checking and repairing a hallicrafters sx-28 gerry o’hara

Checking and Repairing a Hallicrafters SX-28 – Gerry O’Hara

Overview of the SX-28/28A Receiver

The SX-28 was introduced in August, 1940 and stayed in

production through 1944, with production then swapping to the

SX-28A from late-1944/February 1945 through to July 1946,

after an estimated total of 27,150 SX-28/SX28A’s were

manufactured. The receiver was widely used by the military in

WWII and in the post-war period quite rightly became a very

sought-after and popular receiver within the amateur

community.

The receiver is very solidly

constructed on a stout steel

chassis, weighing-in at 78lbs.

The thick steel front panel is

separate to the main chassis,

with some controls being

mounted directly to this. The

RF circuits are contained in a

series of screened

compartments beneath the chassis, reducing heat build-up here,

however, a temperature-compensating capacitor is included, this

being a negative-temperature coefficient ceramic trimmer located in

the tuning gang compartment above the chassis where the four RF

tubes are located. The gearbox in the SX-28 uses gears to drive both

the main and bandspread gangs, however, the SX-28A uses a dial cord

to drive the bandspread gang. Cords are also used to operate band

indicators on each of the two tuning dials on both models when the bandswitch is operated. Some

other differences between the SX-28 and SX-28A include:

- Construction of the RF circuits: in the SX-28A, ‘Hi-Q Micro Set’ coils with phenolic formers were

mounted on phenolic plates, whereas in the SX-28, the coil units were mounted onto the sub-chassis;

- The coil box cover is screwed in place in the SX-28 and is held on with retaining clips in the SX-28A, the

cover in this model also having a louvered top;

- The SX-28 has open ‘spoke’-style tuning knobs and the SX-28A was fitted with closed-in ‘webbed’- style

tuning knobs.

The Hallicrafters SX-28 and SX-28A receivers are 15-tube single-conversion superheterodyne designs

(nominal 455kHz IF), covering 550kHz through 42MHz in six bands:

550kHz – 1.6MHz

1.6MHz – 3.0MHz

Hallicrafters SX-28 Gerry O’Hara

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3.0MHz – 5.8MHz

5.8MHz – 11MHz

11MHz – 21 MHz

21MHz – 42MHz

In addition, an electronic bandspread facility is provided in a separate scale for the following frequency

ranges, covering the 80m, 40m, 20m and 10m amateur bands:

3.5MHz – 4.0MHz

7.0MHz – 7.3MHz

14MHz – 14.45MHz

28MHz – 30MHz

The set is designed to receive AM and CW signals, but with care, SSB signals can be received quite well.

Controls include main and bandspread tuning, a six-position selectivity control (three positions with the

single crystal filter in-circuit), crystal phasing, ANL, BFO, AGC on/off, antenna tune, RF gain, AF gain,

tone, a ‘bass boost’ switch and a standby switch.

Referring to the block diagram above, the circuit comprises two pentode RF stages (6AB7 and 6SK7 as

pre-selectors), the first stage only used on frequencies above 3MHz. A 6SA7 ‘pentagrid’ (heptode)

carries out mixer duty, and a second 6SA7 acts as the local oscillator. Two stages of IF amplification are


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provided, a 6L7 heptode followed by a 6SK7, feeding a

6B8 dual-diode pentode as detector/AGC/S-meter

amplifier. Audio from the detector is fed to a 6SC7 dual

triode doing duty as a pre-amplifier and phase-splitter,

to two 6V6 output tubes in push-pull, giving 8W of

audio. A second 6B8 is used as an AGC

amplifier/detector for the RF and mixer stages, a second

6AB7 as the ANL noise amplifier and a 6H6 dual diode as

the ANL rectifiers (see below). A 6J5 triode is used in the BFO circuit, which is loosely-coupled to the

detector circuit with a ‘gimmick’ capacitor. The power supply is conventional, using a 5Z3 rectifier.

The circuit includes some interesting innovations: there are two AGC circuits – one amplified, feeding

the RF and mixer stages, and the other conventional, controlling the first IF stage. There are also two

ANL circuits: the first provides for shunting of the detector diode load with a capacitor to momentarily

bypass audio signals on noise peaks, the second amplifies the noise signal at the IF frequency and places

an instantaneous negative potential on the 3rd grid of the first IF amplifier (a 6L7 heptode) at a level

selected by the operator thus canceling out noise pulses – this is a version of the Lamb noise silencer

(‘blanker’). The use of permeability-tuned RF transformers using iron-dust slugs was a fairly new

innovation at the time, as was the variable-bandwidth IF and single crystal gate circuitry. Even the S-

meter circuit is a little novel having its own amplifier stage, and the audio output impressively has a pair

of 6V6 tubes in push-pull for great sounding audio, complete with a switchable bass boost circuit.

Background

This SX-28, S/N H164758, originally came from Hans Alderliesten, who has since passed away. Hans was

a former technician at the local Naval Base, and a member of the Victoria Radio Group, of which Lorne

and I are also members. He had sold it to Lorne saying he wanted to make sure it went to a good home,

along with a RACAL RA-17. Hans thought that a resistor had gone bad in the SX-28 but couldn’t

remember. He believed he had replaced the caps at some point but wasn’t sure. Lorne knew it had

been working but did not know how long ago that was. Knowing how meticulous Hans was, Lorne was

sure he had done a good job. Lorne had already undertaken the basic clean-up and cosmetic work on

the cabinet and chassis. Lorne had not tested the chassis and, as he intended to use it fairly frequently,

wanted me to check it over, replace components where needed, and then align it as best I could.

So, this article is written essentially in ‘blog’ format, distilled from the trail of update emails from me to

Lorne over the course of the repair work – so there may be some mixed tenses in parts... You will note a

few gaps in the dates: these were due to trips to the Lower

Mainland or health issues, in particular me recovering from a

broken left wrist and persistent carpel tunnel problems in my

right hand. Lorne brought the set down from Nanaimo on

December 30 2018, a couple of days after I had broken my

wrist, and the day after my cast had been fitted (photo, right) –

maybe not the best timing as I could not even lift the radio – it

had to wait until the cast was off my arm and physio had

worked to the point I could at least turn the radio over on the

workbench – that was late-February 2019.


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Preliminary Inspection, Observations and Checks

(Feb 28 2019): I always like to give a chassis a close visual inspection, plus a few basic electrical checks

before I undertake any further work (see my article on radio repairs in the Winter 2019 issue of

‘Canadian Vintage Radios’). This is always time well-spent: you become familiar with the chassis layout

v schematic, identify any obvious issues, eg. burnt-out resistors, poor insulation, suspect dry joints, or

missing dial cord, assess the presence and quality of any previous repairs, check the continuity of

transformers, etc. On this SX-28, the following was noted during this inspection:

- The majority of the original paper caps had been replaced with high quality metal-encapsulated caps

(mainly ‘JAN’ and Gen. Inst.). I tested a random number of these and found their values to be accurate

and to have zero measurable leakage when tested to within 100v of their rated working voltage. Given

the exceptional low leakage, I assessed these caps as likely being plastic film types and tended towards

leaving the majority of these in circuit. The exceptions would be those between the line supply and the

chassis, which should be replaced with Y2-type safety caps;

- Some paper caps had been replaced with plastic-encapsulated caps. These may or may not be paper

or plastic film types. I recommended

replacing all of these with new plastic

film types;

- There remained at least three original

wax-coated paper caps buried deep in

the antenna section of the RF

assembly. These are very difficult to

access (and even see!), and this is likely

the reason they were not replaced

previously. These needed to be

replaced with new plastic film caps

(part of one just visible as circled on

photo, right – follow the arrow…);

- The electrolytic caps may have been

replaced – some with can-types. These

may be ok (I had not tested them at this point). However, for reliability, I noted that these should be

replaced: C48 and C49 with a dual 32uF can type, the remainder probably with axial wire-ended types;

- A couple of resistors had been replaced. Given my previous experience with these sets, all resistors

should be checked for tolerance and replaced if outside its tolerance value by more than 10% or so;

- A Meissner IF transformer (?) can had been installed under the chassis in the IF section (photo at top of

next page) – the reason for this/purpose needed to be investigated;

- Some re-wiring of the power supply/audio section had been undertaken using thin, plastic insulated

wire - this needed investigating;

- The output transformer looked like a replacement – this also needed investigating;


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- The quality of the recapping

soldering/component layout was not

the best: I noted a few suspect dry

joints. I recommended all joints be

inspected and re-dressed and/or re-

made where needed (may need the

occasional replacement of a metal-

encapsulated cap if leads too short);

- The power transformer had

continuity across its line, HT and LT

windings. The line input has a fuse

fitted as standard;

- Once working ok (I still had not tried to power the set up), the usual general tidy-up/cleaning, switch

and control cleaning needed to be done, along with performance checking, any troubleshooting needed,

and alignment.

The set was powered-up with the speaker connected across the 500ohms output terminals at the rear of

the chassis [though this was not 500 ohms – see later]. It was working reasonably well on Broadcast

Band (not tested on all bands - but seemed very ‘deaf’ on most shortwave bands) – sensitivity was not

what it should be, but not too bad, undistorted audio (and plenty of it, at least on the broader selectivity

settings), and the controls seemed to do what they are supposed to, eg. BFO worked/tunes ok,

selectivity works, antenna trimmer, tone, RF and AF gain (I had not tried the crystal filter yet). The S-

meter was sticking – not sure if its a mechanical or electrical issue (the meter zero controls usually have

tin ‘whiskers’ growing in them which affects its operation). I decided to start work by replacing the

remaining paper caps/plastic encapsulated caps, then checking voltages and resistor values. I also noted

that the dial cord in the pointer mechanism for the bandspread dial was missing.

Repairs (March 5 – April 5 2019)

March 5 2019: I decided to tackle the RF deck first – replacing the remaining wax and plastic

encapsulated caps, checking resistors and replacing when significantly out of tolerance, checking wiring

of the previous recap job and correcting/redressing if needed.

I completed work on the (most difficult) antenna/1st RF amp compartment and then moved onto the

2nd RF amplifier

compartment. This work

is very slow as it takes a

lot of planning/thought

and preparation before

the execution (which also

takes time!) - it reminds

me of rear-tooth

dentistry in a partly

opened mouth… similar

tools used too. I


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prepared a checklist covering all resistors and all paper and electrolytic caps (sample below) – useful

both to make sure all parts are checked and as a record for future use.

March 6 2019: No ‘wiring peculiarities’ noted so far, though some of the replacement caps had been

grounded in odd places for ease of installation – leaving long ground leads/paths (corrected).

Performance was not good though – very ‘deaf’ on the shortwave bands (Bands 3 - 6), though I decided

that I would not check alignment/look for faults until I had worked through the entire radio for caps and

resistors plus the things noted in my preliminary inspection.

March 24 2019:

- Finished replacing the remaining paper caps in the RF section;

- Replaced some more out of tolerance resistors in the RF section;

- Re-dressed some bypass cap grounds in the RF section (and replaced the caps when the leads were too

short);

- Replaced the (separate) power supply reservoir and smoothing caps (older can types) with a new 500v

twin 32uF part (‘JJ’-manufacture, visible near bottom of photo at top of the next page); and

- Replaced electrolytic caps and out of tolerance resistors in the audio section; and

- Replaced line suppressor caps with 275ACvw Y2 rated safety caps (yellow rectangular cases - visible in

the upper right quadrant of the photo at the top of the next page).


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March 26 2019:

Investigated a 4.5kohm 10w resistor

that had been installed in the bias

voltage supply line (photo below – it

has red ‘croc’ test clips attached at

either end for testing). I originally

thought that this had been installed to

replace the 4kohm section of the twin

section chassis-mounted candohm

(R31, 11kohm and R32, 4kohm), then I

noticed it was wired in series with the

4kohm section of the candohm, ie. the

4kohm section was effectively 8.5kohm. The screen

voltage (bias) supply provided by this candohm voltage

divider should be nominally 109v (+/- 10%) according to

the voltage table in the manual, though, strangely, marked

as 100v on the schematic. I assume this was for a nominal

supply voltage of 117vAC.

Assuming the voltage table to be correct, this would allow

a bias supply up to around 120v to be ok. With the

additional resistor in place and exactly 117vAC line

voltage, the bias voltage was noted as being 87v (20%

below nominal, assuming 109v to be the correct nominal screen bias voltage), and without it in place, ie.

per original circuit, it was 122v (12% above nominal). These voltages dropped 2 or 3 volts when the BFO

was switched in due to the small additional current draw.

So, I wondered why someone thought it necessary to install the (additional) 4.5kohm resistor in the

screen supply voltage divider? – possibly a ‘bodge’ installed to get around another issue the receiver

had/has. In my experience, tube circuits are not that sensitive to a few volts on the plate or screen

circuits, so the higher voltage with the standard circuit would seem ok to me and, unless an issue

relating to this was identified later, I decided to revert the circuit to original. The receiver is specified to

operate from 110 to 125vAC – operating it at 125vAC rather than 117vAC would increase voltages by

around 7% anyway. I noted to Lorne that if it is intend to operate the receiver a lot, it may benefit from

having it powered from a 110vAC supply to operate things at slightly reduced voltages.

I then took a look at the S-meter. Two faults were noted:

- the zero adjust pot (500ohm wirewound on the rear panel of the chassis) was intermittent. I removed

the pot, opened it up and cleaned inside with Deoxit. It now worked well; and

- the meter movement sticking issue: I removed and disassembled the meter. Gently blowing the meter

needle indicated that the movement/needle was free when the meter was the right way up, but sticking

when upside down. I could not see any obvious material in the coil/magnet air gap causing the sticking

and the springs looked ok. On re-installing the meter in the chassis, the meter was noted as sticking all

the time at various (random) points on the scale. So, I could not resolve this problem at this point,


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noting that it was possibly a bad/misaligned bearing issue. I noted to Lorne that more extensive fiddling

with the movement may fix it, but in my experience its maybe better to try to find another meter

movement that could be substituted. An external meter connected to the circuit worked ok, so the

problem was definitely a mechanical one in the meter mechanism.

March 27 2019: I operated the receiver for a couple of hours after I removed the ‘extra’ resistor in the

bias line. It was receiving a local Broadcast station without any antenna connected. During various

voltage checks at critical circuit nodes and tube bases, I noted that the ‘cold’ end of the 1kohm 1st RF

stage plate feed resistor (R5) was at the same voltage as the HT supply line, ie. the resistor was not

dropping any voltage as the tube was not drawing current. I felt the 1st RF tube (6AB7) and it was cool

to the touch. There was continuity between pins 2 and 7 of the tube, so the filament was ok. Most of

the tube socket for this tube is very difficult to

access (neigh-on impossible!), so I installed an

adapter above the chassis (photo, right) and

found there to be no heater voltage on the

tube. Pin 7 had voltage (6.2vAC), but Pin 2

(ground return) was not grounded. So, the set

has been working on just the 2nd RF stage –

this explained why it had poor sensitivity on

the short wave bands…(!) – the 1st RF stage is

by-passed on Band 1 (Broadcast Band) and

Band 2. I left the adapter in place and

grounded pin 2 of the adapter and the tube

heated up ok and now drew plate current.

Other points noted:

- the suppressor grid (pin 3) was connected to pins 1 and 2, ie. it was wired to be grounded. According

to the schematic I had it should have been connected to the cathode (pin 5), the voltage on which is

controlled by the RF gain control. Wiring it to ground is not listed as a mod in the Supplement [though

since, I have found a version of the schematic that shows the suppressor grid connected to ground]. It

would work when connected to ground, but likely better when connected to the same potential as the

cathode as the cathode potential varies with the RF gain control. So, I disconnected pin 3 from pins 1

and 2 and connected it to pin 5 (cathode) as per the schematic;

- Pins 1 (screen) and 2 (heater return), although connected together, were not grounded. I could not

see why this was the case due to location of wiring, bandchange switch wafers and several components

– I presumed the reason to be a dry joint or similar problem. I fixed this issue by grounding pin 2 with a

separate connection to the tube grounding lug. As pins 1 and 2 are connected, this also screens the

tube;

- The tube now heated up and drew plate current.

Further measurements showed there were several resistors in the IF section that needed to be replaced

- one 100kohm resistor measured almost 700kohm!


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March 28 2019: All out of tolerance resistors were changed out except one. Amazingly, one resistor in

the Noise Limiter circuit was missing completely(!) – likely from new. It was the anode load resistor for

V9, so the Noise Limiter would not have worked. The resistor not yet changed was inside the T1

transformer housing (R62, which should be 500kohms, but measured 768kohms). It’s the grid leak for

the AVC amplifier, V8, so its value is not critical. Rather than pull T1 apart (unless I need to for another

reason – see below) I decided to leave well-alone. I also found a couple more wiring errors (corrected)

and incorrect replacement capacitor values (also corrected).

March 29 2019: All paper and electrolytic caps now checked and replaced where needed. I also re-

dressed some component leads and checked they had all been installed at the correct points in the

circuit. All remnant caps were replaced by others in the

past with high quality JAN or Gen Inst. types (except

two yellow square body types that were plastic film -

replaced), and all tested good.

On inspecting the wiring, I found that the Meissner IF

transformer under the chassis was actually being used

as a substitute for Choke 3 and associated trimmer C55,

which together act as a 455kHz wave trap in the

automatic noise limiter (ANL) circuit. Perhaps the

original choke burned out at some point – though I

could not see why a component failure would do that,

so maybe it developed a ‘green wire’ corrosion

problem, or someone shorted a high voltage to it when

checking something else – who knows…. I decided that

if it was working ok when I checked the ANL, to leave it

in place.

I then attached an output meter to the speaker as part

of preparing the set for alignment checks, and noticed a

reading on the meter with no audio. I then connected a

scope across the speaker - a nice (inaudible) sine wave

with an amplitude of around 2 volts. I measured the

frequency at 183.8kHz…. (photo, right) – hmmm…, a

supersonic parasitic oscillation in the audio stage. I

needed to find out what was causing that before

continuing with the alignment work. It vanished when the first audio/phase splitter tube was pulled, so

it was very likely to be generated in the audio section.

March 30 2019: The supersonic audio instability was traced to a faulty silver mica cap (C40). I replaced

that and the audio now worked ok with no sign of instability on any setting of gain, tone or bass boost. I

also tidied up the wiring in the audio section with some cable ties – someone had installed a

replacement output transformer at some point and used thin plastic wiring and had left it rather untidy.

On starting the IF alignment, I noted that the top slug (secondary) of T1 would not tune – I would need

to remove the transformer after all and find out why (likely another bad silver mica cap) – at least this

would allow replacement of the V8 500kohms grid leak resistor...


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- Removed T1 from the chassis;

- Disassembled T1 – found that all the components were liberally coated with wax (photo, right - yuk!),

making work on the sub-assembly difficult as the fine wires from the

coils were embedded in the wax;

- I decided to replace all the caps while I was in there as a precaution

(not just the failed capacitor, C104, and R62);

- Carefully removed wax where I needed to liberate wires and parts;

- Replaced C104, C105 C61 and R62 (this is the resistor that had

drifted up to 768kohms). Both of the tuned circuit silver mica caps

were destroyed during removal. I

only broke one coil wire

(secondary) - easily repaired;

- Replaced all rubber-insulated

wires entering T1 as the insulation

on these was brittle and crumbling

(photo, left);

- Replaced T1 in the chassis and

re-wired into the circuit;

- Undertook continuity tests;

- Powered on and rough-tuned T1

primary and secondary windings –

both tuned up nicely.

Note: the IF crystal filter frequency in this set was determined as

454.250kHz while I was starting the IF alignment before I hit the T1

snag.

I placed the receiver on soak test before returning to aligning the IF section.

March 31 2019: Good news and not so good news for Lorne…

- finished tuning up the IF section and also trimmed the responses using the wobbulator.

- tuned up the RF section – all Bands working well. One slug had the slot in the brass tuning screw

burred off, however then Band tuned up ok anyway;

- re-strung the Bandspread pointer mechanism (cord was missing) – worked correctly now (talk about

fiddly!). I noted that someone has restrung the Bandspread tuning dial using ordinary parcel-type

string(!) – but it was working ok, so I left it alone;

- I left the receiver on soak test for the afternoon and it developed some faults:

- the HT line voltage dropped 25v or so, and;


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- the IF section developed a fault that caused a significant drop in signal strength.

Nothing was obviously causing these issues, though I noticed a slight smell - but could not trace it.

April 1 2019: During troubleshooting, I found two issues:

- There was a resistance of 516kohms measured between the IF AGC line and ground, even with all

components disconnected. I could not find what was causing this [at this time] ,and it would not cause

the symptoms I described yesterday; and

- The replacement 250pf 500vw silver mica cap that connects the plate of V3 to the grid (top cap) of V8

(C61) had failed and was now acting as a 318ohm resistor – aaarghhh!! This was a brand new part

installed as a precaution when I had T1 opened up. I have never had a new silver mica fail like that, and

I had even tested it before I fitted it. It was a new 500v part that had around 280v across it in operation.

This failure would have the following implications:

- HT would be applied to the grid of V8;

- V8 would therefore be conducting heavily (which I had noted was running hot after the problem

occurred);

- V8 would be drawing excessive anode current, likely the reason for the reduced HT line voltage;

- The plate dropper resistor to V8 would be overloaded.

This was a new part and, on close inspection, was now

discoloured. It was probably this I could smell… and

- Make T1 behave strangely, ie. the primary would not tune

– which I noted when I was testing the set.

T1 was taken out of the chassis again. I checked the other

two new silver micas that I had installed (one was replaced

again as it was only marginally within tolerance), and I

installed another 250pf silver mica for C61 – this time I

used a NOS part (thinking maybe one made ‘back in the

day’, likely 1950’s in the USA, may hold up better than a

new one made in who knows where…). I tested the

replacement 250pf cap (the reddish-brown cap near the

centre of the photo, right) at 425v for a few hours before I

installed it as taking T1 out and putting back in is a pain… it

showed zero leakage. Phew!

I tested all the tubes on a Precision 10-12 ‘electrodynamic’

tester. Most tested ok, though 2 or 3 were noted as being

weak and should be replaced: I recommended replacing

V7, AVC amp/detector (6B8) as the diodes both test very

weak, and also recommended experimenting with

replacing V2 with a 6AB7 – this has a higher

transconductance than the 6SK7 and is socket compatible.


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April 2 2019:

T1 was re-fitted and the set was working ok, with the HT and screen voltages normal (279v and 110v

respectively). I noted that V8 was still hot to the touch, but maybe that is normal, as V7 (another 6B8)

was also hot.

On re-fitting T1, I noticed that the set had a wiring error, possibly from a repair done years ago: the

‘cold’ end of R62 (V8 grid leak) had been connected to the incorrect tag on a small tagstrip under T1.

Instead of being connected to the ground tag, it was connected to the IF AGC line(!). This is what had

been causing the unexplained 516kohms resistance to ground noted earlier. This was now corrected

(when I re-fitted T1 the first time I just copied where it had been connected to previously). I also

replaced the scorched resistor (R63), even though it was still within tolerance. On powering-up, T1 was

tuning normally again.

After the radio seemed to be running nice and stable for a few

hours, I started to re-align the IF again. All went well until I

started to adjust C31 (the crystal balancing trimmer) – it

adjusted fine last time I had adjusted it. This time it had no

effect (it should peak at the crystal resonant frequency). The

reason is that the centre bush had broken off (a very cheaply

made part – upper trimmer in photo, right – I have had issues

with these in other Hallicrafters repairs). It’s a 4 – 20pf ceramic

trimmer. I had several similar ones in stock (4.5 – 25pf), so

selected a good one - much better quality than the Hallicrafters

one too (lower trimmer in same photo for comparison).

April 4 2019: I fully aligned the RF deck. I noted that Band 6 was not the best above 30MHz or so (but

apparently never was), and age has done nothing to improve things – mind you, there is nothing to

listen to up there anyway.

I completed the checklist and noted that a total of 38 resistors and 28 capacitors had been replaced by

me (some twice!). I did not replace any tubes as my

stock is not accessible since I moved house last

year... one day I will get things organized better!

After aligning the IF using the process described by

Bill Feldmann (better than that in the SX-28 manual),

I used a wobbulator to check the IF response curves

on all six selectivity settings, tweaking T3 (final IF

transformer) slightly on the ‘IF Broad’ position to

widen the response curve – photo, left). I noted that

the sensitivity was above spec on most ranges (spec

is 6 to 20uV for half a watt output). On most ranges

it was around 5uV or better, with <1uV being easily

detectable.

http://www.radiomanual.info/schemi/Surplus_Radioamateur/Hallicrafters_SX-28_IF_alignement_procedure.pdf

http://www.radiomanual.info/schemi/Surplus_Radioamateur/Hallicrafters_SX-28_IF_alignement_procedure.pdf


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After soak testing again for about 9 hours, including some on/off cycles and using the set on various

bands, the following points were noted:

- HT was around 280v and screen supply around 110v, so both nominally at spec;

- The S-Meter worked but still stuck intermittently – a tap usually got it working again;

- There was noticeable mixer noise (a little higher than I would expect), and alignment would not

eliminate this. I suggested to Lorne that he might want to change out the 6SA7s, and also the 6B8 with

weak diodes (V7) to see if there is any improvement;

- Overall, the set is ‘well worn’ and has seen a lot of use (and likely abuse) over the last 77 years or so:

some of the resultant issues are worn threads/slackness in some of the IF and RF tuning slugs,

potentially more problems with the poor quality ceramic trimmers in the future, and a rather sloppy

gearbox with a very free-running mechanism – I suggested trying a little Rocol ‘Kilopoise’ on some of the

gearing to give it a smoother/’tighter’ feel (use very sparingly);

- The workmanship of whoever replaced most of the paper caps was not the best – they left part of the

old components leads in place and soldered the replacement caps to these. I tidied-up and re-dressed

some to look a bit better and also to improve grounding arrangements, but the overall appearance is

still not good. At least they had used top quality

components and these should be very reliable;

- Like any old radio, it could (will) develop faults in

the future, though very few original resistors and

capacitors remain. The original resistors that are left

are within their stated tolerances, but could drift

(likely over decades, but who can tell?), and the only

original caps remaining are silver mica ones, but

these can also develop issues in time (as can brand

new ones, as the fun and games I had with C61

showed!); and

- the crystal ‘activity’ is very likely down a bit as

although a peak is present (photo, above right), when

tuning the ‘phasing’ control, it is not as sharp as I

have seen in other SX-28/28A sets (photo below

right), and the corresponding ‘trough’ between the

main peak and the secondary (minor) peak is very

hard to discern when tuning through the passband.

This is not unusual for a 77 year old crystal. Unless

the set was to be used for ‘DX” listening next to

strong adjacent signals, this should not be an issue.

The crystal could be replaced, or the original unit

disassembled, cleaned and re-assembled to try to re-

invigorate it, but this was not attempted.

April 5 2019: I took the S-Meter apart again and had more success:


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- Made a coil gap cleaning tool as described in an article in the Feb 1943

QST article using a paperclip;

- Removed the meter from the front panel and the movement from the

case;

- Recovered a few very small metal particles from the coil gap;

- Adjusted the rear bearing as best I could (it was loose, so I tweaked the

retaining nut also);

- Cleaned the entire movement and inside the housing/glass with anti-static

fluid;

- Installed new insulating washers on the terminals where they pass

through the rear of the housing (the old ones fell apart);

- Installed terminal washers on the leads to the S-Meter (negative one is coloured black) – better than

bare wires wrapped around the terminals as it was before;

- Installed a new grommet that retains the dial light on the rear of the housing (the old one was in very

poor shape);

- Installed the meter movement into its housing and the complete assembly back onto the front panel;

- Adjusted the zero pot on the rear panel of the chassis.

The meter was now working well across the entire scale, except if the needle gets thrown hard against

the right-hand end stop it sticks occasionally - a slight tap is needed to free it. I think it’s a minor bearing

adjustment or wear issue – though I am not a meter expert!

While packing up the documentation ready for Lorne to collect the set, I noticed some notes regarding

T8 (output transformer – photo, below) on the corner of the fold-up schematic. This seemed to be

notes regarding the replacement output transformer that had been fitted to this set in the past. It

showed that tag 1 was grounded, and tags 3 (8ohms) and tag 6 (60ohms) were connected to the rear

panel speaker connections. The 8ohm winding was connected to the upper pair of speaker connections

on the rear of the chassis, and the 60ohm winding to the lower two.

I suggested to Lorne that it might be

better to re-wire things in the radio

so the upper speaker terminals

connect to the 2ohm tap (tag 2 on

the output transformer under the

chassis), and the upper (now 2ohms)

terminals connect directly to the

speaker terminals (on the driver unit

in the speaker cabinet),

disconnecting the 500ohm

transformer in the speaker cabinet

http://www.rfcafe.com/references/qst/rejuvenating-old-meters-feb-1943-qst.htm

http://www.rfcafe.com/references/qst/rejuvenating-old-meters-feb-1943-qst.htm


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completely. A 2ohm to 3.2ohm mismatch being better than a 60ohms to 500ohms (or maybe even a

5000ohms1) mismatch.

Finally, the chassis was re-installed in its great-looking art-deco metal table top cabinet – complete with

red ‘go-faster’ stripes, ready for Lorne to take away and enjoy.

1 Some information on the PM23 speaker indicates a 500ohm, others a 5000ohm primary on the matching transformer

Above: Re-strung Bandspread pointer arrangement. Below: Tools needed for ‘deep’

component replacement in the RF deck (the deck can be removed as an alternative)

https://antiqueradio.org/halli12.htm


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Above: Under-chassis view after repairs and component replacements


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checking and repairing a hallicrafters sx-28 gerry o’hara

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