receiver is provided in the Appendix.
The front panel controls (photo, below) comprise ‘Frequency’ (tuning), ‘Signal Input’ (RF gain – rather
optimistically ‘calibrated’ to 1uV sensitivity), ‘Volume’ (AF gain), ‘Fidelity’ (tone), ‘Range’ (bandswitch),
Power (Off/On), ‘Beat Frequency’ (BFO pitch), ‘A.V.C.-C.W. Selector’ (mode/standby), and ‘Noise
Suppression’ (threshold level adjustment). A single 600 ohms headphone jack is provided on the front
panel. Standby mode is indicated by a front panel light disguised to look like a second ‘Magic Eye’.
2 There are minor discrepancies in the claimed frequency coverage on some bands between the sales brochure and the technical manual 3 An optional crystal filter unit could be supplied. Instructions on how to install and set it up are included in the Technical Manual
Restoration of an RCA AVR-11 ‘Airport Receiver’ Gerry O’Hara
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The complete AVR-11 receiver/speaker combination when house in its cabinet weighs in at 78lbs and
measures 23.125” high x 21” wide x
16.125” deep – quite an impressive table
top receiver by any standard.
The circuit and physical design of the
AVR-11 is similar to the
contemporaneous RCA ACR-111, which
was aimed at the well-heeled amateur
radio market, and, to a lesser extent, the
RCA 15K-1 high-end domestic receiver –
indeed, it could be considered as a
partial hybrid of these two chassis.
Articles that provide a technical
description and detail refurbishment of
these two RCA models can be found here
(ACR-111) and here (15K-1):
The circuit and main chassis layout (IF stages, detector, audio stages and power supply) of the AVR-11 is
almost identical to the ACR-111 with the exception of the crystal filter (standard on the ACR-111 and
optional on the AVR-11) and a few minor changes in component values, eg. the cathode resistors in the
IF stages, and switching arrangements. The main chassis of the 15K-1, being designed for high-end
domestic use, incorporated more ‘fidelity’ features, including adjustable IF bandwidth and more
sophisticated tone control, a push-pull 6L6 output stage delivering up to 35W of audio power feeding a
12” speaker unit, and the ‘Ultra Short Wave’ band (23 – 60MHz) was included to cover the ‘HiFi’ (wide
band) ‘Apex’ band transmissions4;
The RF sub-chassis (‘Magic Brain’) of the AVR-11 is very similar in design and layout to that in the 15K-1,
though with an additional RF amplifier stage and omission of the ‘Ultra Short Wave’ band5. The RF sub-
chassis of the ACR-111 was a more sophisticated unit that incorporated both the additional RF amplifier
stage and the ‘Ultra Short Wave’ band, together with electrical bandspread tuning using a separate 4-
gang tuning capacitor.
The AVR-11 could therefore be considered as a compromise design that RCA considered to best meet
the needs of the aviation industry of the period, ie. good sensitivity and image rejection provided by the
two RF amplifier stages, omission of the ‘Ultra Short Wave’ band that was not needed for aviation work,
4 In the mid-late 1930’s the ‘APEX’ stations were introduced in certain large US cities that provided wide-bandwidth, uncompressed, high quality audio AM transmissions. More on Apex stations here (“….Finally, starting in 1937, several radio manufacturers began to introduce models that could tune all the way up to the Apex bands. The Raco R-S-R Clipper and several McMurdo Silver models were among the first. That same year, RCA introduced its “Magic Brain” series of receivers which had a top band that tuned up to 60MHz. These early radios proved to be insensitive and unstable at those rarified frequencies.”), and here 5 The dial on the AVR-11 is identical to the dials used on the ACR-111 and 15K-1 receivers, including the ‘Ultra Short Wave’ band, though on the AVR-11 this section of the dial is never exposed through the mask as only four positions of the band change switch are available
Restoration of an RCA AVR-11 ‘Airport Receiver’ Gerry O’Hara
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reasonable IF selectivity that was satisfactory for phone transmissions
used by aviation, as provided by the three double-tuned IF stages, with
an optional crystal filter available to provide additional selectivity
and/or heterodyne/adjacent signal rejection only if needed/specified by
the purchaser, noise suppression and limiting for potentially noisy
airport situations/atmospheric ‘static’, and fairly basic audio circuitry
feeding an 8” speaker well-suited to voice reproduction.
Comparison photographs of the main and RF sub-chassis of the AVR-11,
ACR-111 and 15K-1 are provided in the Appendix.
Restoration Strategy
The answer to one fundamental question needs to be answered prior to starting work on a radio: “What
standard/effort of work is wanted (or warranted)?” – this can range from simply repairing the chassis to
operational condition, through to a full restoration, the latter comprising a complete strip-down and
rebuild such that it looks and works much as it did when it left the factory. The usual level that I will
extend to for a radio is what I term ‘sympathetic’, which is around 75% of fully restoring the chassis.
By ‘sympathetic’, I mean that the set be working close to its original specification, and made to appear
generally as it would as it left the factory, but with some ageing/wear through use, and perhaps
retaining a few ‘battle scars’ acquired through a few years of use – both above and below chassis and,
perhaps, also the cabinet. This level of work would generally entail re-stuffing/replicating paper and
electrolytic capacitors, and replicating/replacing out of tolerance resistors where these components are
visible, replacing broken parts with suitable parts resembling the originals where possible (or recovered
from ‘donor’ chassis of the same or similar type), thoroughly cleaning the chassis and cabinet, repairing
broken mechanical parts/mechanisms, and then aligning/testing/troubleshooting the finished set.
‘Sympathetic’ restoration requires significant additional effort to that needed simply to render the set
operational , ie. ‘repair-level’ work (replacing only those parts that have failed to the point that the set
no longer functions properly), or to render the set operational and more reliable long-term, ie. including
a level of preventative maintenance, such as ‘shotgunning’ (full replacement) of all paper and
electrolytic capacitors and any out of tolerance resistors (but not by re-stuffing or reproducing these to
give an original appearance), in what I would consider ‘refurbishment-level’ work, which may or may not
include some level of cabinet repair/touch-up or refinishing.
In a ‘sympathetic’ restoration, however, I would not expect that every replaced/re-stuffed or
reproduced component, lead dressing and other cosmetic nuances to be indistinguishable from the
original, or that every blemish in the cabinet finish had been removed such that the set looked ‘factory
fresh’ as would be in a full restoration in the strictest sense. Rather, the work would be ‘sympathetic’ to
the look of the original, ie. the chassis would receive thorough cleaning, the mechanical components
cleaned and lubricated (where appropriate), capacitors would be re-stuffed where possible (the originals
may not be present), reproduction parts, eg. dog-bone resistors, would closely match the general
style/colour code method of the originals, and replaced parts, eg. transformers or chokes, should be
chosen, or adapted/modified, to resemble the style or form of the original, though with some ‘artistic
license’, eg. in the design of reproduction labels if the originals are unavailable or unknown. Also, a
judgement call would be made regarding any modifications that had been made to the set, ie. whether
to reverse them or not.
Restoration of an RCA AVR-11 ‘Airport Receiver’ Gerry O’Hara
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In the case of this AVR-11, the owner, given the rarity of the set and its existing condition, (quite rightly)
requested a level of work that I would consider to be ‘high end’ sympathetic, bordering on full
restoration.
Background
This AVR-11 receiver was purchased in the greater
Vancouver area (BC, Canada) by its present
owner. Its condition was very rough, but a good
price was paid, reflecting how difficult this
receiver is to find today. It sat for several years
before the decision was made to bring it to a
condition close to as it was over 80 years ago. It
duly arrived in Victoria for this work in April, 2021
but had to await completion of several other
projects before work could start in June, 2021.
Preliminary Inspection
Some comments from initial visual inspection of
the radio on arrival (photo, right):
- The chassis metalwork on the upper side of the
main chassis and RF sub-chassis was in poor
condition and extensively pitted with rust (photo,
below). Although I could clean this up to look much better than was, I noted to the owner that it would
not be ‘perfect’ – I recommended to
the owner that I remove several
components, mask the remainder and,
after cleaning and degreasing, spray
the chassis with a steel-finish paint;
- The RF sub-chassis screen cover
(‘box’) was missing. The owner agreed
to ask a friend to check if there is a
spare one at the SPARC museum that
could be suitable, eg. off a scrap RCA
15K-1 chassis of the same period;
- The chassis metalwork on the
underside was in good shape,
especially in the RF sub-chassis (and its
lower screen cover was present);
- Most tubular paper capacitors on the
top of the RF sub-chassis (photo, left)
and underneath the main chassis
Restoration of an RCA AVR-11 ‘Airport Receiver’ Gerry O’Hara
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(photo, right) had been
replaced with modern
plastic film capacitors,
likely within in the last
decade or two given the
type of replacements
used;
- Three electrolytic
capacitors had been
replaced on the main
chassis;
- No capacitors had been
replaced on the
underside of the RF sub-
chassis;
- No resistors had been
replaced on the main or
RF sub-chassis with the
exception of part of the
‘Candohm’ multi-section metal-clad power resistor (extreme right of the photo, above);
- The tubes were all in very poor cosmetic condition (I recommended a set of NOS RCA tubes be
obtained to improve visual appearance, even if the existing tubes tested ok);
- The workmanship associated with the capacitor replacements was poor, though the replacement
components used were likely ok. I agreed with the owner that I:
• remove all the replaced tubular and electrolytic capacitors and re-use them where
possible/appropriate in reproduction part bodies, or re-stuff the original bodies/cans
where these were still in place; and
• reproduce replacement resistors where possible and required, ie. when the original part
was out of tolerance and/or in a critical position in the circuit where an out of tolerance
value would likely adversely impact the receiver performance.
- Some rubber-insulated wiring was present and I proposed to replace this with reproduction rubber
insulated wire where visible above the chassis (tube grid
connection wires) using scuffed/’grimed’ PVC insulated wiring,
and cloth-covered wiring elsewhere where the rubber
insulation had degraded significantly and risked shorting to the
chassis or other wiring/components.
- The slow motion tuning shaft wobbled laterally in its bearing
(photo, left) due to considerable wear and tear over its service
life, say airport service for a decade or two, likely followed by
Restoration of an RCA AVR-11 ‘Airport Receiver’ Gerry O’Hara
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radio amateur/shortwave listener use for a few decades. This issue would have to be resolved as it
would significantly detract from the operating experience of the set.
- Apart from the dial escutcheon being
absent and the dials being discoloured
and scuffed (photo, right), the front
panel cosmetics were reasonable, with
only minor paint scuffs and one small
area of missing paint exposing the bare
metal (steel). The owner had sourced a
replacement escutcheon and was
sending this to me, and I confirmed that
a replacement (reproduction) dial set
was available from Radio Daze. The
rear of the front panel was bare steel
and had areas of rust present. The
control fingerplates were generally in
reasonable condition, though the paint had worn/chipped off in some areas. Three of the knobs looked
like replacements, looking newer and having larger pointers than the others.
- The speaker grill cloth (felt) was in poor condition –
threadbare in places and looked like glue had been
spattered against it (photo, left). The speaker panel
paint was in generally good condition, though the
rear (bare steel) was badly rusted (photo, below), as
was part of the speaker frame and magnet clamp.
The speaker panel had been fitted with a switch
marked ‘Cabin’/’Deck’, with a flying lead and plug to
connect to a remote speaker. The switch wiring is
such that the ‘Deck’ speaker is the one fitted to the
panel and ‘Cabin’ is the remote speaker. The switch
is an unusual design, and switches all
the connections to the set, including
the high voltage field coil winding.
Restoration
Initially there seemed to be a dearth
of technical information on the AVR-
11, however, I finally found a copy of
the ‘Technical Information and
Service Data’ for this set here.
However, this is not the best of
scans, and while the text is all
Restoration of an RCA AVR-11 ‘Airport Receiver’ Gerry O’Hara
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readable, most of the component numbering and values are
illegible on the schematic and parts layout diagrams. I found
it necessary to cross-reference more legible scans of the
diagrams for the ACR-111 and 15K-1 , as well as the parts list
for the AVR-11 to ‘fill in the gaps’.
First Steps and Chassis Painting
RF Sub-Chassis
- I removed the front panel from the chassis and the RF sub-
chassis. It was soon apparent that the sloppy tuning action
was due to a worn bushing on the slow motion drive/tuning
control shaft: the inner concentric shaft of the slow motion
drive was a bit worn, but the outer bushing the main culprit.
- I removed the worn tuning shaft bushing and slow motion
drive shaft from the RF sub-chassis and a friend offered to
take the mechanism to a machinist he knew to see if a new
bushing could be made.
- I removed all the coil screening cans and
dial light shade from the RF sub-chassis
(part of the preparation for preparation for
spray painting) – photo, left;
- Neutralized the rust spots on the RF sub-
chassis and than polished it with
microcrystalline silica – this almost brought
the finish to being acceptable if it wasn’t for
three larger rust patches and some
persistent smaller ones;
- De-greased the chassis (again) with IPA;
- Masked-off coils, slugs, tuning capacitor, tube
bases etc.;
- Sprayed with two coats of non-etch grey
primer;
- Sprayed with two coats of chrome finish paint
(photo, right);
Restoration of an RCA AVR-11 ‘Airport Receiver’ Gerry O’Hara
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- Removed the masking tape/paper;
- Worked on the tops of the coil screening cans with
ultra fine steel wool and polishing compound to remove
pitting;
- Left the RF sub-chassis alone for a few days for the
paint to cure before trimming any minor overspray,
plus some cosmetic improvements around the tuning
gang and front of the sub-chassis (behind the dial area),
etc. and then temporarily refitted the coil screening
cans (though not screwed down) to check on how the
finished sub-chassis would look - photo, right.
Main Chassis
- Tested the power transformer on the main chassis. It
tested ok: 350-0-350vAC HT and the 5vAC and 6.3vAC
windings also both seemed ok, however, I noted that
the transformer was drawing around 20W with no load
- a little higher than I would expect, though there was
no continuity between any of the windings and the
laminations, or between windings. I left it connected and monitored the transformer temperature over
several hours and it was still at room temperature, so I concluded that the power transformer was ok;
- Strangely, the power transformer primary had been disconnected from the power switch by someone,
and there was no line cord to the set, or even any
remnants of one – a bit of a mystery;
- Removed the ‘RCA’
plate from the rear
apron of the main
chassis – the chassis
was badly rusted
beneath (photo,
left);
- Cleaned and degreased the main chassis with
naphtha/IPA using Q-tips and cloths. Sanded badly-
rusted areas and then rubbed with wire wool;
- Neutralized the rust spots then polished the top
and sides of the main chassis with microcrystalline
silica;
- De-greased the chassis (again) with IPA;
- Masked-off the IF transformers, power transformer,
flying leads, tube bases, etc (photo, right);
Restoration of an RCA AVR-11 ‘Airport Receiver’ Gerry O’Hara
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- Sprayed with two coats of acid-etch
grey primer;
- Sprayed with two coats of chrome
finish paint;
- Removed the masking tape/paper;
- Removed the power transformer top
shroud, removed/neutralized the rust
present on the shroud, primed it and
spray-painted it gloss black (photo,
below). I left the main chassis for a
couple of days for the paint to cure
before progressing the work.
I then worked on the tops and sides of the IF screening cans with
ultra fine steel wool and polishing compound to remove pitting,
and cleaned up any overspray I found on wiring or components on
both the main and RF sub-chassis. Next, I cleaned and replaced
the ‘RCA’ plate on the rear apron of the main chassis, and
replaced the (painted) power transformer shroud.
Electronic Restoration
Main Chassis – Part 1
First, I re-stuffed the three can electrolytics - the
two larger ones had an unusual construction
with the densest and most removal-resistant
innards (a dense, fibrous sticky mass sealed in
with tar) I have ever come across (photo, right).
I gave up trying to empty the cans of this
material and removed just the tar and enough of
the sticky fibrous material to install a suitable
modern electrolytic (22uF 500vw) beneath the
innards, most of which remains in place. I
installed a plastic diaphragm to separate the
innards from the ‘compartment’ at the base of
the capacitors where the new electrolytics are
installed and sheathed the electrolytics in heat-shrink as additional environmental protection. The cans
of these larger electrolytics are held together with metal tape, the cardboard sleeve and the metal
chassis clamp, so they are really solid. There are a few small dings on the capacitor bodies that I could
Restoration of an RCA AVR-11 ‘Airport Receiver’ Gerry O’Hara
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not remove as I could not extract the innards to allow
these to be worked out (think of them as ‘patina’…). I
removed most of the pitting on the cans using super-
fine steel wool, and managed to carefully polish the
cans leaving the lettering in place. The smaller ‘spigot’
type can electrolytic was straightforward – I used PVC
pipe/epoxy to support it internally after re-stuffing - the
photo, right, shows these capacitors prior to
reassembly. I then re-installed the three can
electrolytics onto the painted chassis (photo, below).
The 1uF paper case
chassis-mounted
capacitor was then
re-stuffed – this
was
straightforward,
with the old
capacitor ‘guts’
pulling out easily after warming the case slightly (photos, right).
The three smaller electrolytics located under the main chassis had all been
replaced by someone previously and the originals removed. In order to find
out what type of capacitor body was used, I found a photo of an ACR-111
chassis that had these in place (follow the arrows in photo, below left) – they
were single-ended cardboard box
types fixed to the rear apron of
the chassis. I found that the
mounting holes for these were in
the correct place on the AVR-11
rear apron, thus allowing me to
size reproduction ones correctly.
Most modern capacitors have lead
wires of 0.6 or 0.7mm diameter. The owner asked for
thicker connecting leads per the originals. I measured an
original RCA capacitor’s wires as 0.9mm diameter. I found
that the wire removed from the (perished) rubber-insulated
wiring removed from the chassis as part of the restoration
Restoration of an RCA AVR-11 ‘Airport Receiver’ Gerry O’Hara
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work was of the correct diameter and was already partly oxidised (ie. ‘looked the part’ perfectly), so this
proved to be suitable for this purpose.
Next, I inspected the Candohm resistor – one section (4Kohm) was open circuit, and had been replaced
by two large brown power resistors mounted on a bent piece of steel strip bolted to the chassis by
someone. Two other sections of the Candohm measured incorrect values, and one of these was
intermittent, with the resistance fluctuating slightly. I would normally replace a faulty Candohm with
multiple power resistors mounted on a tagstrip, however, the owner wanted the underside of this
chassis to look as original as possible, so this solution would not work well given this constraint. I
decided to think about what could be done and progress other aspects of the work in the meantime.
I wired the new electrolytics into the power supply section, replaced the rubber-insulated wiring under
the chassis (blue, yellow and green) as it was perished and crumbling - I replaced it with cloth-covered
wire. The yellow cloth-covered wire was a little ‘bright’, so I dulled it using a rag that I had used to clean
the chassis with (yes, even recycling the dirt!).
Next, re-stuffed the two metal-clad tubular chassis-
mounted paper capacitors (photo, right), and then
tacked in some replacement capacitors in place of the
few remaining tubular paper ones before powering-up
the main chassis to see if it worked and, if so, check
voltages etc. before I start the ‘re-re-cap’ (replacing the
capacitors someone had replaced previously with the
reproduction ones I would be fabricating later).
Speaker Panel
Before powering-up the main chassis, I installed a power
cord and tested the power switch - this worked ok, so I have no idea why it had been disconnected.
Next, I checked-over the speaker panel:
- The speaker was loose and two of the chamfered-head screws either side of the RCA emblem were
missing. I removed the RCA winged emblem and removed the original chamfered-head screws from
underneath and installed new (bright) chamfered-head screws in their place as these would be hidden
when the RCA emblem was reinstalled. I
then used the removed original screws in
the screw holes that were visible;
- There were some wires disconnected and
the ‘Cabin/Deck’ switch (photo, left) was
not working correctly – I repaired the wiring
and re-set/burnished the switch contacts to
regain continuity through each pole;
- On checking continuity through the
connectors/wiring/switch, I then found that
there was significant corrosion on the
connectors and cleaned those as well;
Restoration of an RCA AVR-11 ‘Airport Receiver’ Gerry O’Hara
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- One half of the output transformer primary was found to be faulty, reading around 27Kohms DC
resistance (the other half has a DC resistance of around 700 ohms). Typical 6F6 push-pull output
transformer primaries are between 200 and 500 ohms DC resistance (each section). Unfortunately the
resolution on the schematic I have (and on the ACR-111 schematic) is not good enough to determine the
correct value. To prove it was a transformer problem and not further corrosion in the connectors,
switch or wiring, I disconnected the
transformer and tested it independently
and confirmed it was the transformer. I
had some push-pull output transformers
and I selected a suitable ‘period’ one as a
replacement (~10Kohms plate to plate
impedance);
- The original output transformer is housed
(potted) in a small metal box, however, no
replacements I had to hand would fit into
this. As the speaker panel had already
been modified (with the ‘Deck/Cabin’
switch), I figured that it probably was not a
problem to leave the replacement
transformer ‘exposed’ (being a part from
the same period it looked like it had always
been there) and, if a better replacement was eventually sourced, it could be easily replaced;
I fitted the replacement output transformer (photo, above) and powered-up the main chassis. It was
drawing around 120W and the HT voltage was around 275vDC (a bit on the high-side), but only silence
from the loudspeaker. The fault was traced to an open-circuit primary on the output stage driver
transformer, ie. in the plate circuit of the 6C5 audio
driver tube. This was very unfortunate, and
unusual, as the only three possible failure modes
for this transformer are: 1) corrosion on a
connection or wire inside the transformer, 2)
internal short in the 6C5 tube, or 3) an accidental
(temporary) short-circuit by someone, as there is no
capacitor or other component to cause a fault
(short) on the plate side of the transformer that
would overload the primary sufficiently to cause it
to act as a fuse. Its rather unlucky to have both a
failed driver and output transformer on the same
chassis!
I had three split-secondary inter-stage transformers in my ‘junk box’: two of the possible replacements
(photo, above) would fit inside the original transformer enclosure, the third was too large to fit. I
temporarily jury-rigged one of the smaller driver transformers under the chassis (a NOS one of Stancor
Restoration of an RCA AVR-11 ‘Airport Receiver’ Gerry O’Hara
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manufacture), held in by its leads6. I powered-up the main chassis again and it was now working –
plenty of noise from the speaker. A video of this initial function test of the main chassis can be viewed
here.
The IF, AGC, BFO and audio stages were all working reasonably well, with plenty of IF and AF gain. The
driver transformer I used had a 3:1 turns ratio, which is typical, but the DC resistances of the windings at
581 ohms primary and 2.14Kohms (centre tapped) secondary, are significantly different to the original
spec. (722 ohms primary, 291 ohms secondary). The higher DC resistance of the secondary may affect
the bias condition of the 6F6 output tubes, but I figured that I would investigate that later if there was
low output or distortion problems (there wasn’t).
Next, I manufactured the repro tubular paper capacitor bodies. I experimented with a new
manufacturing method for these, using stiff brown ‘Kraft’ paper for the (slightly oversized) labels with a
supporting ‘roll your own’ cardboard tube underneath. The 'artwork' for this style of RCA capacitor is
very simplistic, comprising only the part number
designation, ie. capacitor value, voltage rating and another
number (purpose unknown to me!). They don't even sport
a ground indicator band like most tubular paper capacitors
from this era, instead, the ground end of the capacitor
(outer foil) appears to coincide with the end of the part
number (as this is how all the remaining original capacitors
were orientated). I added an outline rectangle to the
artwork to facilitate cutting the paper labels to the correct
sizes (photo, right).
The Kraft paper I had was on a large roll, so I had to cut it
into letter size sheets to fit my laser printer paper tray.
Once the labels were printed and cut from the sheet, they
were secured with a piece of masking tape at one end to,
and then wrapped around, a fabricated cardboard tube of the correct diameter. The label was
positioned so the seam was on the opposite side to
the label, and then held in position with stick glue.
The ‘surplus’ width of one side of the label was then
tucked into one of the ends of the cardboard tube.
The new capacitor was then installed into the
cardboard tube, having the larger-diameter wires
attached (soldered on – photo, left) if needed, eg.
the 0.01uF capacitors I used have 0.6mm diameter
leads that look odd emerging from a repro capacitor
body, so these were replaced with the wire
recovered from the rubber-insulated wires removed
6 I had left the original driver transformer enclosure in place when I painted the main chassis – mainly because it is secured using bent-over
metal tabs and these are difficult to get back in place correctly (and tightly). I figured that removing and replacing this would not be easy,
especially retaining the above-chassis cosmetics if I re-stuffed the driver transformer enclosure
Restoration of an RCA AVR-11 ‘Airport Receiver’ Gerry O’Hara
15
from the chassis. However the 0.1uF parts I used are fitted
with 0.9mm diameter leads and these look ok as they are.
The surplus width on the other side of the label can then be
tucked into the other end of the cardboard tube, and the
open ends then stuffed with a crumpled slip of paper as
packing and heat protection for the new part (photo, right)
before squirting in some brown hot-melt glue to seal the
ends. The finished capacitors were then coated with IPA-
diluted
amber
shellac, giving an appearance very close to the
originals (the lower capacitor in the photo, left, is the
reproduction part). The photo below shows the
tubular capacitor ‘production line’…
Once completed, all the (16) reproduction tubular
paper capacitors were installed into the main
chassis, along with the three repro carboard box
electrolytics and two re-stuffed metal-clad tubular
paper capacitors. The main chassis was then re-
tested and found to function much as it did as
before.
RF Sub-Chassis
Before further undertaking any further work on the RF sub-chassis, I
thought it would be a good idea to re-install the five RF coil screening
cans permanently to avoid any damage while handling the unit.
Before I did that, I carefully checked the rubber-insulated wiring from
the coils to the tube bases and band change switch – surprisingly, all
but one (a green one) still had supple insulation and were generally in
good shape. I replaced the one where the insulation was showing
some degradation with scuffed PVC wiring of the same colour (looks
almost identical to the original rubber insulation – photo, right). None
of the coil wiring is close to the chassis or other components and
Restoration of an RCA AVR-11 ‘Airport Receiver’ Gerry O’Hara
16
therefore does not pose a shorting risk, so the remaining wiring to the coils
should last for decades, and, even if (when) the insulation fails, it will not
cause a problem;
- Re-installed all the coil screening cans - a really fiddly job. I had to make a
special tool to install some of the nuts under the chassis – photo, left;
- Replaced the four tube grid connection leads (all perished green rubber
insulation) using scuffed green PVC insulated wiring to mimic the
appearance of the original wire;
- Inspected the remaining rubber-insulated wiring on the RF sub-chassis and
ended up replacing it all - I used cloth-covered wiring to match the most
prevalent insulation type. I also replaced the cloth-covered antenna connection wire above the chassis;
- Replaced the (5) tubular paper capacitors with reproduction ones;
- Cleaned the band change switch with Deoxit and Q-tips.
This left the resistors to be checked on the RF sub-chassis and replaced if needed before installing the
sub-chassis onto the main chassis for a preliminary function test of the complete chassis.
Main Chassis – Part 2
I then moved back to the main chassis and the Candohm. I had a reply to a post on my Facebook ‘Radio
Restorations’ page suggesting I open up and re-build the old one. At first I thought the person replying
meant rewinding it with suitable Eureka wire, but he then explained that he meant removing the guts
and installing modern power resistors, as he had done on a Zenith restoration. I agreed this was an
interesting idea, but initially thought this would be difficult as power resistors are quite ‘fat’ and the
Zenith had a different style of Candohm – broader and with an open side. But then I had the idea of
calculating the dissipation in each section of the Candohm based on voltage readings I took on the very
similar ACR-111 main chassis under varying signal conditions (the voltages vary due to the effect of the
AGC action on tube biasing). I then developed a spreadsheet (see Appendix) that calculated the
maximum dissipation for each section under these conditions. Allowing for an approximate 1.5x factor
of safety (FOS) and ‘rounding’ (up) the wattage thus derived, the calculations showed that the 4Kohm
section needed to be 6W, the 3.6Kohm section 4W, the 1.2Kohm section 2W and the 200 ohm section
4W. I surmised that these values may be achieved using strings of 2W resistors enclosed in a 'fishpaper'
or similar heatproof insulating sleeve inside the old Candohm metal case. The metal Candohm sleeve
(bolted to the chassis) may act as a heatsink - if so, this would add to the FOS. Before I attempted to do
this I asked the owner if he was ok with
this approach as, if unsuccessful, it would
result in the Candohm being destroyed.
Before I attempted this, I jury-rigged a
network of 2W resistors to make up the
required resistances and wattages for each
section of the Candohm, attaching these to
a plastic chopstick(!) – photo, right, to
Restoration of an RCA AVR-11 ‘Airport Receiver’ Gerry O’Hara
17
maintain some physical integrity during testing. The next steps in this process were to:
- Disconnect the Candohm (and the ugly pair of large ceramic power resistors someone replaced the
4Kohm section of the Candohm), and connect the jury-rigged resistor network into the main chassis;
- Power-up the main chassis – it worked just as it did before. I left it running for around 30 minutes and
then made a number of voltage, dissipation and temperature checks on the 2W resistor array. The
checking showed that all the resistors in the array were operating well-within their wattage
specifications per my calculations. The voltage readings were not the same as I recorded for the ACR-
111 – not having the RF sub-chassis installed was likely affecting this, as well as other possible issues
with the chassis not yet identified, but the voltages were all reasonable;
- Check the Candohm resistor string while it was removed from the chassis,: the 200 ohm section
measured 209 ohms, the 1600 ohm section measured 1320 ohms, the 3600 ohms section measured
3540 ohms (but intermittent open-circuit when the connection lugs were moved slightly – not a good
sign), and the 4000 ohms section was open circuit. Given this, and that the connection lugs were all
loose, I didn’t think it was a good idea to re-install the
Candohm in its current state, and the set’s owner agreed to
proceed with the re-stuffing idea. This was done as follows:
- I cut the Candohm metal case open with a Dremel tool
(photo, right), removed the ‘guts’ (photo, below) – its quite
scary to think there is almost 300v across this component
given the insulation they had used (varnish-impregnated
paper/thin cardboard) - and the condition it is now in, ie.
well-baked and crumbling, as well as all the solder lugs
flopping about);
- I installed a strip of thin fibreglass-reinforced silicone
baking sheet on the inside of the metal Candohm
case, holding it in place with a few dabs of epoxy;
- Next, I installed the solder tags: the tags were
modified slightly (one end trimmed off a little), and
the collars of each given two layers of shrink wrap
where they pass through slits made in the silicone
sheet and the corresponding holes in the metal case;
- I then reinforced and spaced the rear of each of the solder lugs with strips of stiff ‘fishpaper’, and
secured these with a little J-B Weld (photo, below);
Restoration of an RCA AVR-11 ‘Airport Receiver’ Gerry O’Hara
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- The 2W resistors were removed from the jury-rigged arrangement on the chopstick and installed in the
prepared Candohm case (photo, below). The resistances between each solder lug was checked and
found to be correct.
The completed re-stuffed
Candohm was then re-
installed in the chassis (photo,
right), and the set powered-
up while monitoring the case
temperature of the Candohm
- after around an hour the
maximum temperature along
the Candohm case was
around 77C.
With the Candohm problem
dealt with, I next set about
removing both the original
and jury-rigged driver transformers from the main chassis, gutting the original one, and re-stuffing it –
thankfully, the NOS Stancor driver transformer that had been working ok jury-rigged under the chassis
just fit inside the original transformer enclosure
(photo, left). I removed the old driver transformer
from the enclosure (it was embedded in tar of course!)
– this needed some heat to be applied and the heat
affected the paint on its enclosure, so I decided to re-
spray it. The transformer enclosure was originally
cadmium-plated, but when I spray painted the chassis,
it was left in place and received the same (‘chrome’)
paint as the chassis. So, as it was now off the chassis, I
decided to respray it with a metallic paint closer to its
original cadmium colour (I used Krylon ‘Satin Nickel’) –
this provides a nice subtle contrast to the chassis
colour. The fixing lugs could not be tightened hard against the underside of the chassis without the use
of a hammer(!) or a large clamp I did not have, so I applied a small dab of J-B Weld under each (between
the lug and the chassis) to fill the remaining small gaps - this secured it tightly, and it was then re-wired
into the circuit.
Restoration of an RCA AVR-11 ‘Airport Receiver’ Gerry O’Hara
19
I then measured the resistors on the main chassis – over half were out of (20%) tolerance, though some
are non-critical and could be left in place. The resistors in this chassis are a bit of a ‘mixed bag’ – some
ceramic-bodied ones, some regular cylindrical carbon compo ones, and even a couple of ‘dogbones’ – all
looked original. So, the question was how best to repro them? – my suggestion to the owner was to
replace the ones that needed to be replaced (whatever type) with repro ones all of the same type rather
than make a variety of repro types. I cannot reproduce narrow colour bands accurately (as per the
ceramic bodied ones), so I suggested using either the body-end-spot colour code (per the dogbone), or
the broad colour bands on plain cylindrical types as per the non-ceramic bodied ones. It was agreed to
use the latter method.
Over the years I have tried various methods to repro resistors of a variety of styles – always trying to
improve appearance while expediting/simplifying construction and shortening the time needed to make
the reproductions (as I do with capacitors) – I am not fixated on a particular method and am always
willing to learn from others and from experimentation. This time I tried another fabrication technique
for the resistors:
- Cut a plastic drinking straw7 of the
correct internal diameter to the
correct length to act as a mould for
the resistor;
- Partly fill the straw mould with
two-part quick-setting epoxy
wood-filler (‘Bondo’ or similar
fillers would likely work just as well
– just make sure they don’t
conduct electricity!);
- Coat the new resistor with the
wood filler;
- Insert the new resistor into the straw
mould;
- Top up the straw mould with wood filler
and tamp out any air spaces using a needle;
- Wait for the epoxy filler to set8 (photo,
above);
7 Single-use plastic items such as drinking straws are not available for purchase in several jurisdictions these days, however, paper straws will work just as well for this application 8 Around 15 mins for the wood filler I used, which only has a ‘working’ time of around 3 – 5 mins. I would prefer this to me more like 10 mins to allow larger batches of resistors to be made at a time – I found I could only make 3 or 4 resistors before the filler became unworkable. A slower-setting epoxy, such as ‘Milliput’ modelling epoxy, available from most hobby shops, would likely be much better – almost one hour working time but takes four hours to set hard
Restoration of an RCA AVR-11 ‘Airport Receiver’ Gerry O’Hara
20
- Cut the straw mould longitudinally with a craft knife (photo, left), and peel it off from the set filler - any
score mark left by the knife blade in the filler looks like a moulding line on the repro as per the original,
so are ok to leave in place;
- Trim any surplus filler from the
moulded repro resistor and then
roughen the surface with
sandpaper;
- Paint the repro resistors with
the correct colour code, allow to
dry, and then overcoat with IPA-
thinned amber shellac to age and
give a 'satin' finish to the resistor,
similar to the originals (photo,
right). Note - they do not have to
look 'perfect' as the originals
most certainly were not - especially after 80 years!
I fitted all the repro resistors into the main chassis (20 replaced initially), plus 7 more on the RF sub-
chassis – photo below. A photos of the chassis with the repro components installed is shown below
(lower RF sub-chassis cover removed. A photo with the cover installed is on shown on page 33).
Restoration of an RCA AVR-11 ‘Airport Receiver’ Gerry O’Hara
21
Troubleshooting
The main chassis was then tested after changing out the resistors – it was working much as it was
before, including an AGC fault that I had noted previously, ie. no AGC action at all.
The AGC circuit uses the 6R7 pentode/dual-diode tube, but changing the original tube for one of the
NOS RCA 6R7 tubes did not fix the problem – so I decided to fit the RF sub-chassis onto the main chassis
and connect it into the circuit to see if that made any difference. After fitting, both chassis seamed to
be functioning ok, though the AGC fault persisted. I decided to troubleshoot the problem:
- Noted that the ‘AGC’ voltage that is derived separately using the 6H6 detector for the eye tube was
working well, with over -15vDC generated on a strong signal on the Broadcast Band;
- The problem therefore seemed to be a biasing problem on the 6R7 tube – this was a bit odd as I had
changed out all the resistors in that section of the circuit: one of the original 1Mohm resistors was open
circuit and I would have put money on that being the problem – but it wasn’t. I double-checked all the
resistor values and the wiring in that section of the chassis and all was ok. A short video of the complete
chassis being tested at this stage can be viewed here.
I finally traced the AGC problem to the first NOS RCA 6R7 tube I had installed in the chassis being ‘dead’
– the heater was working but there was zero emission on both the pentode and the diode sections –
possibly an open circuit cathode as this element is common to all the sections in the tube. If so, this is
an unusual fault. Fitting the second NOS RCA 6R7 completely cured the AGC issue, with well over -
15vDC AGC voltage generated on a strong local signal on the Broadcast Band and no 'blocking’ of the
receiver at all, even on a very strong local signal with the RF gain fully advanced.
Given this issue with the 6R7 tube, I
tested all the remaining NOS RCA
tubes before I installed them in the
chassis (photo, left). All were ok,
except one of the 6C5s which only has
55% emission and a gm of 1450um.
The other had over 80% emission and
a gm of 2100um (2200um is typical for
a good 6C5 tube). I put the weaker
tube in as the push-pull driver for
now, and the supplier agreed to send
a replacement 6C5 tube.
I decided to replace the resistor in the
BFO coil screening can and the two
resistors located in the 3rd IF
transformer can. These were all marginally out of tolerance and non-critical, but I thought I may as well
change them out as they are awkward to get to if there is a problem in the future. In addition, I also
installed three new reproduction resistors in the IF amplifier circuit: a 680 ohm resistor in each of the IF
Restoration of an RCA AVR-11 ‘Airport Receiver’ Gerry O’Hara
22
amplifier cathodes, and a 220 ohm resistor across the bias switch9 - I had noted that these were all
marginally out of tolerance (high), and as these could affect the overall gain of the receiver, decided to
change them out. Doing this increased the AGC voltage on a strong signal by around -3 volts, so worth
doing as this indicates additional gain has been obtained. I also replaced the 1Mohm resistor in the eye
tube holder.
Slow Motion Tuning
I found that the new bushing that had been
machined for the slow motion drive was not
quite correct: the original was made out of
steel and had a slight internal taper, whereas
the replacement was machined from brass
with no taper. However, I thought of a way of
using it together with the old bushing to
prevent the shaft wobbling laterally like it
used to in the old (worn) bushing. The original
bushing actually comprises three parts: the
flange, an outer bushing and a slightly tapered
and stepped insert, all made out of steel. I
suspect the insert is hardened steel as the three spring-loaded ball bearings forming the epicyclic speed
reduction contact this when assembled (I think brass would wear prematurely where it contacted the
hardened steel ball bearings);
The epicyclic slow motion
tuning mechanism (refer
to diagram, right),
comprises an inner shaft
which fits concentrically
into a hollow outer shaft.
Three ball bearings pass
through three holes
spaced at equal intervals
around the outer shaft
that, when assembled,
contact both the inner
shaft and the external
bushing under spring
compression. The inner
shaft has a (fourth) ball bearing attached to its end. Rotating the inner shaft rotates the ball bearings
that carry the outer shaft around with them as they contact and rotate against the bushing. The outer
shaft thus rotates slowly relative to the inner shaft (approximate 3:1 ratio) through this action.
9 The ‘bias switch’ adds an additional resistor (220 ohms) in series with the cathode resistor (680 ohms) of the 1st IF stage to reduce its gain slightly on bands ‘X’ (‘Long Wave’) and ‘A’ (Broadcast Band). Interestingly, the ACR-111 uses lower value IF amplifier cathode resistors (390 ohms), that would result in higher gain in that receiver design
Restoration of an RCA AVR-11 ‘Airport Receiver’ Gerry O’Hara
23
To assemble the mechanism, the inner shaft is pressed down against a (captive) spring in the base of the
outer shaft. The end of the inner shaft behind the fourth ball bearing is shaped such as to allow the
three rotating ball bearings that contact the outer shaft to retract slightly within their corresponding
holes in the outer shaft. When positioned in this 'sweet spot', it allows the slightly tapered bushing to
slide over the ball bearings and against the step in the bushing. When released, the compression in the
spring then acts against the shaft and pushes the three ball bearings outwards, which stops the inner
shaft from pulling out of the tapered/stepped bushing and thus maintain contact with the inner shaft,
such as to allow the epicyclic slow motion action between the inner (slow) and outer (fast) tuning shafts.
The outer shaft has the logging scale attached to it only, the tuning knob being attached to the inner
(slow motion) shaft.
I decided to use the new (brass) bushing as a ‘stabilizing’ collar
on the outer shaft (yellow arrow on photo, right), with the old
(steel) bushing located in its original position (green arrow),
contacting the ball bearings. Amazingly, the new bushing was
just the right length to allow this to work, placed behind the
flange where the bushings secure to the RF sub-chassis, and
not interfere with the pinion that engages the quadrant gear
fixed to the tuning gang shaft. Before assembly I lubricated
the inner shaft and bushings with lithium grease. Once
assembled, I added a fillet of J-B Weld around the
bushing/flange function on both the original and new bushings to provide additional stability.
Final Assembly (almost)
With the slow motion drive refitted, I installed the new (repro) dials onto the RF sub-chassis (photo,
below), and re-installed the linkage to the band-change rotary switch. I found that the ‘bias switch’,
which changes the bias on the 1st IF amplifier, and hence its gain, was faulty. The shaft of this switch is
the actual band change switch shaft on the front panel. This is an unusual switch design in that it trips
around half way around its shafts travel. Luckily, a
good squirt of Deoxit and working the switch several
times fixed it, but adjusting it so it switched the bias
between bands ‘X’/’A’ and ‘B’/’C’ correctly/reliably
took a while to sort out. The switch introduces a 220
ohm resistor into the cathode circuit of the 1st IF
amplifier to reduce the gain slightly on bands ‘X’ and
‘A’.
I re-used the original rivets (flipped around as I had
drilled them out) to fix the main dial in place, each
held in place with a dab of epoxy. I did not have any
suitable rivets for the logging dial (the originals were
destroyed during removal), so I used some small
screws instead (photo, right). I also cleaned up all the
dial bulb holders and installed a set of five new #46
dial bulbs.
Restoration of an RCA AVR-11 ‘Airport Receiver’ Gerry O’Hara
24
Alignment and Testing
Alignment of the IF was straightforward – simply peaking six slugs on the IF transformers to 460KHz and
zero’ing the BFO. One of the 1st IF transformer slugs was jammed, but I managed to coax it free and all
was then ok.
I then moved on to the RF alignment – all went well, apart from a few of the locknuts on the plunger
type trimmers being stiff and I could not tune the IF trap for a dip when feeding an IF (460KHz) signal
into the antenna terminal.
The specified RCA alignment tool would not turn several of the RF trimmer locknuts, and a socket
wrench would not fit into some of the available spaces. With a bit of effort, I managed to free the
recalcitrant ones sufficiently with some bent-nose pliers to allow adjustment of their plungers ok.
Having a functional IF trap is usually not a significant cause for concern, but can, in some circumstances,
result in unwanted heterodynes and/or unwanted breakthrough of strong low frequency signals. So I
decided to investigate this problem:
- A continuity check on the IF trap coil showed it was open
circuit. I could not see any damage or disconnected wire
with the coil in position, so I removed it from the RF sub-
chassis;
- On turning the IF trap coil upside down, it was obvious the
coil had been zapped by static – black discolouration on the
former and one of the wires had burnt out (photo, right).
Thankfully the burnt-out wire was right at the point where it
entered the coil, so I was able to tease some good wire out,
tin it and attach another piece of wire to it, extending it to
the terminal on the coil former. The coil then measured 23
ohms DC resistance with an inductance of 2.6mH, which
both seemed reasonable values. I also
tested the resonant (series) silver mica
capacitor and it was good/within
tolerance;
- The IF trap was then reinstalled into the
RF sub-chassis (photo, left), and I was
able to obtain a very deep dip in the
receiver's response to 460KHz fed to the
antenna terminal, so that issue was now
resolved.
Restoration of an RCA AVR-11 ‘Airport Receiver’ Gerry O’Hara
25
The chassis was now performing very well with good
dial accuracy and sensitivity on all bands. By this
time, the chassis had been operating continuously for
around 5 hours, so I took the opportunity to measure
some temperatures (ambient 25C), eg. power
transformer (50C), output transformer (28C), speaker
field coil (33C), and the chassis side adjacent to the
re-stuffed Candohm (40C – photo, left)) – these all
seemed ok.
Cosmetics
The AVR-11 was in poor cosmetic condition on
arrival: the cabinet had several dings and the
paintwork was in very poor shape, the dials were
worn and discoloured, the dial escutcheon was
missing, the fingerplates were worn, some knobs did
not match, and the chassis and rear side of the front panel were rusted. The speaker panel was also
badly rusted on its rear side and the speaker frame was also rusted. Clean-up and painting of the
chassis and fitting of a reproduction dial set is described elsewhere in this article. The following
describes work to render the cosmetics of the set into a much more acceptable condition:
- The control fingerplates were removed and the paintwork touched-up on each. I then polished them
with Novus #1 and #2;
- Scuff marks and missing paint on the front panel were touched-up with colour-matched grey acrylic
paint;
- I tried to resurrect
the black felt speaker
cloth, but it was too
far ‘gone’ – I only had
green felt (baize) in
stock, so I fitted that,
then cleaned and
touched-up the
winged RCA emblem
to see how it looked
(photo, right);
- Rust was removed
from the rear of the
speaker panel and it
was given a couple of
coats of satin grey
spray paint;
Restoration of an RCA AVR-11 ‘Airport Receiver’ Gerry O’Hara
26
- The rust on the speaker
frame was removed and
the speaker panel
reassembled (photo,
right).
- Cleaned all the knobs
and touched up the paint
on the (metal) tuning
knob. I noted that some
knobs had longer
pointers than the others.
Also, some had jammed
set screws that
stubbornly refused to
cooperate… so I decided
to try to source a matching knob set (the knobs are a fairly common style for communications receivers
and electronic equipment of the 1930s). Luckily a friend had a set of eight matching knobs that were
very similar to the originals, though with larger pointers;
Next, I tried fitting the new dial escutcheon to the front panel - it had been removed from an RCA model
10K-1 domestic receiver, but is an identical design to the one that the AVR-11 was fitted with.
Unfortunately the front panel has oversized holes, so I had to secure nuts in them using J-B Weld. Once
the J-B Weld had set, I removed the rust from the rear of the receiver front panel and gave it two coats
of the same satin grey spray paint as on the rear of the speaker panel;
The front panel was then reinstalled
onto the chassis and the dial
escutcheon fitted. However, I found
that it fouled both the pointer and the
logging scale dial. Perhaps the 10K-1
escutcheon design is slightly different,
eg. it was deeper such as needed to
pass through the wooden panel on a
domestic cabinet. There were two
options to mitigate this problem: 1)
space the front panel away from the
chassis, or 2) install a spacer between
the escutcheon and the front panel. I decided to opt for the latter as the former would have shortened
the effective length of all the control shafts and some were a little on the short side already. I cut a
double-thickness gasket from sheets of neoprene to act as the spacer (photo, above). This worked well
and also provided a ‘cushion’ between the front panel and the escutcheon. I had to use longer screws,
and then found that two of the longer ones fouled the logging scale(!), so I trimmed them all down to
the optimal length. The heads of the screws were painted bronze to match the escutcheon colour. The
Restoration of an RCA AVR-11 ‘Airport Receiver’ Gerry O’Hara
27
knobs were then
fitted, along with
the phones jack, eye
tube and standby-
light (photo, right)
before testing the
chassis again.
A Zoom call was
then set up with the
set’s owner to
discuss some
aspects of the work
completed so far,
including:
- The touched-up front panel control fingerplates and (less so) the winged RAC emblem are not ‘perfect’.
I had touched them up using marker pens and then toned them down with Novus #2/#1 polish before
applying Armor All. Although much-improved, I advised the owner that the only way to render these as
‘perfect’ would be to find a firm than could refinish them professionally: they are all ‘reverse etched’ (it
even says that in the brochure), ie. the lettering is embossed – this should allow enamelling, powder
coating or some sort of etching/anodizing by a specialist firm to be undertaken;
- I had noticed occasional minor/intermittent 'hesitation' in the slow motion mechanism. This was likely
due to wear. I had not noticed this until the front panel was re-installed, the large tuning knob fitted
and I started to rotate the dial frequently to test the chassis. The slow motion tuning is certainly
useable, but again, not 'perfect' and could become annoying (and possibly worsen) during frequent use;
and
- I only had green baize (felt) in stock, so I used that for the speaker grill (I placed it over the original
black felt, so could be removed to expose the original if desired). I also have a strip of green baize I
intend install inside the rear of the cabinet where there was a strip of very tatty black felt prior to me
removing it to paint the cabinet. The colour of the baize could be changed using dye if the owner
wanted this.
Finishing Touches
As a result of the Zoom call with the owner of the RCA AVR-11, it was decided to:
- Dye the green baize on the speaker grill black to render it the original colour;
- Remove the slow motion drive and try to eliminate the intermittent slight 'hesitancy';
- Paint the dial escutcheon black (the owner had found a photo of an AVR with the original escutcheon
present and it was finished in black).
On this basis, further work undertaken included:
Restoration of an RCA AVR-11 ‘Airport Receiver’ Gerry O’Hara
28
- Removal of the dial escutcheon from the front
panel, removal of the plastic dial cover from the
escutcheon, then roughening the surface of the
escutcheon with wire wool, cleaning it with IPA,
then applying two coats of satin black spray paint
(photo, right);
- Applying a half bottle of ‘Feibings’ black leather
dye to the green speaker baize (photo, below);
- Removal of the knobs, eye tube, phones jack,
front panel, and lower RF sub-chassis screening
cover to allow removal of the slow motion drive.
The following work was then undertaken to
mitigate the occasional hesitation of the pointer
while tuning the receiver:
- Disassembled the slow motion drive, cleaned all
the grease off all bushings and shaft surfaces as
well as the ball bearings, and then degreased all surfaces with naphtha. On close inspection for
any issues that might be the cause of the
problem, or at least may be contributing to
it, I noted that the bearing surface on the
inner shaft that contacts the ball bearings
had uneven wear/flat spots (tip of arrow in
the photo, right). Ideally a new inner shaft
would need to be machined to fix this,
however, I decided to proceed with
reassembly to see if any improvement
could be made by careful assembly and adjustment;
- Applied a light coating of lithium grease to the insides of the bushings, and a tiny spot on the
end (thrust) ball bearing on the inner shaft, taking care that no grease contacted the three
epicyclic drive ball bearings or contact surfaces to maximise friction at these points and hence
limit any slippage. Reassembled the slow motion drive, lightly greased the pinion, and re-
installed it into the chassis;
- Adjusted the position of the quadrant gear on the tuning gang shaft slightly to avoid any
possible fouling with the end of the brass bushing;
- Adjusted the pointer clearance slightly to avoid any possibility of contact with the heads of the
rivets securing the dial; and
Restoration of an RCA AVR-11 ‘Airport Receiver’ Gerry O’Hara
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- Adjusted the position of the slow motion drive relative to the quadrant gear to minimize
friction.
I then tested the slow motion drive for any slippage – it was still smooth and wobble free,
however, over multiple rotations I detected a very minor hesitation a couple of times. It was
therefore agreed with the owner that the drive be taken to a local machine shop to see if they
could machine a new bushing and inner shaft. Unfortunately, the machine shop was not 100%
confident they undertake the work successfully and turned the job down. Luckily, I recalled one
of my radio friends saying he knew a good machinist, so I had
a word with him – a couple of weeks later the re-machined
slow motion drive was in my hands: a new inner shaft and a
new bushing (both in steel) had been made – photo, right -
and were a perfect fit. The drive now felt very smooth, but I
felt it needed some lubricant – I found that a thin machine oil
worked better than a light grease for the roller-bearings.
After fitting the drive to the chassis, only some minor
adjustments were needed to render the tuning action
‘smooth as silk’ and there was no sign of any hesitation that
bugged the worn-out original.
- Finally, I had obtained a set of matching knobs from a friend, however, these had large pointers
than the ones originally fitted to the set and the owner thought these did not look quite right. I
had five of the original knobs with the smaller pointers, so I trimmed the pointers on three of
the set of knobs I obtained from a friend to match these. I then painted all the pointers an off-
white colour so they matched and looked slightly ‘aged’.
Cabinet
The cabinet was powder blasted (silica and walnut shell powder), and then checked for dings – finding
several, plus some severe rust pitting, it was decided to
fill the dings and pitted areas before refinishing. Also,
the owner had noted that one of the side strips where
the receiver and speaker fix to was bent outwards,
such that the mounting screws for both the receiver
and speaker unit were difficult to insert. I measured
the width I found it was a quarter inch ‘out’ at the
centre point. I also found a couple of small drill holes
in the top of the case. I filled the holes in with JB-
Weld as a first step, then sanded any remining minor
areas of rust (there was a little remaining after the
sandblasting). I then de-greased the cabinet with IPA
before applying an initial two coats of grey etching
primer to prevent formation of any surface rust as
further work on the cabinet would be delayed by a few
weeks due to other commitments.
On returning to the cabinet work a few weeks later, I:
Restoration of an RCA AVR-11 ‘Airport Receiver’ Gerry O’Hara
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- Filled a deep ding on the top front
corner and pitting where the
previously badly-corroded patches on
the top of the case were with two-
part epoxy filler (photo, right);
- Sanded the filled areas flush with the
surrounding paintwork and feathered
the edges;
- Straightened the bent front
radio/speaker mounting strip back
into place using a large clamp (photo,
below). Following this, the pairs of
mounting holes were now all
18.25” apart, which matched the
receiver and speaker mounting
hole distances;
- Applied three additional coats
of high-build primer/filler to the
top and top edges of the
cabinet;
- Applied two additional coats of
regular grey primer to the
remainder of the case;
- Wet sanded the high-build
primer with 800 grit wet and dry
paper;
- Filled in a small ding on the
upper right corner and some
minor pock-marks on the top of
the cabinet with epoxy filler;
- Wet sanded the
newly-filled areas with
120 then 800 grit
paper;
- Applied three more
coats of high-build
primer to the top of
the cabinet;
- Wet sanded again with
800 grit paper;
- Dried the cabinet with
a hairdryer and
cleaned it with IPA
(photo, right);
Restoration of an RCA AVR-11 ‘Airport Receiver’ Gerry O’Hara
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- Applied three coats of satin
black spray paint to the bottom,
inside, outer side panels and the top
and left it for two days, then
- Lightly sanded the cabinet and
gave it a final coat of satin black
spray paint;
- Installed a strip of felt in the
rear of the cabinet at the receiver
rear apron height – photo, below
(the cabinet had a similar strip of
felt installed that had to be removed
for the refinishing work).
Finishing-up
The chassis and speaker panel
were then installed in the cabinet
– I realized that the screws that
had been holding thes in place
were of different types, so a set of new screws were obtained and the heads painted with satin finish
grey enamel to blend in with the colour of the front panel (photos on pages 34 and 35). The owner has
a cover for the ‘Magic Brain’ RF chassis and this will be cleaned-up and fitted at a later date.
Closure
The RCA AVR-11 is a somewhat a compromise design, tailored to suit the specific needs of the niche
market segment it was aimed at. Maybe not quite as ‘leading edge’ as the contemporaneous ACR-111,
but an impressive receiver nevertheless, and it must have been a revelation to those used to many
earlier receiver designs. Most impressive in my book though is its imposing ‘presence’ in its dual-height
desktop cabinet – it simply looks very, very ‘cool’… (and it works like a charm too!!).
Restoration of an RCA AVR-11 ‘Airport Receiver’ Gerry O’Hara
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Above-chassis views of the AVR-11 after cleaning, painting, fitting reproduction tubular paper
capacitors and resistors on the tuning gang (RF sub-chassis), and re-stuffing the can electrolytics
on the main chassis. All the tubes fitted are NOS, manufactured by RCA, and selected for
combined good performance and cosmetics. A cover will be fitted over the RF sub-chassis
Restoration of an RCA AVR-11 ‘Airport Receiver’ Gerry O’Hara
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Under-chassis view of the RCA AVR-11 after fitting reproduction tubular paper and box electrolytic
capacitors, reproduction resistors, and re-stuffing the can electrolytics and Candohm resistor
Restoration of an RCA AVR-11 ‘Airport Receiver’ Gerry O’Hara
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The restored AVR-11 chassis and speaker installed in the re-finished dual height desktop cabinet
Restoration of an RCA AVR-11 ‘Airport Receiver’ Gerry O’Hara
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Oblique view of the restored AVR-11 chassis and speaker installed in the re-finished dual height
desktop cabinet – a very imposing radio to have on your desk!
Restoration of an RCA AVR-11 ‘Airport Receiver’ Gerry O’Hara
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Appendix
- Comparison of RCA Chassis (ACR-111, AVR-11 and 15K-1) – Main Chassis Top/Bottom*
- Comparison of RCA Chassis (ACR-111, AVR-11 and 15K-1) – RF Sub-Chassis Top/Bottom*
- Candohm Resistor Calculation Spreadsheet
- AVR-11 Schematic
- AVR-11 Brochure
*Photos of AVR-11 chassis before restoration work undertaken
Restoration of an RCA AVR-11 ‘Airport Receiver’ Gerry O’Hara
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Restoration of an RCA AVR-11 ‘Airport Receiver’ Gerry O’Hara
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Restoration of an RCA AVR-11 ‘Airport Receiver’ Gerry O’Hara
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Restoration of an RCA AVR-11 ‘Airport Receiver’ Gerry O’Hara
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