The CJD ELF/LF Receiver

 
 

That meter was odd.. the zeroing screw had been tweaked and resetting it proved simple, but later when testing I found the coil was open circuit. It's NATO code is 6625-99-971-7538. 

 

This outline sketch shows a complete CJD receiver and you'll note a panel mounted above the main receiver. This box carries some essential parts of the receiving set-up, such as the power supply and their electronic assemblies.

Whether the basic receiver chassis can be encouraged to work as a self-contained item remains to be seen.

Click the box to see more.

The basic receiver is pretty heavy, weighing in at 97 pounds which is not quite as heavy as another Navy receiver, the DST100 which manages 110 pounds. Anything designed for the Royal Navy needs to be solid enough to withstand the effects of the pyrotechnics used at sea and I guess the CJD needs to be depth charge proof.
  

 The chief reason for developing this receiver was obliquely referred to in my True Story concerning the British nuclear deterrent as it was in the 1960s. Only low frequency broadcasts can penetrate deep seas and if one tunes across the VLF band many strange broadcasts can be identified. These if listened to with a BFO turned on seem to be jingly rather than carrying speech. Because intelligence can be transmitted only commensurate with the basic carrier frequency, once you get to the low end of VLF means it's typically teletype style signalling that has to be used.

This CJD model can receive AM, CW, facsimile plus single and double sideband. It can be tuned normally or at the flick of a switch, and some carefully fiddling, a synthesised oscillator can be used in place of its tunable local oscillator.

The receiver above carries the NATO code 5820-99-916-4903 and to the best of my knowledge contains the front end and the majority of the IF amplifier. The plain fronted unit with its code ending in 04 contains the power supply, a final IF amplifier stage, detector, AGC, and audio output circuitry. Oddly, because of its history the CJD uses a mixture of valves and transistors. Whether this is just a matter of timing I don't know, but valves are much more robust when it comes to EMP as well as having superior technical aspects compared with early transistors. The DST100 designed well before the CJD uses an ancient RF amplifier valve (CV21=VP41) just because it works better than newer types for pragmatic reasons I won't go into here.

 

 The important detail for a listener are the five wavebands covered by this set. The range is 10KHz to 200KHz. The top end conveniently covers Radio 4 on 198KHz. Was this deliberate? It delivers 10mW for a signal of 1uV which is pretty good. What isn't run of the mill is its IF amplifier (61.5KHz and 21.5KHz) with the lower frequency amplifier more akin to hi-fi audio than RF. The circuit configuration has the highest frequency range (Band 5 carrying Radio 4) as a dual superhet using 61.5KHz followed by 21.5KHz, but as a single superhet using an IF of 21.5KHz for the other wavebands.

 

 Band

KHz

 1

 10 to 23

 2

 23 to 41

 3

 41 to 71

 4

71 to 119

 5

120 to 200
 

 Above, never mind the width.. look at the quality.. two drums which are connected via complicated bits of stuff to a 3-gang tuning condenser and something else? The tuning dial has a rotatable assembly a bit like that used in the CR100 that carries the KHz markings for the 5 wavebands and a steel strip threaded around the drums. This strip carries a pointer indicating the tuned frequency. It's clearly never going to go wrong even if a bomb dropped on it unlike the really flimsy arrangements used in Army kit of similar vintage. Compared with the RA17 the above is an order of magnitude mechanically more advanced. Even over-engineered one might be inclined to think... but then again because of the reliance on ELF kit for launching WW3 missiles maybe not...

Now, how difficult will it be to reproduce the upper box. Using intelligence collected from a number of sources it seems a circuit diagram is not too easy to lay ones hands on. I heard one solution for copying the RF section of the upper box revolves around a chip (LM373 probably the H, or can version which is easier to use) introduced in the 1960s and improved in the early 70s. I've now ordered one, whose date is yet unknown, from a supplier in Italy. It arrived in mid-September 2021. The original circuit used a simple diode detector for AM and a ring modulator for CW and SSB. I also have a circuit diagram for a test unit used for the CJD power unit from which I've gleaned most of the connections to the flying lead emerging from the rear of the receiver box. An adjacent coax connector probably has the IF feed.

 

 Pins numbered: Right to left, top to bottom : B F L R V Z DD JJ NN, D J N T X BB FF LL, A E K P U Y CC HH MM, C H M S W AA EE KK

So, for example, the mains circuits are the 4 pins on the extreme right, top to bottom B, D, A and C

No problem trying to figure out the mating half as Pete G4GJL kindly sent me one.

 

 

 PIN

 FUNCTION

 NOTE

B

 Mains on/off

 Mains N

F

 6.3VAC

- 

L

 AGC long/short

 Long=+24V

R

 minus 55V

-

V

 BFO on/off

 On=ground

Z

Screen

For Pin U

DD

 Ground

-

JJ

 Ground

-

NN

Spare

-

 

 PIN

FUNCTION

NOTE

D

 Fuse F1

from Pin B

J

 Rx Ready

Relay ON=0

N

 600 ohm o/p

-

T

 Audio out

 Wiper of pot

X

 +235V

-

BB

 minus 43V

-

FF

 +150V

-

LL

 Ground

-

 -

 -

 -

 

 PIN

FUNCTION

NOTE

A

 Mains on/off

Mains L 

E

 6.3VAC

-

K

 +24V rough

Lamps

P

 600 ohm o/p

-

U

 Audio in

 End of pot

Y

Screen

For Pin T

CC

 +24V smooth

-

HH

 Ground

-

MM

 AGC1

-

 

 PIN

FUNCTION

NOTE

C

 Fuse F2

from Pin A

H

 +24V var

Lamp dim

M

 AGC on/off

 Off=+24V

S

 +20V

-

W

 +20V

-

AA

 RF level

To meter

EE

 AGC3

AGC to RF

KK

 AGC2

 -

 -

 -

 -

 Information is beginning to arrive from Pete G4GJL (thanks for the pair of 34-way connectors), Peter G8BBQ and Paul G8GJA so what seemed like a hopeless task is now starting to look quite promising and my £230 ("inc delivery") investment may be saved...

Provisional pinning for 34-way connector above.

The drawing below is that of the test set for the CJD receiver which I've reproduced to indicate the power supply requirements, hence a key to designing a suitable PSU for the receiver sub-assembly plus the ancillary parts located within the upper panel. Allowance for the latter needs to be added to the power requirements below because the test set connects only to the receiver sub-assembly.

 

 

 Voltage

 Test Load

 Load Wattage

 Current

 Notes

 6.3VAC

 4.7 ohm

 10W

 1.34A

 Valve heaters

 20V

 130 ohm

3W

 154mA

Transistor power

 24V rough

 220 ohm

5W

 110mA

 Dial lamps

 24V variable

 220 ohm

5W

 110mA

 Dial lamps as above

 24V smooth

 100 ohm

10W

 240mA

 Relay power

 minus 43V

 150 ohm to -55V

1W

 80mA to -55V

Oven power + AGC

 minus 55V

 47 ohm

40W

 1.18A

Oven power etc

 150V

 10 Kohm

 6W

 15mA

 Valve local oscillator

 235V

 4.7 Kohm

30W

 50mA

 Valve HT

 The receiver design is slightly puzzling, almost as if it were the result of a committee decision. It's either fitted into a cabinet for bench mounting, called CJD(1) or as a five receiver setup CJD(2) or CJD(3) in a floor-standing rack. The main receiver(s) carrying the controls is either powered by a completely separate power supply mounted above the receiver CJD(1) or in a rack with each receiver carrying a small chassis bolted to the rear of its chassis. Almost as if the designers ran out of space the PSU part is accompanied by extra receiver circuitry again either in the box above the receiver as CJD(1), or bolted onto the rear CJD (2) or (3). These differences make some of the documentation a bit confusing.

To clear up which bits are where, the main receiver includes a synthesiser made up from four parts viz.a Phase Demodulator, Ring Modulator, Frequency Divider and Harmonic Frequency Selector, secondly an IF Amplifier, thirdly a BFO, fourthly the RF front end using a set of valves and an oven for stabilising frequency, and it seems an audio amplifier driving the loudspeaker. The parts not fitted into the main receiver are the Power Supply with its various voltage stabilisers, the AM/SSB/CW Detectors, an Audio Amplifier specifically for driving a 600 ohm line and something called a Receiver Gain Equaliser (developing AGC voltages from its own IF amplifier). These items except the PSU wound components and those requiring heatsinks, are fitted onto three printed circuit boards. Most of the parts in the main receiver are sub-assemblies which can be removed from their main chassis.

One of the slightly mystifying features of the receiver is control of its overall sensitivity. Not content with various gain controls and powerful AGC, hidden under the rear of the chassis is a switch for enabling or disabling the first RF amplifier. AGC is derived in the outboard circuit which accepts the IF output of 21.5KHz from the IF amplifier where it is also processed to derive AM, CW or SSB using the BFO output as appropriate. Following this, also outboard is the AF amplifier used, not for the internal speaker but, for broadcasting around the vessel. Communication between the outboard circuits including the power supply and the main receiver chassis is via a cable with a 34-way connector (as covered previously) and a pair of coax leads (IF + BFO).
 

Next topic to investigate, now I have the requirements (above), is the practical implementation of a suitable power supply.

Below is the original design which has slightly awkward transformer voltages.

 

 From an intial view of the requirement I envisage using three mains transformers including an HT transformer rated at a minimum of say 30VA to provide 235 volts (at 50mA=11.75W) and a stabilised 150 volts run from the 235 volt supply (15mA=3.5W). Heater supply for the valves from the same HT transformer at 6.3 volts AC (at 1.34A = 8.5VA). This has a secondary winding of 185V in the schematic above and will produce a peak voltage of about 260 volts DC.

The second transformer has a secondary of 70V providing a peak of 98 volts DC. This needs to supply 55 volts at 1180mA and 43 volts 300mA (1480mA=145W peak) and so will be much heavier than the first. The diodes in the original design were CV7313 which are similar to the BYX38-600 or the common P600K 6A diode. A suitable alternative for these however will be a standard full wave bridge rectifier such as the SB2510. I'll need to search through my transformer collection to identify something suitable for these two but the other lower voltages aren't a problem and a small, junk box, transformer can be used. Of course I'll use modern regulators to simplify the design of the above.

 The IF and audio circuitry will be based on the LM373H chip, an early device used typically in the circuitry shown below, taken from a 1972 article by K4DHC. As you can see this includes a BFO circuit which is not required as this is already included in the main CJD case. Also included within the main CJD receiver case are the various switches and controls. All interconnections are carried via the 34-way connector. Below is based on an IF of 455KHz but of course the CJD final IF is much less at 21.5KHz whose frequency will match the filter shown below.

 

 It will be a case of interfacing the existing circuits into those required by the LM373H. The filter shown in the sketch above will need to be in keeping with the method used in the IF amplifier carried on the main chassis and maybe a simple affair as it looks from the markings on the front panel as if the IF response will be shaped before the detector in the LM373H. I suspect the amount of audio power required for the main chassis will need to be tailored to suit the small loudspeaker. Mode switching is carried out via small 24-volt relays.

Below, I've marked the 34-way connector table in purple where connections are required for the new detector/audio circuit. Note there are four AGC connections associated with the front panel AGC switch positions "Short" and "Long" plus a separate feed to the valved front end. Markings in red are for power supply connections. Keeping the main chassis to its original design will unfortunately mean mixing mains connections, DC and AC supplies plus low level signals and control connections.
 

 

 PIN

 FUNCTION

 NOTE

B

 Mains on/off

 Mains N

F

 6.3VAC

- 

L

 AGC long/short

 Long=+24V

R

 minus 55V

-

V

 BFO on/off

 On=ground

Z

Screen

For Pin U

DD

 Ground

-

JJ

 Ground

-

NN

Spare

-

 

 PIN

FUNCTION

NOTE

D

 Fuse F1

from Pin B

J

 Rx Ready

Relay ON=0

N

 600 ohm o/p

-

T

 Audio out

 Wiper of pot

X

 +235V

-

BB

 minus 43V

Oven control

FF

 +150V

-

LL

 Ground

-

 -

 -

 -

 

 PIN

FUNCTION

NOTE

A

 Mains on/off

Mains L 

E

 6.3VAC

-

K

 +24V rough

Lamps

P

 600 ohm o/p

-

U

 Audio in

 End of pot

Y

Screen

For Pin T

CC

 +24V smooth

-

HH

 Ground

-

MM

 AGC1

-

 

 PIN

FUNCTION

NOTE

C

 Fuse F2

from Pin A

H

 +24V var

Lamp dim

M

 AGC on/off

 Off=+24V

S

 +20V

-

W

 +20V

-

AA

 RF level

To meter

EE

 AGC3

AGC to RF

KK

 AGC2

 -

 -

 -

 -

 I'll need to incorporate a 12 volt regulator with the LM373H as the 24 volt rail is too high for its 18 volt max rating. I understand the CJD design has two printed circuit boards covering this area viz. an audio power amplifier and the second for AM/SSB/CW detection and development of AGC to control the RF and IF circuits. As the LM373H is designed as a complete IF amplifier I may need to provide a suitable attenuator at its input and because IF shaping is within the main chassis its filter characterisitics can be relatively broad, perhaps even as minimal as a simple coupling capacitor. For example the impedance of a 100nF capacitor at 21.5KHz is 74ohms. From a perusal of the collection of pages I have in hand it appears the IF output will be around 1mV.

 Below is a set of drawings (kindly supplied by Paul G8GJA) showing how the external detector, AGC and audio circuitry connects to the CJD main chassis. You can see in the first drawing, on the left the SSB/CW and AM detectors selectable by a pair of relays; centre the provision for remote volume control and the audio amplifiers, (the lower) one on the main chassis for its loudspeaker and headphone sockets and the second for 600 ohm output. Click the picture to see more detail.

 This next picture shows the detector circuitry in detail where you may recognise the straightforward envelope detector for AM plus a diode ring using the BFO signal from the main chassis for SSB/CW detection. In order to keep switching of sensitive signals local to this circuit a pair of relays is used. Click to see more detail.

 This third picture shows the AGC circuitry. For some technical reason perhaps a completely separate IF amplifier is used to develop AGC. The reason is probably to simplify the application of AGC to SSB/CW reception. In a standard superhet AGC and AM detection are closely coupled but in this case the two functions are governed by two different circuits. As with the detector circuitry, a pair of relays is used to keep sensitive signals local. Click the picture to see more detail.

 My intention is to use the LM373 device, as described previously, to handle both detector and AGC functions although I'll need to add some circuitry to provide the delay feature and a fixed potentiometer to provide different degrees of feedback (AGC1, 2 and 3). I can see the need for some experimentation when it comes to exact voltage levels associated with the new detector circuit and AGC output, but this might be simply achieved by using preset pots.

Physical implemention of the external features needs some thought. In the CJD rack the PSU is carried on a chassis which also carries a cardframe for the detector etc on printed circuit boards.

 
 

 

 I can understand having an external power supply but I'm puzzled about why the designers couldn't find room in the main receiver for the detector, AGC and external 600 ohm audio line driver. Maybe there was a very strict limit on the case dimensions, bearing in mind the requirement for co-locating a number of receivers? There's also the unduly complex design of tuning. This is very reminiscent of the much earlier CR100 which must have influenced the writers of the procurement spec. The use of valves in the front end almost certainly would have been the result of MoD concerns about EMP. If not EMP, the design was initiated in the days when transistors were limited in performance and predominantly using germanium, although this surely would have had little bearing considering the low frequencies involved.

 Building the Power Supply

 

 The voltages used in the CJD Receiver are a mixed bunch and a search through my transformer collection yielded a couple that should do.The first is a standard HT/LT transformer (T1), once removed from a receiver, which has a 256V-0-256V secondary (W3) plus 6.3V (W1) and 5.0V LT (W2) windings. The second is an LT transformer (T2) carrying three 24V windings (W1/W2/W3). Power outputs are taken from the Test Supplement.
  

 Voltage

 Current

 Notes
  Source

 6.3VAC

 1.5A

 Valve heaters
  T1 W1

 20V stabilised

 150mA

Transistor power
  T2 W3

 24V unsmoothed

 200mA

 Dial lamps
  T2 W3

 22V smooth

 200mA

 Relay power
  T2 W3

 minus 43V

80mA

AGC
  T2 W1+W2 +T1 W2

 minus 55V

 1.18A

Oven power etc
  T2 W1+W2 +T1 W2

 150V

 15mA

 Valve local oscillator
  T1 W3

 235V

 80mA

 Valve HT
  T1 W3

As I develop the circuit for the PSU I'll edit the drawing below which initially showed the original design shown earlier. 

 

 I've decided to build the PSU on a handy piece of scrap steel. It's not aluminium so it takes longer to work, but for heavy transformers it's pretty good and I did get 65% in "O-Level" Metalwork back in 1957.

 

 

 For appearance and safety all the transformer wires will be under the chassis and terminated on tagstrips even though it meant lots of drilling cutting and filing. It probably doesn't matter too much but I arranged the two cores to be at right angles. I'll drill as many holes as necessary before mounting the two transformers.

The chassis measures 11" x 7.5" x 2". I'll be fitting an IEC connector and an umbilical cable will terminate in Pete's (G4GJL) 34-way connector to mate with the CJD receiver.

The RF circuitry will be mounted using a combination of veroboard plus components soldered directly to tin sheets. The latter allows for very easy modifications and the material is free from our recycling bin.

 I've realised that with some fiddling the CJD receiver, once powered from its basic supplies, will produce an IF output of 21.5KHz which if fed into an SDR will prove its basic functionality. That way, as I usually do when faced with a fairly major job is to check on the practicalities of continuing and completing the job without wasting too much effort. In fact I could have taken a shortcut and powered the receiver from bench supplies. The 55 volt supply heats the oven running at something over 1 Amp until the temperature reaches 69C. At this temperature a "Ready" lamp comes on and a relay operates. The relay adds an additional 1.2Kohm resistor into the series regulator drive. I'm unsure about this feature but it may be used to merely reduce the dissipation of the 55 volt regulator or, alternatively to reduce the voltage to a level where the oven current of maybe a couple of hundred mA allows the temperature to just remain constant. I can delay the design of the new 55 volt circuit until I've made some measurents.

A possible critical factor might be the grounding arrangements. For example, mains wiring is carried in the 34-way cable connecting to the on/off switch and fuses on the receiver front panel using Pins A, B, C and D. Grounds are Pins DD, HH, JJ, and LL. These latter connections cater for safety earth as well as return paths for DC power and signal earths. In addition there are two coax cables used which have screen grounds. Poor choice of grounding will result in modulation hum. A prudent change to the original design (without modifying the main chassis) will be to use a mains on/off relay in the PSU which is driven from the receiver on/off switch. This, with the addition of a mains on/off switch plus a fuse in the PSU, removes the requirement for mains connections in the umbilical cable. This is slightly more complicated than it first seems because, if the PSU is turned on via the front panel switch via a relay, power must already be available for the relay coil. In commercial equipment this power is usually provided by a small standby PSU so I'll add a small low voltage transformer/rectifier for this purpose. I fitted two mains switches, one to enable testing without the main receiver and the second, for normal use, to turn on the standby mains transformer for the relay coil supply. Mains power will be provided by a small relay operated from Pins B and D in the umbilical cable (which will now carry 24 volts for the relay coil). I'll add a blue LED to indicate standby mode and a green LED to indicate the main receiver is switched on. A feature shown in the drawing below is to bar the PSU test facility if control has been passed to the CJD receiver ie. testing of the external circuitry can only be done if the standby switch is turned off.

 

 

 

 A couple of views above showing construction awaiting wiring up. The two transformers are pretty heavy and conservatively rated for the job but a junk box hasn't got ideal bits and pieces hence its name. The taller transformer seems to have three 24 volt windings plus a winding for 142 volts, the other just a standard mains radio transformer. I'll need to test the 24 volt windings to measure their power output so that the 55 volt requirement of over 1A can be achieved without overheating of windings.

 

 Above is the circuit diagram for the mains wiring of the new power supply which removes mains from the umbilical cable and allows testing, and below the progress so far (most of the AC wiring). The standby transformer has a primary winding of about 2Kohm and produces 18VAC off load and is ideal for a relay with a 24VDC coil. The small LED works from the standby transformer secondary using a 5.6Kohm resistor to set the current at about 3 to 4mA. The 6.3VAC (yellow) wires will be anchored later when I'm wiring the 34 termination tags for the umbilical cable. I fitted the PSU control relay between the two large transformers..

 

 

 

This is the terminal board (whose position is shown in the picture above) to which the umbilical cable to the main receiver will be wired. One option is to terminate this in a chassis mounted socket but it will be more flexible to use a mating cable of about twelve inches that will allow the power supply to be mounted away from the receiver in case work on the latter needs to be done whilst powered up. I found I'd missed out "V" so used the pin next to "B".

 As work on the PSU progressed I discovered one of the windings on the LT transformer was linked to the mains primary winding so was unsuitable for providing the 150V supply. I also found the HT was a bit high at 325V. Initially I added a bleed resistor and this together with a small LF choke dropped the HT slightly. I then added a 1Kohm resistor in series with the LF choke bring the HT down to 300V, still a bit high. I then decided to solve both this and the missing 150V by adding an OA2 stabiliser fed with a 7.5Kohm feed resistor. This worked and brought the HT down to 250V. Once the receiver draws its HT current this should drop further to the correct voltage. The downside of all this was having to fit a B7G valve holder. I'd also fitted the HT choke and smoothing condenser on the upper surface of the chassis. I need to fit a pair of coax leads terminated with BNC plugs for the IF and BFO inputs.

 Scant information and lack of familiarity is a problem with this CJD receiver, as it is with many projects concerned with working on an old piece of electronic equipment. The CJD receiver designers had limited semiconductor devices at their disposal so developing a modern substitute for the missing parts is interesting. I'm looking at the power supply for the oven and reminded of an exercise many years ago concerned with regulating something like 80 volts for a teleprinter circuit. In those far off days the designer had seen that a huge stud-mounted 80 volt zener was available and had used this to stabilise the supply. Alas, although the voltage was nicely stabilised when the teleprinter was running and drawing power, the zener diode was expected to handle much the same amount of power when the teleprinter wasn't running. The engineer hadn't read the small print in the zener diode spec about "safe operating area" so things went from bad to worse once a batch of equipments had been delivered. Of course the blame was placed on a "bad batch" of zener diodes and all ended well but it was silly for a programmer to be appointed to run the project! Make a programming mistake and it's dead easy to just issue a patch but a hardware design error impacts on printed circuit layout and component changes.

Looking at the slightly mystifying -55V section of the PSU (above) it would seem that the designer (or most likely the designer's boss?) had noticed a heating problem and added a relay (RL2A) to circumvent this. It's worth mentioning that even today, it's not easy to find a suitable linear stabiliser to develop minus 55 volts, so is it possible to simplify matters? The circuit diagram refers to a series stabiliser which I first assumed was a large PNP transistor, but I spotted a drawing showing a "TR1" mounted on a heatsink with its collector grounded.. meaning it's probably an NPN device. This would need to handle around 1.5A and with a bridge rectifier output of say 85 volts the transistor dissipation would be quite high.. not as high as it first seems (45W) because circuit resistances come into play and at full current draw the dissipation could be as low as say 20W... but with a germanium transistor this is still pretty high.

The problem here is the designer needed to use worst case parameters such as the maximum transformer output and stabiliser circuit parameters once the full current demand had dropped hence the addition of the relay which is apparently used to reduce the stabiliser dissipation once the full current has decreased. Nowadays the choice of stabiliser is only slightly easier. A silicon power transistor (2N3055) or even an IGBT with a suitable heatsink could be used, but before I can complete the design I need to see how my particular transformer/bridge rectifier works with the oven. Below are the results of testing the 55 volt PSU. Clearly lots of power is available from the bridge rectifier as we need only about 1.13A at 55V.. Using the figures given below the source resistance of the transformer and rectifier is around 14 or 15 ohms.

 

 LOAD

 OPEN

 21 OHMS

 27 OHMS

 47 OHMS

 VOLTAGE

 88 VOLTS

 51.4 VOLTS

 58 VOLTS

 64 VOLTS

 CURRENT

 0A

 2.45A

 2.13A

 1.36A

 POWER

 0W

 126W

 124W

 87W

 

 This might be the circuit of the original oven power supply?

In order to reduce the zener dissipation once the oven has reached its correct temperature the extra 1.2Kohm resistor is switched into circuit. Once the oven turns on current through TR2 rises and TR2 demands more base current. The voltage from the bridge rectifier supplying Vin drops, transistor TR1 then turns off, relay RLA deactivates and switch RLA1 shorts out the 1.2Kohm resistor.

With a junk box transformer the voltages will be different to those from the original transformer and component values will be different.

A bench test will allow me to work out new values.

Transistors will be selected for ease of mechanical mounting so TR2 can be something like a 2N3055 or BUT11 with TR1 2N5415 or BD244C.

Another drawing I spotted shows two multi-turn pots meaning worst-case calcs were not essential.

 

 Here are some rough and ready calculations if the general form of the above circuit is realised (max oven current = 1.3A) Ref. pictures below whose voltages are used as a general guide. After testing I'll revise the relay switching zener value to suit the bridge off-load voltage. The initial figure of 88 volts compared with voltages on-load reflects the internal resistance of the transformer secondaries plus that of the bridge rectifier together with the performance of the reservoir capacitor.

At full load the transistor will dissipate something like 1.3A x (68V-55V) = 17W.

The combined zener and base current will be (68V-55V)/330 = 39mA. Assume the zener draws 5mA leaving 34mA for TR2 base current. Zener dissipation being 55 x 5mA = 0.3W

With a gain of say 38, TR2 emitter will manage 38 x 34mA = 1.3A. When the oven switches off the PSU voltages rises to its off-load level of 88V and TR1 turns on bringing the 1.5K resistor into circuit reducing the current to (88-55)/1.83K= 18mA. The zener will sink most of this.. say 16mA and dissipate about 1W.

If the 1.5K resistor were not to be brought into the circuit the total current would be (88V-55V)/330 = 100mA causing the 55V zener to dissipate 5.5W.

 

 

 

 All these figures are close enough to try the layout above and make some measurements. Note the relay is slugged by the 47uF capacitor which will give a measure of hysteresis to its operation. In practice I'd use a combination of zener diodes in series to improve their power handling. One unknown is the gain of TR2 which is possibly the reason the designers used a couple of potentiometers in their implementation. I selected a 2N3055 and a 2N5415 for TR2 and TR1 respectively and I'll using a 680uF reservoir and a 33uF smoothing capacitor.

Below you can see a little more progress as the oven supply is being fitted top right on the second picture. It seems there's less and less space becoming available for the electronics section!

 

 

 

Oven supply test results 

LOAD

 OPEN

 OPEN**

 150 OHMS

 100 OHMS

 50 OHMS

 INPUT VOLTS

 88

 83

 68

 68

 68

 INPUT CURRENT

 15mA

 30mA

 420mA

 640mA

 1.14A

 INPUT POWER

 1.3W

 2.5W

 28.6W

 43.5W

 77.5W

 OUTPUT VOLTS

 58

 59

 57

 57

 57

 OUTPUT CURRENT

 0mA

 0mA

 380mA

 570mA

 1.14A

OUTPUT POWER 

 0W

0W 

21.7W 

32.5W 

 65W

 With the circuit wired up I connected a variable DC power supply across the bridge rectifier and cranked it up until RLA switched off. This happened at about 87 volts. I found that without its capacitor the relay tended to oscillate at the switching point. Using different load resistors I found the results listed above. I used a 24V zener instead of the original design's 27V and a 24V + 33V for the 55V zener hence the elevated output voltage. I'll substitute a 39V + 15V for these zeners later to reduce the power dissipations. I used a 48V relay with a 4.8K coil in series with 1.2K. Once the testing is moved to the chassis supply I'll check the off load voltage to see if the 24V zener needs changing. With the 57V output the relay switching point is a couple of volts adrift also.

** I guess heating of the various components resulted in the changes particularly the open circuit voltage. This effect should be reduced once the 43 volt supply is wired up.

Besides the oven supply the minus 55V rail also supplies the minus 43V for the AGC and RF level indication circuitry. This latter area is due for redesign once I start the analysis using the LM373H so its requirement of up to about 280mA is presently fluid. The 43 volt supply test figure shows a 150 ohm load across the 43 to 55 volt supplies rather than 43V to ground which is a bit odd. Given a 43V zener and a 360 ohm feed resistor to the 55V supply the zener off-load current is 12/360= 33mA so the current drain might allow for 5mA zener current leaving only 28mA max load so I'll use a 330 ohm feed resistor to the zener and test using a load of say 1.2Kohm.

 

 Testing the 55 volt power supply with a bench DC supply which worked but then with the PSU transformer proved the difference in voltages between open circuit and loading to 1A was insufficent to guarantee the relay switching (because my circuit uses a lower AC voltage with lower transformer resistances) so I decided to try the obvious solution which surely the original designers should have adopted rather than the relay circuit with its adjustments namely using a Darlington arrangement. I fitted a small NPN 2SC2236 transistor into the 2N3055 base circuit. A quick test showed it worked fine with the output dropping from 54.3 to 54.2 volts with a 50 ohm load drawing about 1.06A. The zener voltage remained constant using a 1.5Kohm feed resistor and inserting a 330 ohm resistor into the driver emitter so I could check its current, gave 4.5V representing 13.6mA zener current with the 50 ohm load. This represents a gain for the 2N3055 of about 1046/14=77. The off-load drain through the zener(s) is 10mA so its dissipation is only 550mW.
  

 The next stage was tidying up the new circuit and testing it further with the correct transformer/rectifier. Off load the rectifier produced 75V dropping to 64V with a 50 ohm load at the -55V output. The output voltage was 54.4V off load and 53.9V on load. The 2N3055 was dissipating about 10W with the 50 ohm load.

I used a bridge rectifier for the 24V output and with 2200uF the winding produced 27VDC. The normal load for supplies derived from this amounts to 550mA. If I understand correctly the only stabilised output is 20V at 150mA. I'm unsure of the various 24V-based rails as they appear to carry different voltages in the connector cable/test set compared with the power supply markings. As the raw DC output isn't too far from 24V I guess some feeds can be taken to this rather than the stabilised level, but anyway I fitted a 24V stabilised output and I fitted a series resistor of 27ohm as per the original circuit for the 22V output.

To follow the original design I need to figure out how to easily provide the Receiver Ready signal which is used to light the front panel lamp. This is done in the 55V area where a rise in the off-load voltage is detected once the oven has warmed up and turned off. The voltage across the series regulator transistor could be used as this goes from 20V off load to 9V on full load. Something like a small relay with a 5V coil fed via a 10V zener diode might work, however when I looked at detailed relay specs I realised that once triggered a typical relay would not unreliably unlatch whilst the coil voltage was at all positive hence the reason for the designers use of a PNP transistor and zener diode. It seems the primary use of their relay was to provide signal to the Ready Lamp and the power saving was just a bonus. In fact reducing the main zener current aided the voltage increase and hence made the relay more reliable. My problem is the overall resistance of the 55 volt supply being much lower than theirs reduces the voltage swing between full load and no load making a more sensitive circuit necessary. Why couldn't the designers use the oven control circuitry to light the Rx Ready Lamp? I guess the additional zener resistance was important to them after all because of their simple voltage stabiliser drawing too much power in standby.

After experimenting I discovered a typical relay indeed failed to turn off as the 2N3055 VCE dropped from 20V to 10V as sufficient holding current was always present. This meant that from say 24V to 5V the relay stayed on, but adding a small zener diode in series with the coil resulted in only a very small change being necessary for the relay to drop out. For the moment I'm using a SPCO relay with a 24V coil at 2.4Kohm and a 6.8V zener diode. The relay kicks in at 16V and drops out as soon as a 50 ohm load is applied to the minus 55V output. I'm using a red LED on the front of the chassis to indicate the oven is on with the NC contact used as shown above. A ground at the Rx Ready wire should light the red lamp on the receiver front panel. Once everything is connected it might be necessary to change the 6.8V zener to say 4.7V because residual 55 and 43 volt standby current may reduce the measured off-load voltage from 75V to a bit less thus failing to turn the relay on.

 

 Above is a view of the new 34-way umbilical cable to be used between the power supply and the main receiver chassis. I had enough different coloured stranded wires to make the job easier than using a single colour used in typical equipments of this vintage. The key problem with constructing this equipment is preventing accidental damage from wiring errors because the wiring carries a mixture of valve power supplies including HT, low voltage rails for transistors, general control wires and even mains wiring (although I've in fact designed out the need for mains wiring for safety reasons). Apart from stranded single wires two signal wires for use in the volume control area are carried by miniature screened cable. Wires handling HT have better insulation and those carrying a degree of current are heavier. At this stage as you can see the soldered connections are awaiting insulation.

Below is a picture showing current progress (usually termed a "rats nest" method). Note centre-top a bleed resistor for discharging the HT which I'd inadvertently found twice to be present several days after bench testing. The cable above will be connected to the tagboard. As yet, none of the addtional circuitry needed to handle demodulation etc is present. My general idea is to power the main receiver and monitor the IF output if this is possible before proceeding with the LM373 design. That loose relay will soon be soldered in place with a red LED (top left) to indicate oven current.

 

For my own benefit I'm recording the wiring detail below because my computer screen is next to the bench where I'm doing the wiring.

 

 

 PIN

 FUNCTION

 WIRE

B

 Mains on/off

WHITE

F

 6.3VAC

BROWN

L

 AGC long/short

PURPLE/RED

R

 minus 55V

GREYISH BROWN

V

 BFO on/off

WHITE/BLACK

Z

Screen (U)

SCREEN+BLUE

DD

 Ground

BLUE

JJ

 Ground

BLUE

NN

Spare

BLACK

 

 PIN

FUNCTION

WIRE

D

 Fuse F1

WHITE

J

 Rx Ready

ORANGE/BLACK

N

 600 ohm o/p

YELLOW/BLACK

T

 Audio out (Y)

SCREENED BLUE

X

 +235V

RED/BROWN

BB

 minus 43V

YELLOW

FF

 +150V

DARK PINK

LL

 Ground

BLUE

 -

 -

 -

 

 PIN

FUNCTION

WIRE

A

 Mains on/off

PURPLE

E

 6.3VAC

BROWN

K

 +24V rough

GREY

P

 600 ohm o/p

YELLOW/RED

U

 Audio in (Z)

SCREENED RED

Y

Screen (T)

SCREEN+RED

CC

 +24V smooth

WHITE/RED1

HH

 Ground

BLUE

MM

 AGC1

ORANGE/RED

 

 PIN

FUNCTION

WIRE

C

 Fuse F2

PURPLE

H

 +24V var

PINK

M

 AGC on/off

GREEN/WHITE

S

 +20V

ORANGE

W

 +20V

ORANGE

AA

 RF level

GREY/BLACK

EE

 AGC3

WHITE/RED2

KK

 AGC2

GREEN/RED

 -

 -

 -

 

 The umbilical cable is wired in place and connections tested. Plugging it into the receiver and turning on the power resulted in a fairly loud 100Hz hum from the loudspeaker because none of the RF/AF circuit is in place. The HT was OK at the rectifiers and measured around 240Vat the output pin which slowly dropped to 190V (presumably as the valves warmed up and suggests either too much resistance in the HT supply or too much HT drain in the receiver. Maybe the floating AGC connections are the reason as these should be somewhat negative? The 150V supply also dropped to around 135V following the HT drop. LT voltages seemed about right. There's a few things to test. I could remove the speaker hum by grounding the audio pin and see if I can get any indications on the front-panel meter.

Alas the meter which should have registered the HT for example refused to budge and after removing it I discovered an open circuit coil. I did notice the voltage across the rear connections rose from under a volt to 13V in one particular switch setting. Is that the problem or is it just a false reading on my high impedance multi-meter? I need to discover the reason for the meter failure before I fit an alternative.

 

 Lots of the 34-way connector pins were bent and the worst snapped off but this turned out to be unwired and marked as "spare".

Once I'm happy with the wiring I intend to tidy it up somewhat.
 
 

 Eventually, if all goes well, the chassis will be mounted on a panel to be located above the main receiver chassis much like the original equipment layout.

 pending

 Return to Reception