•

To the average amateur, surplus is synonymous withoutdated American military electronic equipment withlittle thought to the military part or what the equipmentwas actually used for. The following article concernssurplus of a different origin, how it works, was used,and is used today.

Ge rma n World War IICommunications

ReceiversTechnica l PerfectionFrom A Nearby Past

Part I

BY DICK W. ROLLEMA·, PAflSE

•

R ad iOcommunication equ ipment,as used by the allied forces duringWorld War II, is well known among col -lectors and lovers of wireless equip-ment from the past.

Communicat ion equipment that wasused by the German forces dur ing thewar is much less known; that appliesnot only in the United States, but forEurope as well . This may look remark-able as that same Europe formed oneof the main theatres of war. Why was itthat so much of the all ied equipmentwas left behind after the Ge rm anforces capi tulated that war surplusshops flourished for many years after,whilst at the same time German equip-ment has been rela ti vel y rare rightfrom the start of the post war per iod?

Not because these units were manu-factured or used in limi ted quantities.One of the main suppliers of radioequipment, the Telefunken firm, em-ployed some 40,000 workers, spreadover 350 different locations, by the endof WW II. How many sets of different

• v.a. Marckstfaat 5, 2352 RALEIDERDORP, Netherlands

20 • co • August, 1980

categories were made is unknown tothe author. But it has been put onrecord that of one radio used in tanks,the so called "Boqeqerat" , some180,000 units were manufactured.

The reason that so little of the vastGerman production remained may wellbe that the allied forces that occupiedGermany after it co ll apsed orderedthat all rad io equ ipment that wasfound was to be demolished. By theend of 1945, and the beg inn ing of 1946th is order was changed in that equip-ment rema ining at that time was not tobe destroyed but dismant led. The com-ponents that resulted from that actionformed the basis for German produc-tion of consumer rad io equ ipment, aproduction that came to a falteringstart at that t ime.

Not only thousands of radio's endedunder the crash hammer, also all draw-ings and other documents that sup-ported design, development and pro-duction of the equipment went thesame way so that hardly a trace of Itwas left . What we know about thebackground for the wartime produc-tion of German radio equipment hasbeen reported orally by some of the

leading men who were involved. Pleasedon't get the idea that the author con-demns the decision to destroy all war-time German equ ipment. It was a com-pletely understandable and justifieddecision. But it does explain why mostof us radio amateurs, both in the USAand outside, are in general rather unfa-miliar with the communication equip-ment that was used "on the other side."And that is certainly to be regretted asthis equipment was of except ionaltechn ical perfection and beauty.

Of course some German WVI/ II rad ioequipment escaped destruct ion andpart of what remained found its way in-to the hands of collectors , some ofwhom specialize in German radio ap-paratus. By sheer coincidence yourscribe came into contact with one ofthose specia lizing collectors, ArthurBauer, PA0AO B, wh o lives nearAmsterdam. Arthur owns a most beau-tiful collection of German WW II radioequipment. When I met him tor the firsttime in 1977, his collection comprisedmore than ninety items. By today itmust have passed a hundred easily. Itis certainly no simple feat to bring to-gether so much of this rare equ ipment.

Say You Saw It In CO

Photo 1- Arthur Bauer, PAfJAOB, amidst some items 01 his extensive collection01 German WW 1/ radio communication equipment. He actually uses

several of these sets for making QSO's on the h.f. bands.

Photo 2- Here PAfJAOB is typing on a "Hel/schreiber, " an ingenious system oftele typing over radio. The system was extensively used by the German forcesduring WW II. It is now commercially obsolete. But a group 01 enthusiastic Euro-pean amateurs have revived the system and are using it in regular skeds on the

h.f. and v.h.l. bands.

Germans used t.r.t. sets. But they cer-tainly knew how to make them in supe-rior form. A great advantage of thestraight set is that spurious responsesare non existent, even in the presenceof extremely strong signals as in ship-board use, where several t ransmittersmay be active at the same t ime recelv-ers are operated. Another advantagefrom a military point of view is that thet.r.t, set does not use oscillators and so

some general characteristics of Ger-man radio equipment. One feature wewill discuss in more detail : a quartzcrystal intermediate frequency filterwith continuously variable bandwidth.This type of filter was used in severalreceivers of the superheterodyne type.Following it we will discuss two receiv-ers of the tuned radio frequency varietyand two superheterodynes.

You are probably surprised that the

Arthur scans the whole of Europe for itand he has his contacts in most of thecountries of our part of the world.

PA0AOB not only col/ects the Ger.man equipment, he also uses it for hiscontacts with fellow amateurs.

Photograph 1 shows Arthur,PA0AOB, making a eso via a powerfulGerman transmi tter of WVV II vintage.The receiver at the reft of the transmit-ter will be described later In this series.Single sideband does not exist for Ar-thur, of course. Only c.w. and a.m. anda system of teletyping over radio, In-vented in the twenties by Or. RudolfHell. The machines that use this sys-tem are called " H e ll schreiber"r'scnretcer' means "writer" in German;The system is different from RTTY aswe know it in that the characters aretransmitted in a form of simplified fac-simile (FAX). The received charactersare printed on a paper tape as they arereceived as " pictures". The charm of itis that the receiving machine does notdecode the characters as in normalteletype. The decoding is done by theoperator who Interprets the "pictures"on the tape as characters. Interferenceand or fading on the rad io path cannever result in a wrong character beingprinted. The picture of the charactersmay become blurred or smeared by theInterference. But due to the sup remecapabilities of t he hu man eye andbrain a lot of interference can be ac-cepted before the received charactersbecome unreadable. The pr ice for th isimmunity against interference is in-crea sed bandwidth. The signal isabout six times as wide as a radioteletype signal of the same tranamls-sion speed in characters per second.This is partly compensated by the factthat the Hell sys tem of te leprintlngdoes not need FSK as modulationmode. Simple onloff keyi ng of a c.w.transmitter gives fine results. So in theend the Hell signal occupies about thesame bandwidth as a teletype signal ofthe same speed.

The Hell system is commercially ob-solete. But it has been revived by agroup of European amateurs, who pos-sess the machines for it. Photograph 2agai n shows Arthur typing on a " Hell-schre iber" as they we re ex tens ive lyused by the German forces during WVVII. Your scribe is so lucky as to have a"Hellschreiber" on loan from PA0AOBand so he meets Arthu r and severalother amateurs in Europe every Sun-day afternoon on forty meters, usingthis nostalgic way of communicating.One of the members of the "Hell group"is Hans Evers, OJ0SAIPA0CX and hedescribed the Hell system of teleprlnt-Ing in Ham Radio Magazine, December1979 ("Hellschreibers rediscovered").

But back to the subject of this series:German WVV II communication receiv-ers. In the next section we will describe

Say You sew II In c o AuguSt, 1980 • CO • 21

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that made It extremely stable, bothfrom a mechanical and an el ec t ricalpo int o f view. Inside the box was divid-ed in completely screened compart -ments that housed the different stagesbelong ing to that un it. The units werecombined to a complete set by mount-ing them into a frame, again made ofcast alloy. The electrical connectionsbetween the units consis ted of mu lti-pole c o n nec t o rs on t he mod u les,mating with sim il ar devices o n theframe. The whole became an extremelystrong combination with almost idealelectrical charac teristi cs and easil yaccessible on all sides for servic ing.The whole assembly sli des from thefront into a sturdy cabinet that In Itselfalready forms a solid and stable basis.

Thi s battleship-like constructionalso works out in a negative way; mostof the German pieces of radio gear areextremely heavy, no doubt partly dueto the fact that aluminum was not usedfor the boxes and frames. The exac tcomposition of the alloy Is unknown tothis author, but very likely zinc formeda major component of it.

The moving parts such as gears, tun -Ing capacitors and switches are mas-terpieces of mechanical engineering.Moving a coil turret from one positionto another, e.g., is done by a big solidcrank and it feels like openi ng a safe orthe door of an expensive otdttme auto-mobile. Regardless how complicatedthe mechanical d evices, t hey canalways be dismantled in a few secondsby loosening one or two screws. Thatthe mechanica l l inkage can be compli-cated is evident when one rea lizes

Fig. 2- Prototype bandpass filter sec-tion of the so-ceuea constant·K type.

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Fig. 1- Quartz crystal bandpass filter with variab le bandwidth as can be found inseveral German World War II superheterodyne receivers.

of di fferent types of radio tubes. Mostof the receivers used the same type oftube in all stages! Th is posed sometricky problems for the des igners. Wewill meet an example of this later on.From a logistic and maintenance pointof view it is of course a clear advantageto limit the variety of tubes to the ab-solute m inimum. The tubes were es-pecially developed for this militaryequipment. They were miniature types,certainly gauged by the standards ofthose days, and very robust. The tubesf itted special sockets that completelyenveloped them and in which they hadto be inserted top first. Nevertheless,the hope that a few types of tubes forall receivers and transmitters wouldsuffice was not fu lfil led; by the end ofthe war some 100 different types couldbe found on the li sts of m ilitary tubes.

Another feature of German radio re-ceivers and transmitters is that thedesigners certainly were generous asto the total number of parts used,especially if by doing so potentialsources of trouble could be avoided.One finds, e.g., decoupling devices andscreening liberally applied in all stagesof a receiver or transmitter. Another fea-ture is that even the simplest piece ofequipment has the possibility of hav-ing its proper operation checked by theuser. A built-in vol tmeter w ith selectorswitch to measure voltages at differentparts of the ci rc uit is the minimumalways found. Sometimes quite elabo-rate bui lt-in operational checks can beencountered. Again we will meet an ex-ample of this later on in the series.

The most striking characteristic o fGerman equipment is no doubt themechanical part of it. The traditionalchass is, so familiar to radio equipmentof the past , was never used as such bythe Germans. In stead they filled thespace in a cabinet in three dimensions.The circuitry of a radio set was dividedin a nu mber of un its, " modules" wewould call them in our day, that couldbe easily removed and replaced . Usual-ly such a module took the shape of acompletely screened box of cast alloy

the chance of location by the enemyusing a direction fi nder on spuriousradiation of the set Is negligible.

This article could only be preparedthanks to the assistance of PAMOB.Not only did he make the receiversavailable for photography, he also gavethe author the opportunity of usingsome of the sets in his own shack for aconsiderable period of time.

The fact that PAMOB could providethe original technical manuals, or exactreplicas of them, was also of great helpIn the preparation of this art icle.

General Characteristics ofGerman Radio Equipment

The oldest company that manu-factured military radio equipment inGermany is undoubtedly Telefunken.During the first World War (1914-1918)this firm supplied radio communlca-tion equipment for the German army.Production of military equipment wasforbidden in Germany under the Ver-sailles treaty that ended WW I. Bu twhen Hitler came to the fore in the ear-ly thirties the situation changed eras-tlca lly. When general conscription wasannounced in 1935, product ion of wea-pons and other war material came intoful l swing. New communication equip-ment was to be developed and pro-duced. Again Telefunken was the lead-ing firm. Also Lorenz, a German branchof the American In concern, startedextensive activities in the field of milt-tary radio. In 1937 the German govern-ment invited tenders for a new radio formilitary aircraft. It came as a shock toTelefunken that Lorenz emerged as thewinner with their FuG 10 set. This con-sisted of beautifu lly made separate reocelvers and transmitters fo r di ffe rentfrequency bands that were combinedin a rack. Photograph 3 shows part of aFuG 10 installation, at the left a longwave receiver, in the center a shortwave receiver and at the right a shortwave transmitter. The antenna wasmatched by a remote controlled tuner,that is visible in photograph 1 on theshelf; it is the box with the rounded cor-ners at the left. The superiority ofLorenz was especially evident in themechanical engineering of thei r equip-ment. In the toltowlnq years the dl-viding line between Telefunken andLorenz products became less clear asequipmen t was ma nufactured undermutual licensing contracts. Also otherbig firms. l ike Siemens and companiesIn countries occupied by Germa nytook part in the production.

We will now lake a took at the Ger-man radio equipment and see whetherwe can find some characteristics thaimake it so unique and different fromsimilar gear used by the allied forces.

In the first place we observe thetendency to use a minimum number

22 • CO • August, 1980 Say You Saw It In CO

Fig. 3- Constant-K type bandpass filter with component va lues calculated for apassband 01 10 kHz wide and a center frequency of 1 MHz. Note the "imoos-

sible" vafues of the capacitor and coif in the series tuned circuit.

Photo 3- Some units of the FuG 10 aircraft radio set, as it was developed by theGerman Lorenz factory, part of the ITT concern, in the year 193 7. The radio wassubsequently manufactured by other firms as well. The remote controlled anten-na tuner of this set can be seen in photograph 1; it is the box with the rounded

corners at the lef t on the shelf.

R - 1I3 kfl•

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L118C1/JH

L,SH

C1140pF

L23.6H

'"O.OO5pF-~~w.,.,

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Fig. 4 - Equivalent electrica l circuit ofa quartz crys tal resonator. The ete -:ment values indicated are typica l for

a 1 MHz crystal.

pedance Z I( and loaded by an lm-pedance ZI(. ZI( has a different value forevery frequency in the pass band andstop bands. Because th is is almost im-possible to realize in practice the fil teris used between a source with in ternalresistance A and also loaded by A. A istaken as the value of ZI( at frequency to'

In fig . 3 a constant-K bandpass fi lteris shown that has been designed for acenter frequency of 1 MHz and a passband of 10 kHz wide, just as the crystalfilter in the " K6In" receiver. The com-ponent values for L1 and C1 have beentaken equa l to L1 and C1 in f ig. 1 for the" K61n" f i lter. Now look at the seriesresonant circuit l2C2; don't C2 and l2have " impossible" values? Indeed, es-pecially l2 could never be construc tedwith a self-inductance of 3.6 henry at 1MHz. To produce a capacitor of 0.007pF isn 't simple either. But wait , let ustake a look at the equivalent electricalcircuit of a quartz crystal resonator, asshown in fig . 4. This consis ts of aseries tuned circuit and a parallelcapaeltor that represent s the capacl-tance of the crystal electrod es. Thevalues for the elements of the equiva-lent circuit shown in fig. 4 are typicalfor a 1 MHz crystal. These values arevery near to those of the series circuitC2l2 in f ig. 3 ! So it looks like we couldreplace C2l2 by a suitable quartz crys-tal and so obtain a bandpass filter at 1MHz with a 10 kHz wide pass band. Butwhat about the earlier statement thatthe elements of a constant-K filter weresuppose to be without losses? As of 5kilo-ohms in fig . 4 certainly looks like ahigh loss. But be careful wi th that con-elusion; what real ly matters is the Q ofthe series tuned ci rcuit. And a is equalto the reactance of c , or L, at 1 MHz,divided by As- And that works out toa Qof 6280_That value is so high that thecrystal, acting as the series tuned clr-curt, can be considered tosstess. Andhow about the loss es in the para llel

C20.007 pF

C1140pF

L118C1/JH

A ~ 113kn

The circuit d iagram of this filter isshown in fig. 1. The filter is positionedbetween Lf. amplif ier tubes. A secondfilter of the same configurat ion followsthe tube on the right. Upon casual ln.spect lon one could easily concludethat th is filter is of the famil iar type tobe found in many popular Americancommunicati on receivers and intro-duced in the early thirties by JamesLamb in his " Single Signal Superhet."These filters feature a sharply peakedresponse plus a rejection notch thatcan be moved up and down in frequen-cy by means of " phasi ng capacitor"C3. But this conclusion would not becorrect. The German fit ter exh ibits areal bandpass response, that is to saya flat passband and symmetrical fl lterslopes at the transitions between passand stopbanda. How is it possible torealize such a response with j ust asingle quartz crysta l resonator?

For explanation we turn to an ele-mentary form of bandpass filter, the socalled "constant-K type," depleted Inf ig . 2. This fi lter consists of two par-alle l t uned circuits connected by aseries tuned circuit . All three circuitsare resonant at the same frequency fo>the center f requency of the pass band.The ci rcuits are supposed to be ideal(without losses). The f ilter must be fedf rom a sou rce wi th an Interna l tm -

that , e.g., variable capacitors or bandswitches that are ganged are some-times found in different modules thatcan be easily taken out of the frame, inspite of the mechanical gears. Never-theless the mechanisms operate withextreme precision. Examples that dem-onstrate these princip les will be shownwhen we come to the discussion of thefour receivers that will be covered inth is articl e.

The German sense for perfection,that is reflected in even the smallestdetails like terminals, is also demon-st rated in the inst ruction manuals thatcome with the sets. These not only pro-vide very com plete information for theoperational use of the set, but also themaintenance man f inds everything heneeds to know for performing his jobproperly.

That the des igners certai nly hadservice abil ity of their products in mindis already clear from a simple visual In-spection of a radio set. One finds, e.g..screws that are surrounded by a redri ng. The se have to be loosened toremove a complete unit (module). Ifone wants to take a unit farther apartthe screws marked with a blue ringhave to be removed. It is these detailsthat make it a real joy to dissect a Ger-man WW II radio.

As announced in the previous sec-tion we wil l now discuss one feature inmore detail: an intermediate frequencycrystal filter with continuously variablebandwidth.

Such filters are found in several Ger-man communication receivers and theywere made for different l.f .'s . The"Kotn" receiver for instance, that wewill meet later, has an Intermediate f re-quency of 1 MHz. The bandwidth of thecrystal filter working on this i.f. can bevaried between 0.2 and 10 kHz!

Say You Saw II In CO August, 1980 • CO • 23

Photo 4- Tuned radio frequency receiver Torn E b. The lower cabinet houses the2 v. filament battery and a vibrator power pack for the 90 v.h. t. The empty com-

partment is used to store the headphones and other accessories.

tuned circuits L1C1? The coils in thesecircuits certainly don't have such ahigh a that they can be considered tobe without losses. But as shown in fig.3. these circuits have to be loaded byresistors of 113 klfo-obms, and part ofthis loading can be provided by theloss of the circuits themselves. Isn'tthat beautiful? The loaded a of theparallel tuned circuits has to be equalto the center frequency fo of the filter,divided by the width of the pass band.In our case 1000 kHz divide by 10 kHz.So the loaded a of circuit UC1 mustbe 100. with good quality componentsthis can be easily obtained. If the actuala of the circuits turns out to be higherthan 100, extra loading by resistors (orby the output impedance of the preced-ing tube and the input impedance of thefollowing tubet) can be provided.

So now that we have seen how thesingle crystal filter can really work as abandpass filter let us go back to fig. 1,the filter of the "Koln". We see severalextra components, as compared to fig.3. In the first place the crystal is notconnected between the upper ends ofthe parallel tuned circuits, but betweentaps on the coils. This is done becauseit would be very difficult, if not impossi-ble, to manufacture a quartz crystal re-sonator where the parameters of theelectrical equivalent circuit have ex-actly the values required . So in prac-tice the crystal is measured to find outwhat the actual values of C. and L, areand then the proper tap on the call iscomputed.

The required value for C2 and l2 ofthe series tuned circuit changes withthe square of the tap ratio on the coils.If for example the taps were madehalfway up the coils then the value ofC2 would be four times as big and of l2four times as small as In case of con-nections to the top of the calls. So byselecting proper taps on the calls theactual crystal can be matched to thefilter. In the "Kotn" several taps areavailable, obviously to cater for manu-facturing tolerances in the crystals.

Another new element is trimmer ca-pacitor C3. This is a neutralizing ca-pacitor for the parallel capacitance ofthe crystal and its holder. Once proper-Iy set it needs no further adjustment.

You will also note that the anode ofthe tube feeding the filter is connectedto a tap on the coil. That is obviouslydone to decrease the loading of the in-put circuit of the fitter by the outputresistance of the tube. Undoubtedly thedesigner had selected this tap in such away that the correct value for the loadeda of the input circuit is obtained.

The grid of the tube following thefilter is also tapped. From a loadingpoint of view this seems unnecessaryas the Input impedance of a pentade at1 MHz Is very high. But the 'tube is con-trolled by the automatic gain control

24 • CO • August, 1980

system. And under influence of thea.g.c. voltage the input capacitance ofthe tube changes slightly and this coulddetune the output circuit C1U of thefilter. Hence the tap. This leaves us withthe function of C4 and C5 to explain.They are sections of a two-gang vari-able capacitor, but a special one; theconstruction is such that when one sec-tion increases in capacitance, the othersection decreases by the same amount.Now assume that at a certain positionof the capacitor the input and outputcircuits of the filter have been aligned tothe same frequency. The filter then actsas a bandpass filter of 10 kHz wide inour case. Now turn the capacitor, say insuch a direction that C4 increases andCS decreases. This means that the twoparallel tuned circuits become detunedfrom 1 MHz by equal amounts and in op-posite directions. Now the whole circuitis no longer a proper bandpass filter. In-

deed what remains is a crystal, actingas a series tuned circuit, connected be-tween two impedances. These Imped-ances become lower as they are de-tuned farther from 1 MHz. The result is anarrowing of the passband that ulti-mately approaches the response of thecrystal alone, which is a very narrowone. Because the input and output cir-cuit are detuned in opposite directions,the response always remains symmetri-cal. There you are; a crystal filter wherethe passband can be smoothly variedbetween a few hundred cycles and 100rmore kiloHertz by simply rotating asingle knob!

In the "Kotn" and other receivers aswell, two of these filter sections wereused in cascade, separated by an i.f.amplifier tube. Th e sections of thecapacitors fo r varying the bandwidthare in that case combined to a four-gang unit.

Say You Saw It In CO

formulas that can be fo und in for in-stance the ARRLAmateur Radio Hand-book. For the variable capac itor withcoun teracting gangs we could prob-ably use two vancaos on which thecontro l voltage acts in opposite ways.

It is perhaps well to explain that thesuccess of these early crystal filters isfor a large part due to the fact that theGermans knew how to produce stablehigh Q coils, usi ng powdered iron cup-cores and such at a t ime when in othercountries ai r coils were still used as arule, with a simple powdered iron tun-ing slug used on others.

Fine coils were not found exclusive-ly in t he r.t . and t. t. parts of Germanrecei vers . In the variable frequencyoscil lators of transmitters coils can befou nd in which silver turns are burnedinto ceramic coil formers. As a resultthese osci llators show a remarkablemechan ical and electrical stability. Ac-cording t o PA0AOB th is techniq uereduces the temperature coefficient bya factor of 200, as compared to a con-ventionally wound coil . The same con-st ruction techniques were used in themanufacturing process of trimmer andfixed capacitors in tuned circuits.

NoW; that we know something ingeneral about German radio equip-ment from WW It days it is gettingtime to take a closer look at some ofthese fine radios.

Tornisler·Empfanger bWe will use the original German

designation of the receiver we are go-ing to discuss now. Photograph 4shows a front view of the set. You seethat the set consists of two units aboveeach other. The cabinets- meant to becarried on the back of a soldier-thathouse the units are called "romfster''in German. Actually in photograph 4you see two half "Tornlster'', one hous-ing the receiver and the other the powersupply. "Emptanqer'' is the Germanword for receiver. And the letter "b" inthe designation simply indicates whichreceiver. Mostly the des ignator wasshortened to "Torn E b" and that iswhat we will use. The Torn E b wascreated around 1935/36. It was ingeneral use with the German SignalCorps, but also at higher army staffs,police and traffic control authorit ies. Itwas a popular set, produced in greatnumbers and one of the few sets thatfoun d their way to amateur service inEurope in post -war stations.

It is a tuned radio frequency (t.r.t.) setwith four filament type tubes that wererun from a 2 volt battery at 0.2 a. each.So the whole radio consumed about 0.8a. from the battery. The anode currentcame from a 90 v. dry battery at a con-sumption of about 12 rna. It was alsopossible to generate the h.t. from the 2v. battery via a vibrator unit and that iswhat you see in photograph 4. The

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CIRCLE 4e ON FlU.OER SERVIC E CARD

TU-170

(To Be Continued)

received very well. S.s.b. can be re-solved too, but tu ning gets a bit t rickythere, as al ready mentioned. Cross-and tntermodutatton forms no prob-lem . Even In Europe's extremelycrowded forty meter band, with itsmany Intruding st rong broadcast sta-t ions, amateur c.w. signals can beeasily copied al night, a test that manymodern superhets fai l. The forty meterband Is not within the range of the setaccording to the officia l specs, as itdoes not tune beyond 6970 kHz. Never-theless, In practice some 30 or 40 kHzof the 7 MHz amateur band is in factcovered.

One is st ruck by the quiet operationof th is t.r .t. set. The input noise Is justnoticeable on a qu iet band. And thebackground level is steady, no doubtcaused by the fact that there Is noa.g.c. As wa well-known Dutch radioexpert stated before vm II: "auto-matic ga in control moves the fadingfrom the signal to the background."

The set is rather small, 36.5 emwide, 24.5 em high and 22 em deep.This Is for the receiver alone. Togetherwith the battery and accessor ies "Tor-ntster" height is 46 em. But the weightof the complete un it is surpris ing, 24kg (52.9 Ibs).

be read, the correct voltage reading Isalso indi cated by a red marker! Bypushing the button on the meter frontthe instrument reads the h.t., thatshould be 90 v. and the correct value isagain Indicated by a colored markeron the meter face, this t ime in blue.

Finally we find on the front panel anonloff swi tch that controls the fi la-ment current, a plug for the powercable and sockets for two sets ofheadphones. And of course two bigand easy to use terminals for antennaand ground.

As I already mentioned, Arthur,PA0AOB, gave me the opportunity touse the radio for considerable time inmy own shack. Although the design Isnow over forty years old, the set stiliperforms remarkably. As to be ex-pected the receiver is at its best onc.w. Especially on the 500 kHz marineband it leaves nothing to be desired.But also on the long wave and mediumwave broadcast band many more sta-tions were copied than on a modernrun of the mill superhet. Quality of theaudio is rather limited, but then the setwas certainly not meant for mus ic.Selectivity is excellent. In fact on thelong and medium wave bands the re-generation control shou ld not be ad-vanced too far, otherwise serious topcut Is experienced.

Also on the short wave bands c.w. is

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the frequency In increments of 5 d ialdivisions. But for a finer reading wecan extrapolate between these In-crements; In a fourth window, im-mediately above the one that showsthe subband number, we read howmany kiloHertz correspond to onedial division. For subband 1 thi s is 0.8kHz, as you may be able to read fromthe photograph.

The knob called " Ruckkoppl" attop right contro ls the regenerat ion ofthe detector stage, number 55 in thec ircuit d iagram. Any of you who haveused a t.r.f. set know that the joy ormisery you derive from it is deter-mined in a major part by the action ofthe regenerat ion contro l. The setshould slide smoothly into oscilla-tion, without thumb or backlash, thatis to say the detector should start andstop osc illation at the same positionof the control. These desi rable tea-tures are dependent upon a numberof factors in the ci rcuit, as any of youoldtimers can test ify. But the Germandesigners of t.r.f. sets certainly knewthe secret of making a fine reqenera-ttcn control. The one on the Torn E b,or any German t.r.I. set for that mat -ter, is a pure joy to use. Fine control isass isted even more by a slow mot iondrive on the regeneration capacitor!

The knob with the crank tu rns thetuning capacitor via a 1:19 slow mo-t ion drive. Th is operates with greatprecis ion and wi th a smooth feel.Nevert heless it was at th is point thatthe author had some criti cism on thereceiver; at the high end of the tre-quency band covered by the set near7 MHz, one revolu ti on of the tun ingcontrol changes th e frequency bysome 200 kHz and thi s Is too much foreasy tuni ng of s.s.b. signals. Thi scri ticism is quite unfair, of course, asat the time the receiver was devel -oped s.s.b. was un known, at least formilitary applications. Sti ll I found away of fi ne tuning the set; as withmost t .r.t . sets at high frequenciesthe regenerat ion control pulled therece iver tuning somewhat and thiscou ld be used as fi ne control.

The control marked "Lautst " is thegain control. It varies the screen-gridvoltage of both r.I . amplifier tubes.This works very well and avoids thepossibility of over loading the detector.Top center we find the antenna trim-mer with screw driver adjustment. It ismarked 20 in the circuit diagram.

For te legraphy an audio fi lter, tunedat 900 Hz, can be brought into the ct r-c uit by means of t he switch"Tonsteb." The filter consists of a par-allel tuned circuit with coil 62 and cap-acitor 63. It Is very effective on c.w.

Top left on the front panel we find avoltmeter. It reads the filament volt-age. Not only is the meter calibratedso that the proper voltage of 2 v. can

Say You Saw It In CO 27

In Part /I of this expanded series we take up the Lo 6 K39a receiver. Many of the receivers that have survivedthe war and time are still seeing service by amateursthroughout the world.

German World War IICommunications Receivers

Technical PerfectionFrom A Nearby Past

Part II

BY DICK ROLLEMA ' , PAQSE

In Part I of this series, we saw someof the ingenious thinking that went in-to the design and cons truction of Ger-man equipment used during WW /I .We pick up where we lef t off las t timewith a discussion of the La 6 K 39ashor t wave receiver. The pictures at-test to the fabled metnoatoism of Ger-man cra ftsmanship , and one can onlysurmise that the accompanyingmanuals told o f the number of peoplerequired to either lift or move some otthis equipment. Some of it looks l ikeit should have crane hooks welded tothe cabinets. -K2EEK

Short Wave ReceiverLo 6 K 39a

T he receive r t hat we are going todiscuss now must be the ult imate intuned radio frequency receiver design.But before we start to describe it we willfi rst expl ain the type designat ion, justas wedid forthe Torn E b in the previoussection. " Lo" stands for "Lorenz", thefirm that designed and buil t the set.Lorenz was the Germa n branch of the

' v.a. Marckstraat 5. 2352 RA LEIDER-DORP. Netherlands

Say You Saw It In CO

American In concern. The f igure " 6"means there are six tuned circui ts . The" K" means " Kurzwellen," German forshort wave. The set came into opera-tional use in 1939, hence " 39" in thetype number. And the " a," finally ,means that c apac i tors o f st andardtypes were used.

The Lo 6 K 39a was used by the Ger-man Navy during WW II. Photograph 6gives you a f irst im pressio n o f thi sreceiver. It cannot be called small , asthe Torn E b. with its dimensions of 53.8cm high, 50 cm wide and 32 cm deep.That is without the shock absorbingfeet, on whi ch it was normally fitted forstnptome use. Aga in, it is bu ilt like theproverbial battleship - it was meant forthe Navy - and thai refl ect s i n i tsweight, a sol id 65 kg (143 Ibs). ArthurBauer, PAMOB, the owner of the set inthe photograph, kindly gave it to me onloan for a practical evaluation and I hadto enlist the help of my neighbor, a con-tractor with bulging muscles. to helpcarrying the monster up two flights o fstairs to my shack in the attic.

I got the orig ina l technica l manualwith the set and the first line in thatcomprehensive booklet nicely summa-rizes what it is. In trans lation it sayssomething like "The short wave receiv-er Lo 6 K 39a is a f ive tube tuned rad iofrequency recei ver with six gangedtun ing circui ts . A sixth tube serves for

frequency checking." The radio covers1.5-25MHz in eight subbands that cov-er the following ranges according tothe speci fica tions:

Range 1: 1.50-2.135 MHzRange 2: 2.135-3.05 MHzRange 3: 3.05-4.33 MHzRange 4: 4.33-6.16 MHzRange 5: 6.16-8.74 MHzRange 6: 8.74-12.40 MHzRange 7: 12.40-17.60 MHzRange 8: 17.60-25.00 MHz

In practice th ere is some overlap be-tween ranges . A s you see the sub-bands cover relatively narrow frequen-cy ranges and that. together w ith the1:50 slow motion drive on the tun ingcapacitor makes for very easy tuning,up to the highest frequencies. Butmore about that later.

The circuit diagram is shown in fig . 7in simplified form, as presented in themanual. As you see the input is pro-tected by a neon tube. This already in-dicates that the radio was des igned foroperati on near transmitters, a st tua-tion encountered on board naval ships.The lamp is alight.

There are not less than six tuned cir-cu its ! Of these the first three precedethe first r.t. tube in the form of a tripletuned bandpass fi lter. This w as nodoubt another protection of the input

December, 1980 • CO • 17

-- --------

Photo 6- Tuned radio frequency receiver Lo 6 K 39 a. This set features no less thansix tuned circuits. all ganged to the tuning control!

tube agains t t he st rong signals ofnear-by transmitters. Further tuned err-cul ts precede the second and third r.t,amplifying tubes and the leaky griddetector with regeneration . Each of thesix sets of eight coils occupies aseparate screened section of a ver -tically mounted coil turret, clearly visi-ble in photograph 7 which shows theinner works. The turret is rotated viathe crank at the bottom of the fr ontpanellhrough a set of gears. The band-swi tching act ion by means o f thi scrank is very pos itive and imparts astu rdy fee ling, like the exclusive auto-mobile door we ment ioned in an earliersection 01this art icle. The six sectionsof the tuning capacitor are divided overtwo screening boxes, wi th part of thegear train in between. The uni t can beseen at the right in photograph 7. Aswith the coil turret the shaft is verncat .

The dial on the tuning capacitor isdirectly calibrated in f req uency fo reach of the eight subbands. There is nodanger that the wrong frequency scalecan be read because of a ci rcular maskwith separate wi ndows for each bandwhi ch rotates with the c o i l turretthereby allowing on ly the frequencyscale of the selected subband to beread. In photograph 6 this is the middleone of the three windows in the blackmask. The mask also clearly indicatesthe frequency range and the subbandnumber and even in which direction thecrank must be rotated to reach the ad-joining subbands. In the photographsubband 5 (5.97 - 9.00 MHz) has beenselected. Subband 4 can be reached byturning counter clockwise and number6 by turn ing clockwise. Another exam-pie of German perfection !

The lower wi ndow disp lay s a log·ging scale and the top window has noglass, which makes it possible to enterpencil marks on the dial.

We already ment ioned that tuning ismade easy through the slow motiondrive and the relatively narrow sub-bands. But it is made even easier bymeans of a fine tuning control thatvaries the tuning of the detector circuit(Which determines the tuned frequencyon c.w. and s.s.b. with an oscillatingdetector) plus or minus 3 kHz aroundthe frequency selec ted by the maintuning control. The tuning range of thefine control is independent of the set-ti ng of the main tuning capac itor or thefrequency range selected. This cou ldonly be achieved wi th a coil of variableself·inductance that is changed withthe subband in use; it is designated 74in fig. 7. If a variable capacitor hadbeen used for fine tuning its influencewould be dependent on the total capa-city in use. Thus the fine tuning rangewould be much greater on the high tre-quency end of a subband than on thelow frequency end. The variable coilsare operated by a big control knob, the

18 • CO • December, 1980

outer one 01 two concent ric knobs onthe lower left Iront panel. The inner onemarked " L" (vt.autstarke") cont rols thereceiver gain by varying the screen gridvoltage 01 the first two r.f . tubes.

Here, on the subject of gain, we findanother of those special measuresthat makes the operation of this Ger-man WW II communica tion receiversuch a pleasure. The impedance atresonance 01a parallel tuned circuit ishigher when the ci rcuit is tuned to thehigh end of it s tuning rang e (minimumcapacity) than at the low end. This af-fec ts the gain of the tube that precedesthe ci rcui t . That gain is the product ofthe mutual conductance of th e tubeand the impedance of the tuned ci rcuit.The net resu lt is that the stage gain in-creases when the circuit is tuned fromlow to high within a subband. As thereare three r.l. tubes, each wi th a tunedci rcui t fo l low ing , t he effec t is ag-gravated and we would have to adjustthe ga in con t rol continuously whentuning to keep the gain and signal

handling c apabili ty o f t he set t hesame. Indeed, this wou ld have been aproblem were it not for the ingenuity ofthe designers of the La 6 K 39a, whofound a clever remedy. If you take agood look at photograph 7 again youwill notice a potentiometer at the rightbetween the boxes containing the tun-ing capacitor. This potentiometer iscoupled to the shaft of the tuning gangby means of a set of gears, so it rotateswi th it . Th is pot cont ro ls the sc reengrid voltage of the th ird r.f. tube (notshown in the simplified diagram of fig.7) in such a way that the voltage be-comes lower when th e set is tunedtowards higher f req uencies, so de-creasing the gain. This compensat ionis so effective that the user does notnotice any obvious change in receiversensitivity when tuning within a sub-band. Neat eh?

The detector features the alreadyfamiliar smooth regeneration cont rolby varying its screen grid voltage. Thedetector drives the a.f. f inal stage via

Say You Saw It In CO

Photo 7- This view from the back of tuned radio frequency receiver Lo 6 K 39 showsthe coif turret with six compartments, each of wh ich houses eight coifs. one loreach frequency range. The two boxes at the right contain the sections of the six-gang variable tuning capacitor. Between the box a pot can be seen tha t is drivenIrom the gears for the tuning capacitor. The pot varies the screen grid voltage ofone of the tubes, in order to keep receiver ga in cons tant when tuning over a fre-

quency range.

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goes top first in to a holder that com-plete ly envelopes it. In order to retractit a specia l tool is screwed into i tsbase. Otherwise the bottom 01th e tubeis flush wi th the panel that carr ies th eholder. A photograph of the RV 12 P2000 appears later in this article.

As you see in fig. 7 the receiver has abuilt-in pow er supply with a doublesmoothing section. The supply voltagefor the detector stage is stabilized by aneon tube.

The Lo 6 K 39a is a sophistica tedradio in every re spect. Th is reflect s

an audio fi lter tuned to 1000 Hz with a 3dB bandwidth of 200 Hz. Because thisis a double tuned bandpass filter itsslopes are steeper than for the one inthe Torn E b receiver.

The six th tube is used in a crystalcalibrator on 100 kHz.

All tubes are of the same type, anespecially designed tube for mili taryapplications, a miniatu re pentooe w iththe type number RV 12 P 2000. Thisuniversally used tube has an indirectlyheated cathode that takes 0.075 ampsat 12.6 volt heater current. The tube

CIRCLE 63 ON READER SERVICE CARD

Say You Sa... It In CO 19

also In the provision for metering thedifferent stages. Top right in photo-graph 6 we find the test instrumentthat features not one, as In the Torn Eb, but two pushbuttons. By pushing theyellow button we read the heater vol-tage and the meter pointer should In-dicate within a yellow sector on themeter face. The anode voltage can bechecked by operating the red push-button and the meter should read with-in a red segment on the meter face. Butit doesn't end there. We can also checkthe anode currents of the six tubes. Asa preparation to this the controls forgain and regeneration should be turnedfUlly clockwise and the tuning controlon "0" (the tuning control effects thescreen grid voltage of tube numberthree, as we saw). Under and tothe rightof the meter we find a six positionswitch marked "Anodenstrom" (anodecurrent). It is normally held In an off-position by means of a spring. Whenwe move the switch against the springinto positlon "I" the meter reads theanode current of tube number 1. If thetube is in normal condition the meterpointer is within a blue sector on themeter dial. The same happens for tube2 with the switch in position " II" . Andso on for all six tubes . Speaking ofperfection ...

As could be expected the electricalspecification of the receiver is verycomplete, be it that In the days of WWII some of the electrical characteristicswere specified differently from nowa-days.

Output is stated as the voltage overa pair of headphones with 5000 ohmImpedance at 800 Hz. The receivernoise should produce between 0.2 and3.0 v, with the gain control at maximumand the detector just In oscillation formaximum sensitivity for c .w. Withmaximum 0.3 v. receiver noise 2 micro-volt Input for A1 (c.w.) and 4 microvoltfor A2 (m.c.w.) should produce 1 v. out-put at optimum position of the regen-eration control. According to testsmade by the author a signat of 0.4-0.8microvolt generator emf behind 50ohm Internal generator resistance pro-duces a readable c.w. signal.

You probably wonder what sort ofselectivity can be expected from a t.r.f.set with six tuned circuits. The specsstate the selectivity as follows that at± 0.85% detunlng a 10' times increaseof signal Is required to restore theresponse from a 2 microvolt signal at4.62 MHz (signal generator modulatedwith 400 Hz at 30% modulation depth,mode A2, regeneration control set Justbefore oscillation). We would say that0.85% detuning causes 60 dB attenua-tion, which certainly Is not bad. Thereare many more interesting points in thespecs but we will leave It at that.

The practical evaluation In theauthor's shack fully confirmed what

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would be expected from the specs. Theset had been inactive for many years.Ow ner PA0AOB did not rea lign t hereceiver before it was handed to theaut hor. The only thing Arthur did wasto clean the moving con tact s of thecoil tu rret and capacitor gang withcontac t spray . Nevertheless the tre-quency ca librati on was found to beremarkably correct. The 100 kHz crys-tal in the calibrator was spot on. Tun-ing is as easy as can be, even sidebandon 21 MHz. thanks to the Itne-tumnqcontrol. As with the simpler Torn E b.the set is beaut ifully quiet. As to be ex-pected the set is at its best on c.w.Aga in no trace of i nt er- o r c ros smodulat ion was found, not even duringnight hours in the crowded 40 meterband. And not a single spurious signalor birdie was observed, quite normalfor a t.r.I . set of course. But it is areassuring feeling to know that when asignal is heard you can be sure it is areal one on the frequency the rad io istuned to.

The author remembers that at thetes t equ ipment department of the elec-tronics firm, where he was employedsome 15 years ago, a Lo 6 K 39a wasused to check the purity of the outputfrom a signal generator under test.

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Say You Saw It In co December. 1980 • CO • 21

Part 11/ of this elongated series concerns itselfwith the Kw E a superheterodyne receiver whichwas a basic Telefunken design.

German World War IICommunications Receivers

Technical PerfectionFrom A Nearby Past

Part III

BY DICK ROLLEMA · . PA~SE

Part Iff now takes up with a 1938desig n lo r a s up erhe terodynereceiver. The Kw E a as you wfll seefrom the photos Is a marvel ofmechanic al engineering. WhilePAUSE rega les us with the theoreticalaspects of the design, one can onlyimagine how that was tra nsformed In-to the mechanical beauty we see un-fo ld. Stili, the thought of lugging 77pounds of ha rdware Braund leaves alot to be desired. -K2EEK

Short Wave Superhet Kw E aThe first of the two superheterodyne

receivers we are going to discuss is theKw E a (Kw = " Kurzwe lle n" = sho rtwave, E = "Emptanqer" = receiver, ais serial indicator). This radio wasdesigned by Telefunken and it becameoperat ional in 1938.

Photog raph 8 gives you a f irst im-press ion 01 what it looks like. Photo-graph 9 shows the receiver out 01 itscabinet and photograph 10 a rear view.

· v.d. Marckstraa t 5. 2352 RA LEIDER-DORP. Netherlands

The dimensions of the rad io are 69.2em wide, 27.4 em high and 34.6 emdeep. It weighs 38 kg (77 Ibs) . Notdirect ly a set to take on holidays.

The total frequency span of 1 10 10MHz is covered in f ive rang es, a sfollows: 980-1610 kHz; 1560-2550 kHz;2470 - 4060 kHz; 3940-6395 kHz and6205- 10000 kHz. Again the subbandsare kept relatively narrow which im-proves the ease of tuning and the fre-ouency read-out.

l ooking at photograph 8 you noticeseveral by now famil iar features. Leftof the dial the band-selector controlabove ("Grab" = coarse) and main tun-ing control with crank (" Fein" = fine)below. The d ial features the rotatingmask we already met on the previousreceiver with a slot tha t displays onlythe frequency scale in use. The maskalso shows the limits of the selectedrange and the frequency incrementthat corresponds to one dial division,10 kHz on range III , as seen in th eph otograph. The receiver uses thesame f ilament t ype tube in a ll 11stages. It is the tyPe RV 2 P 800 pen-tode we also encountered in the Torn Eb. The radio consumes 1.6 a. at 2 v. fOfthe fil aments and 15-20 rna. at 90 v.anode current. The bottoms of thetubes with extract ion knob can clearlybe seen in photographs 9 and 10.

The simpli fi ed diagram in the tn-

struc tion manual is too complicated toreproduce here. Therefore we present ablock schematic diagram as fig. 8.

The mixer is preceded by two r.f. am-plifier stages. There are five tuned r.f.c ircuits th at are ganged to the oscn-tater tuning circuit. The user has theoption to use a single or a double tunedcircuit between the antenna and thefirst 1.1. tube. Normally one circuit isused. But the manual says that wheninterference is experienced from a verystrong near-by transmitter the secondci rcuit between antenna and first r.f .tube should be brought into operation.The swi tch for th is is the top one of thetwo controls at the right on the frontpanel.

The lower one of these cont rols is anantenna attenuator. Not a resist ive onebut a va riable serie s ca pacitor be-tween antenna and input circu it. Toavoid detu ning of th is circui t a secondsection of the capacitor adds just asmuch capaci tance in parallel with thetuned ci rcuit as the series capacitordetract s. In other words the input at -tenuator is a differential capacitor. It iscalled " Ankopplung" (= coupling) onthe front panel. There is also a seriestrimmer in the antenna that compen-sa tes fo r different antenna ca pact -tances. It is set once and tor all tor aparticul ar antenna and therefore hasscrewdriver adju stment (top right just

Sa yYou S.w It InCa

to the left of the antenna connector inphotograph 8~

The six sets of coi ls that have to beswitched for the five frequency rangesare mounted in a coi l turret. This one isof parti cular beauty. Photograph 11shows the turret , taken out of t hereceiver, which is a simple operation.The turret Is moved from one posit ionto the next by means o f a Maltesecross mechanism that can just be seenat the ext reme left of the turret. Butbefore th e turret starts to rotate th econtact f ingers are lifted from theirpartners on the turret by mean s of acamshaft that can be observed in pho-tograph 11 in front of the turret. Whenthe turret has come to rest in the newposi tion, th e cont ac t finge rs a relowered onto the t urret again. Thefingers make a sl ight wip ing move-me nt whe n pu sh ed on to th e r ingshaped contacts on the turret, therebyremoving possible di rt deposits.

The receiver uses the relat ively lowintermediate frequency of 250.9 kHz.But because f ive t uned ci rcuits areused ahead of th e mixer the imageresponse is suff iciently supp ressed(on the order of 80 dB).

The oscillator is of the tuned anode

circuit variety. A coil in the grid ci rcuitis induct ively coupled to it. A secondcoi l, coupled to the anode circuit of theosci llator is in series with the coil thatforms the r.f . tuned ci rcuit connectedto the grid of the mixer tube. In this waythe osci l lator signal is injected into themixer.

There are three 1.1 . ampli fier tubes.They are preceded by double tuned Lt.t ransformers on 250.9 kHz.

The receiver offfers the selection ofseven different bandwidths, of whichfour are meant for telephone, the f ifth,si xth, seventh and eighth are for c.w.on ly. The bandwidt h in positions sevenand eight is identical, but in the eighthposit ion the b.f.o. is switched to theother side of the passband. The band-width selector control can be seen inphotograph 8 di rect ly under the meter.

The principle of the bandwidth varia-tion is ind icated in f ig. 8 in simpl if iedform. In posit ions 1-5 the bandwidth ofthe i.l. ampli fie r is changed by varyingthe coupli ng between the tuned c ir-cuits of the Lt. tra nsformers. By goingfrom position 1 to 5 the coupling ca-pacitors between the tuned circuits aremade smaller and smaller. This wouldalso shift the center frequency of the

passband slightly. But this is compen-sated for by add ing extra capac ity inparallel with the tuned circuits as the("'()l Jpling capacitors become less. Alsothe dam ping resistors in parallel witht he tuned ci rc uits are Inc rea sed invalue as th e bandwi dth narrows. Inposit ion 5 no extra damping is used. Inposit ions 6-8 of the bandwidt h controlthe i.f. bandwidth remains th e sameb u t t he a.t . ba ndw id th i s re-duced. This is done by a tuned circuitthat resonates at 900 Hz between thedetector and the a.f. final amplifier. Inpositi on 6 it is brought into the circuitbut the response i s broad ened bymeans of a para llel resistor. In posi-tions 7 and 8 the resistor is removedand the bandwidth is at it s narrowest.

The b.f.o. is crystal contro lled andworks at a f ixed frequency of 250 kHz,thereby generating a beat note of 900Hz with the 250.9 kHz i.f. signal. Theb.t.o. can be brought into operation bymeans of a separate switch. It is direct-Iy under the bandwidth selector switch.If i n po si ti on seven interferenc e isexperienced t he opera tor can go toposition eight. The b.f.o. is now chang-ed from 250.0 to 251 .8 kHz, aga in gen-erat ing a beat note of 900 Hz but now

Photo 8- Front view of short wave superheterodyne receiver Kw E a.

Say You Saw 11/" co May, 19B1 • co • 21

Photo 9- Receiver Kw E a taken out of its cabinet.

Photo 10- Short wave receiver Kw E a as seen from the rear. The two neon lamps at the right protect the input circuitsagains t high voltages when the set is operated near a transmitter.

22 • co • May, 1981 Say You Saw It Inca

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Fig 8- Btock schematic diagram of short wave superheterodyne receiver Kw E a.

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without a.q.c. but is only operative Inthe a.q.c. posit ion. The manual says Itshou ld only be used in case of verystrong interference.

The combined I.t .la.f. gain controlcan be seen to the left of the lett han-die that is used to pu ll the receiver outo f its cabinet. The separate I.f . gaincont rol that only works in case of a.g.c.is to the left o f the right handle Inphotograph 8.

The last item of the Kw E a we willdiscuss is the metering fac ility. It Islike the one on the La 6 K 39 a. Themeter is at the top left o f the frontpanel and to the left of it you will noticea switch w ith 13 posit ions. In the firstthe fi lament vol tage Is read and in the

using lower sideband reception in-stead of higher sideband in pos itionssix and seven.The b.f.o. has a separatecrystal at 251.8 kHz for th is.

The detector is of the leaky grid type.Both i.f . and b.t.o. signals are intro-duced on the control grid of the RV 2 P800 pentode tube that is used through-out the receiver. The detec tor tube iscoupled to the final a.f. tube by meansof the tu ned circuit at 900 kHz that wealready ment ioned. In positions 1- 5 ofthe bandwidth control it is rep laced bya Retype coupling.

According to the manual the setshould normally be used with manualgain cont rol. Th is varies th e screengrid vallage of the first and third l.f.amplifier tubes. But in case of fad ingautomatic gain control can be used.There is a separate l.f. amplifier for thea.g.c. It receives the same l.t. signal asthe leaky grid detector and It feeds twooiooes in a voltage doubling rect ify ingcircuit for the a.g.c. voltage. This is fedto the second r.t. amplifier and the sec-ond i.f. tube. The manual gain cont rolis made inoperative when the a.g.c. isswitched on and rep laced by an a.t .cont rol that is on the same shaft as themanual l.f. gain control. But the U. gaincan still be controlled manually whenusing a.g.c. by means of a separatepotentiometer that has the same func-t ion as the manual gain control for use

M.y,1981 • CO • 23

Photo 11- This is tne coit turret used in the Kw E a. Nofte the Maltese cross drive at the left and the camshaft at the front thatlif ts the contact fingers from the turret contacts before the turret is rotated.

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s eco nd t he an ode voltage . Thevoltages should be within a red or bluesector on the meter face for the twovoltages. The remaining 11 switch posl-tlons are for metering the anode cur-rents of the 11 tubes in the receiver . Ablack mark on the meter dial indi catesthe minimum reading lor a serviceabletube.

The frequency dial also carries redmarkers for frequency checking. Thecalibration signal for this comes fromthe 250 kHz crystal in the b.t.o. Har-monies of the b.f.o. signal can be fed tothe input of the receiver . For th is apushbutton must be operated that canonly be reached when the set is out ofits cabinet. In case the dial reading isnot ok this can be corrected by rota t inggreen encircled adjustment screws onthe coi l turret.

It is remarkable that the manual forthe set does not contain performancespeci fi cations. I have not yet had anopportunity to test the receiver in myown shack. But this is going to happenin the future and I am quite convincedthat the radio will come up to my highexpectations. 3!l

(To Be Con tinued)

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24 Say You Saw It In CQ

German World war 2 communications receivers part 1 1980.pdfGerman World war 2 communications receivers part 2 1980.pdfGerman World war 2 communications receivers part 3 1981.pdf