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.
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	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

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 plus 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

I394G 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 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)

Model 394F schematic. Download for full size image. ( Schematic courtesy Geoff Wooster)

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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Dec 28/20