The Marconi M361
The Marconi Type 394A Communications Receiver used on Fishing Trawlers in 1938.
It was used in conjunction with Marconi Transmitters, Type 517 and 527.
|I have included this web page by request. I have been sent a number
of E-mails from people world-wide, having visited
my website and noting that I used a Marconi M361 in my schooldays, which was over forty-five years ago.
They requested further details of this historic equipment. It appears that although the equipment is very outdated, people
still want the thrill and excitement of getting these receivers to work. I don't know how they get hold of these remarkable receivers, but I regret loaning my own M361 receiver to a friend 40 years ago, never to see him or the receiver again.
I would like to make it clear that I don't have the Service Manual or the Handbook. The Circuit Diagram of my M361
was located FIXED to the underside of the hinged lid. The Photograph above and also the Diagram shown below is that
of a Marconi Type 394A Receiver, and I am sure that they are very similar to the M361.
|It is important to check the Valve types. This can be done using a
Valve Data Book. (Another item of history).
My M361 Receiver was supplied with 2-volt valves; these were directly heated ( 2-volt valves were fitted as standard).
Some receivers were fitted with 4-volt or 6-volt valves.
|Both H.T.3 and H.T.2 terminals should be joined together and to the
positive terminal of a 120 volt H.T. battery.
Terminal H.T.1 should be connected to a 72 volt H.T.battery. (I used a 67·5 volt H.T.Battery available at that time).
The grid bias battery, which is connected externally, supplies the negative bias required by the output valve. The positive
terminal of the grid bias battery must be connected to the L.T. negative. For an anode voltage of 120 volts on the output
Valve, then the grid bias voltage required is approximately 3 volts.
It is very important to check the valve types and also the heater voltage. If directly heated valves are used they must be supplied with a SMOOTH Direct Current supply, (I used a 2-volt accumulator). It is very difficult to provide this from a Mains Power Supply. There have been many circuits in Practical Wireless Magazine for pure d.c. rectification from the
mains supply, but there is always a risk of 'a.c.ripple' on the output audio. Later receivers Marconi 394C/12 and 394C/30
used indirectly heated valves which don't give rise to the same problems.
It is very similar to that of the Marconi Series 394A.
The receiver consists of two stages of high frequency amplification, a detector on which
reaction is available and one output stage. The output stage may be either of the pentode type or the power valve type, according to the duties required of the receiver.
The aerial coupling is arranged to suit the various wave-ranges of the receiver, and gives improved results over the basic 394 receiver.
The three tuned stages are all carefully ganged, and a single dial only is required for fine tuning within any given range. There are four wavebands covered on most versions of the
receiver, but the frequency ranges can vary on different models.
On the Marconi Model 394A the ranges are:
3000 - 1100KHz (100 - 270 metres) 1250 - 260KHz (240 - 650 metres) 546 - 200KHz (550 - 1500 metres) 215 - 150KHz (1400 - 2000 metres) It is important to mention however, that the fourth wave band range is obtained, not by the use of a fourth tuning coil, but by adding a parallel capacitor of 0·0004 microfarad capacity
in parallel with the normal tuning capacitor of approximately 0·0005 microfarad maximum capacity. This parallel capacity must be added on all three tuned circuits, and consequently
it was necessary to have the three 0·0004 microfarad capacitors accurate to within ± 1 per cent of one another to ensure correct ganging.
Reaction is obtained by feed-back through the 0·0003 microfarad variable capacitor
to a coil magnetically coupled to the tuning coil in use in the anode of the second screened grid valve. (This gives good amplification).
Volume control is obtained by means of the grid leak bias potentiometer, which varies
the negative voltage on the grids of the variable-mu screened grid valves.
A local-distant switch on the left-hand side of the receiver reduces the input from strong
signals if required.
A small lamp has been included in the front of the panel, which illuminates the dial when
the receiver is switched on.
Photograph of a Marconi M361 Communications Receiver This receiver operated on the following wavebands. 100 - 235 KHz 215 - 510KHz 460 - 1080KHz 1000 - 2250KHz 2000 - 4500KHz 3500 - 8500KHz