CMC Products - Vacuum Tube Era - 394 Receiver Family

394, 394A, 394F and 394G Receiver Family

This document traces the evolution of the 394 receiver family. Because the 394Freceiver version was made for the British Admiralty by Canadian Marconi, it has been cataloged in both of my Canadian and Brisish Marconi web pages.
Some people refer to the sets as having the letter "M" ahead of the 394 model number. Marconi only seems to use the word "Type" as the prefix and not M.

Type: Four valve regenerative receiver
Frequency Ranges: See table

GENERIC MODEL	A/P #	FREQUENCY RANGE	COMMENTS

394	None	100 meters to 2000 meters (150 kcs to 3,000 kcs) Four bands: 100- 200 m 220- 500 m 500 - 900 m 900 - 2,000m	This receiver type was originally supplied to British fishing trawlers and small craft. Built by Marconi International (UK), The 394 was normally supplied with 2 volt valves but it could be used with 4 or 6 volt valves. An output jack makes the receiver compatible with Marconi repeater equipment. 394 schematic (From Wireless Telegraphists).
394C/12	None	Same as 394	This version of the 394 uses indirectly heated valves with a 12 volt supply. The circuits between 394C/12 and /30 are slightly different. 394/C12 schematic (From Wireless Telegraphists).
394C/30	None	Same as 394	This version of the 394 uses indirectly heated valves with 30 volt supply. The circuits between 394C/12 and /30 are slightly different. 394/C30 schematic (From Wireless Telegraphists).
394A	None	Four bands: 3,000 - 1,100 kc 1,250 - 260 kc 546 - 200 kc 215 - 159 kc	Built by Marconi International (UK) for commercial use. This is an improved type 394 which has been redesignated to 394A.Marconi says this set has a better range and selectivity than the 394 394A schematic. (From Wireless Telegraphists). In this receiver, the coils are no longer selected individually. It uses one large coil with lots of taps. Various segments of the coil are used depending on the range selected. It employs two HF RF stages, one detector and one output stage,
394E	?	150 to 3000 KHz	Used with either the TW12 A or TW12 B transmitters, Requires +150 VDC for the valve anodes.Valve lineup: 2 x ARS8, 1 x NR39, 1 x NR42 and 1 x ND3 394E schematic.
394F	A/P M312	Five bands: 100 - 235 kc 215 - 510 kc 460 - 1,080 kc 1,000 - 2,250 kc 2,000 - 4,500 kc	This 394F variant was built to A/P M312 specs by the Marconi Wiress Company of London. It is not known as to why Canadian Marconi and Marconi International both built the 394F variant. for the British Admiralty. 394F schematic
394F	A/P 342	Five bands: 100 - 235 kc 215 - 510 kc 460 - 1,080 kc 1,000 - 2,250 kc 2,000 - 4,500 kc	This is the Canadian Marconi version of the 394 receiver called 394F. It entered service with the British Admiralty as A/P 342. Valves used were VS2 for the RF amps, Z21 or SP210 for the detector and type P2 for the output valve. The 394F is built of aluminum plate 1/8" thick, while the British version 394G used steel plate of 1/16" thickness. It is also puzzling as to why Canadian Marconi did not supply the 394F set to the British Admiralty with the extended band range up to 8500 kcs.
394G	A/P 361	Six bands 100 - 235 kc 215 - 510 kc 460 - 1,080 kc 1,000 - 2,250 kc 2,000 - 4,500 kc 3,500 - 8,500 kc	Built by Marconi International (UK) for the British Admiralty. . It entered service as A/P 361 (394G). Circa- August 18, 1940. 394G / AP361 schematic
394G	A/P 361		Built by Canadian Marconi for the British Admiralty using Canadian parts.

The is the 394A, the commercial version of the 394 built by Marconi Marine International (UK) The 394 receiver was made in Britain in the late 30's and designed mainly for fishing trawlers and small craft. The British Admiralty wanted the same radio but the coverage ended at just over 4 Mc/s. So Marconi added some extra coils that extended HF coverage to 8.5 Mc/s. The Admiralty version then became the AP361 ( 394G) It had a different dial and much finer and more precise tuning. (Courtesy Handbook of Technical Instruction)

394E receiver (Courtesy Commsmuseum.co.uk) .

The 394F receiver was built by Canadian Marconi for the British Admiralty . It was designated 394F and Admiralty Pattern 342. Some of the pieces that were missing on this example were "Photoshoped" to give the perception of a complete receiver, Otherwise, this receiver is being restored. . The receiver is finished in dark green, wrinkle finish paint . (Download image to enlarge. (Photo by Geoff Wooster)

394F nameplate. (Photo by Geoff Wooster)

394F Serial 105 has been seriously restored in December 2019 with just a bit more work left, (Photo by Geoff Wooster)

This Youtube video , (about 20 minutes long) is a fine film about a true rescue story at sea. One can see the 394 radio in action.

394F PHOTOS

NOTE: ALL PHOTOS IN THIS TABLE RELATE TO TYPE 394F, A/P M312

394F A/P M312 by Marconi Wireless , London. Front panel details.

394F A/P M312 by Marconi Wireless , London. Top view

394F A/P M312 by Marconi Wireless , London. Underside view

394F built by Marconi Wireless , London - nameplate.

All 394F photos in this table by Bruce Macmillan. Download images to enlarge.

394G PHOTOS.

This pristine example of an A/P361 (394G) is owned by Geoff Wooster G3YVF. The photo also provides detail on the tuning dial. The red plate says: SWITCH MUST BE OFF BEFORE CHARGING.

394G Interior view #1

394G Interior view #2

394G interior view #3 showing "square wiring" which consists of bare, tinned copper wire that interconnects the components .Square wiring is a technique where we bend the solid copper wire using only right angle bends and route the wire connections that way. It looks pretty but it has its drawbacks! A to B means point A is connected to point B using a straight wire. It does not look so good but prevents a lot of stray capacity so using it in an RF coil pack makes sense. The British version is all "square wiring" while the Canadian version A to B, uses tinned copper, sleeved. Square wiring is also called buss wiring by radio collectors.

All photos in this table by Geoff Wooster G3YVF. Download any image to enlarge.

394G built for the British admiralty by Canadian Marconi using Canadian parts

This 394G (A/P M361)was built for the British Admiralty by Canadian Marconi using Canadian parts.Click on image to enlarge. (Photo by Geoff Wooster )

394G chassis view 1

394G chassis view 2. There is a shielded cable just above the large black transformer. This cable ends in a plug which is inserted into a holder. The plug is stored there to prevent damage when allowing the front panel forwards to gain access during repairs. Look at the plug and you can see it firmly secured with white webbing to prevent it coming out during shipping. If you didn't do that it could easily break a valve or two!

When the set is in use, this plug is inserted into a socket on an assembly that is bolted to the left rear cabinet wall. This assembly is the antenna signal input attenuator and cannot be removed unless unbolted.

394G chassis view 3

394G chassis view 4

394G chassis view 5

394G nameplate.

394G calibtation chart

394F and 394G Operating notes

394G ACTUAL SCH EMATIC

(All photos in this table by Geoff Wooster)

394G BAND SELECTION INSTRUCTIONS
The waveband switching knob on the 394G has the following instructions imprinted right on the knob and needs to be followed when wanting to use the highest frequency range of the set,
8500 -3500KC RAISE LID INSERT PLUG INTO EACH "IN" SOCKET ON EACH COIL. FOR OTHER RANGES PLUGS MUST BE IN "OUT" SOCKETS.
At the end of each big brass coil container are connections to the wave switch. On the 394G Admiralty Pattern 361, a small extra coil is fitted across the end of the brass coil container with a pair of sockets marked "in" and "out" and a jumper plug patch lead (permanent fixture at the end of each coil) has to be inserted into the correct socket. Plugs "out" gives the normal tuning range and plugs "in" gives the 3.5 to 8.5 Mc/s range. The Marconi 394G is the set that has these plugs and is the one used as Adm.Patt.361
This process must be reversed in order to restore operation to the frequencies between 100 and 4500 kcs. Nothing can be mounted over the receiver since the lid is very big and requires at least 2 feet of clearance,

394G LINK SWITCH
Both the 394F and 394G (A/P342 and A/P361) receivers employ a link switch which is found under the chassis. Under the metal link is a label which says:
" LINK TO RIGHT FOR AUTO-BIAS " . This link has to be closed for auto-bias. With this link closed, R14 (see the Marconi 394G schematic) is connected from HT-ve to earth. The HT current now passes through this resistor thus developing a voltage drop across it. Since it s the -ve lead to chassis/earth it's used to provide a -ve bias voltage for the grid of the output valve. This voltage is also supplied to the volume control and here a varying amount of this bias is applied to the R.F. Amplifier valves thereby controlling R.F. Gain and hence the output volume. Depending on which output valve is used, the resistors R6 and R7 have to be changed and Marconi supplied different resistors with each set for this purpose.
With the Link open, this auto-bias is not available. Therefore a Grid Bias supply, generally a battery, must be used. This of course complicates matters... One can only speculate as to why Marconi did it this way.

This is how the link switch looks. Just slacken the nuts then move the link to the desired position. (Photo by Geoff Wooster)

394G COMPONENTS UPDATE
As provided by Geoff Wooster, t he following is a correction listing for the 394G receiver Ser.No.609 M361.
Values where possible were taken from looking at the available information (which was incomplete) and what was found by looking inside the set without dismantling it! Not always possible to read the information on the component body or can....some capacitor values are the “best informed guess”! C1 does not appear to exist, not on any drawing or in the set?
T1 3:1 transformer. T2 45:1 and 15:1 windings.
R1 250K, R2 1M, R3 100K, R4 1K, R5 220K, R6 20K, R7 10K, R8 1K,
R9 2K? R10 250K, R11 30K, R12 250K, R13 15K, R14 1K.
CR1/2/3/4/5 ARE DECOUPLING UNITS CONSISTING OF 1K AND 1UF, C2 0.IUF, C3/C4/C5 100P TRIMMERS?, C6 40p, C7 100P, C8/9/10 0.1UF,
C11 100P, C12 300PF VARIABLE, C13 2UF, C14 100P, C15 2UF, C16 100P, C17 0.1UF, C18/19/20 ganged, C21/22 25UF, C23 2UF, C24 0.1UF?
394G COMPONENT EXPLANATIONS
The images in the table below explain the purpose of the components in the 394G receiver.

394G view 1

394G view 2

394G view 3

394G view 4

394G view 5

Provided by Geoff Wooster

394G BIAS RESISTORS FOR OUTPUT STAGE
From page 686 of the book "For wireless telegraphists:
"The bias for the output valve is obtained from two resistors in series, these resistances having engraved on their holders the letters "A" and "B." When a Pentode valve is used as an output valve, resistance "A" should be 1,000 ohms and resistance "B" should be 3,000 ohms. If a P2 power triode valve is used as an output valve, resistance "A" should be 2,000 ohms and resistance "B" should be 1,000 ohms. Normally the receiver would be sent out with this resistance set for a power valve type P2 but a spare 3,000 ohms resistance will be included with the holding down screws for the receiver".

Contributors and Credits:
1) Geoff Wooster G3YVF <gw.woo@btinternet.com>
2) Handbook of Technical Instruction for Wireless Telegraphists. Dowsett and Walker. Second reprint. 1944. Seventh edition
3) Bruce macmillan <wirelessset(at)hotmail.com>
4) 394E photo: http://www.commsmuseum.co.uk/transmitters/132%20Type%20Marconi%20TW12/pdfs/marconitw12photos.pdf

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Feb 1/25