Marconi Model 730 TRF Receiver

 

  This TRF receiver, designed in the early 1930s was used in ships until superseded by more modern superhets.

Like a handful of manufacturers, Marconi decided to use plug-in coils rather than a complex switching arrangement. A few Eddystone and National HRO receivers had similar arrangements.

It uses four old battery-powered 7-Pin British valves, type W21 pentodes popular during the mid to late 30s before widespread use of octal types. One is missing (my guess is that this is for monitoring the old 16KHz channel from Rugby, now defunct). There's also a 6-pin socket that's missing something. Looking at the circuit at the bottom of this page, this was a coil for a preset circuit tuned to the old international distress frequency of 500Kc/s or 600 metres and selected by the mode switch on the front panel when in the "STAND BI" position.

Arrayed along the top and under the hinged top section is its set of plug-in coils. There are 10-wavebands, Range 8 coils are in place on the chassis, but maybe the Range 4 set was surplus to requirements or just got mislaid? I constructed the table below on the assumption that Range 0 is the highest and Range 10 the lowest frequency range and "M" is metres. Once I've tested the receiver this will become clear. If my assumption is correct then Range 8 will cover Radio 4 on 198KHz. I suppose I could weigh a Range 0 and 10 coil when the heavier will be for the longer wavelength? Indeed, Range 0 was 90gm and Range 10 was 138gm. I used the wavelengths given in a technical article which listed the 10 wavebands (below) but inspection of the coil labels (0 to 10) surely shows eleven wavebands. In that case could the missing Range 4 coil be the one that plugs directly into the chassis covering a specific wavelength in the 600 metre waveband? The number doesn't actually make sense so some investigation is needed. I count ten pairs of holes for the coils, so 10 wavebands seems right, however I found on the Net, pictures of a couple of other 730 examples, one of which had a pair of Range 4 coils, all the spares in place, and a pair in use (making a total of 12 wavebands including the fixed frequency coil. I'm sure that once the receiver is powered up and the various coils tested, all will become clear...

 RANGE

 FROM

 TO

 RANGE

FROM

TO

 0

 15M

35M

 6

275M 

500M

 1

 30M

55M

 7

 460M

1100M

 2

 50M

100M

 8

 1100M

2300M

 3

 85M

180M

 9

 2,300M

8,000M

 5

 170M

330M

 10

 8,000M

20,000M
 
 

 

 Here are the three valves fitted in the receiver. The centre valve is an Austrian S217 (2v x 0.18A, Va 150v @ 3mA, Vg2 150v @ 1mA, gm 1.7) similar to the variable mu VP2. Interestingly, the W21 and similar types were introduced with a choice of B4 or B7 base which must have resulted in more than a few puzzled customers. Both the VP2 and S217 draw 180mA filament current compared with the W21 100mA.

 

 

 

 

 The set is built into a wooden case fitted to rubber shock mounts. As can be seen below the top section, holding the spare plug-in coils, hinges upwards for access to extra coils and maintenance.

 
 

 I'll take a better photo soon. This shows an early crystal detector, above the Reaction control because, in an emergency situation, the receiver could be pressed into use as a crystal set. Note the rather stout cat's whisker!

Another interesting facility is a 16KHz fixed frequency setting originally tuned to Rugby Radio Station. This station was built in 1925 for the transmission of timing signals for keeping clocks in synchronism. Alas the station was finally dismantled in August 2007, over 80 years after its first transmissions on 1st January 1926. Transmissions which ceased on 1st April 2003 were replaced by signals from Criggion, Powys which for many years had acted as standby station, however Criggion, in turn, was dismantled very soon afterwards and transmissions taken over by Anthorn in Cumbria.

These low frequency transmitting stations were also used, on slightly different frequencies, to communicate with submarines as the extremely low frequency (ELF) signals could be received under water. If you look carefully, elsewhere on this website I've mentioned this in connection with my work with Plessey during the "cold war".

Below is the circuit diagram from the manufacturer's operating manual.

Power is provided from external batteries. 2 volts for valve filaments, -10 volts for grid bias and 110 volts HT.

A puzzle must be the hole in the top left of the front panel (seen in the picture above). This might be explained by the neon lamp shown in the circuit diagram. In keeping with other receivers used by the Royal Navy, or in a marine environment ( for example the DST100 and the R1116), some form of limiter was placed across the RF amplifier. This was done to prevent too great a signal from a local radio or radar transmitter from damaging the valve.

 

 Click to see an excerpt, describing the 730, from a technical instruction handbook

Read about the problem of interference on marine receivers

 The Marconi 730 was also used for listening to enemy transmissions until being superseded by the more advanced DST100 borrowed from the Army.

 

 Below, I've marked component identifications to aid testing.

 

 Resistor

 Ohms

 Resistor

 Ohms

 Condenser

 Value

 Condenser

 Value

 Component

Description 

 R1

 500K

 R13

25K 

 C1
 

 C11

 100pF

 CR1

 Crystal rectifier

 R2

 5K

 R14

 750

 C2

 0.01uF

 C12

 0.1uF

 J1

 Heaphone socket

 R3

 10K

 R15

 2M

 C3

 0.1uF

 C13

 0.1uF

 TC1-TC4

600 Metre Rx trimmers

 R4

R16

 1K

 C4

 C14

 500pF

VC1 

 HF stage tuning

 R5

 20K

 R17

 -

 C5

 C15

0.01uF 

VC2 

Main tuner 

 R6

 20K

 R18

 -

 C6

 C16

 200pF

VC3

Reaction control 

 R7

 30K

 R19

 -

 C7

 C17

 100pF

VC4 

 16KHz Rx tuning

 R8

 500K

 R20

 -

 C8

 50pF

 C18

 S1

Mode switch 

 R9

 500K

 VR1

 25K

 C9

 0.1uF

 C19

 S2

16KHz Rx on/off 

 R10

 10K

 VR2

 500K

 C10

 0.1uF

 C20

 S3

On/off switch 

 R11

 50K

 C11

 100pF

 C21

 100pF

 T1

Interstage transformer 1:4

 R12

 2K

 C12

 0.1uF

 C22

 0.1uF

 T2

Output transformer 12:1 
 

 I worked out the power supply wiring. There's a large connector block from which emerges a set of wires. Green which is chassis, white, LT+, orange HT+ and black, HT-. Like many receivers from the 1930s and 1940s, without a separate grid bias battery, the HT return is taken to the chassis via resistors, the current through which automatically determines a more or less fixed bias voltage. In the circuit above VR1, the RF gain control, allows a varying grid bias to the control grid of V1. A second connection from HT- to R14 and R12 plus R13 sets the grid bias for V3, the audio amplifier.

I applied power from my home-brew PSU and set the HT to around 120 volts. Using the coils marked "8" and a short aerial revealed two stations, Irish Radio 1 near the high end of the band and Radio 4 around the middle. I added a long wire and continued testing. There was some interaction between controls but this is not unexpected as this is a TRF receiver. Everything appears to work as it should except the audio output varied dramatically depending on the control settings and it was easy to get deafened as tuning and reaction were set to optimum. Many old receivers prevent this by fitting a limiter in the headphone circuit.

Because it's been decades since the set has been used I needed to use switch cleaner on the mode switch but once cleaned the set was relatively stable. Overall though I found it to be not too pleasant an experience especially when compared with a suphet receiver employing AVC.

I decided to check the circuit, in particular to see if any components needed changing. During this exercise I found an anomaly. RF gain control, in common with other battery receivers (such as the R1116) is achieved by placing a suitable negative grid bias on the control grid of the RF amplifier (note...in a receiver using valves having heaters and separate cathodes the much simpler technique of using a variable resistor in the cathode(s) is used). Anyway, I found the wiper and live end of VR1 were transposed. This has the effect of progressively shunting HT- to chassis as the gain control setting is increased and simultaneously reducing the bias to the audio amplifier. The overall result spells trouble for the operator (bearing in mind the interaction of the various controls which already makes it easy to get deafened when attempting to tune in a station) by accentuating any interaction of controls. Because the set uses separate variable condensers for tuning and RF amplification, headphone output can vary by a huge amount and for example, having tuned a weak signal in conjuction with the reaction control, retuning to an adjacent more powerful station results in excessive audio output.

 

 

 If my memory serves me correctly the wiper is connected to the centre pin. The right hand pin is minimum resistance clockwise and left minimum resistance anti-clockwise.

Top right is ground and that means the 5Kohm resistor circuit is reversed with that on the centre pin.

 

 

 

Below, a picture of the tagstrip located centrally on the back of the chassis. Note a modern electrolytic capacitor added by a previous owner.

 
 

 Below, this terminal board carries the power supply leads. Above this a tagstrip carrying R1 (500Kohm), R14 (750 ohm), C19 and C20. The blue component appears to be a modern capacitor added by the previous owner.

 

 Below, T2 (the output transformer which looks brand new) and T1 (the interstage transformer). The rear of the headphone jack socket is top left and R16 top right.

The brown condensers are, left C15 (0.01uF mica), lower right C17 (100pF), lower centre C14 (500pF). The tubular metal condenser is C13 (0.1uF) and all these are Type 4703 and without a maker's name.

 

 Still no firm dating evidence... 1936 to 1945 ??

 pending...

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