Type 75


1929 but modified in 1930 and again much modified in 1933.


48400 - 75000 kc/s.


200V Mains Supply





MCW. The original set (75X) had trialled modulating the transmitter using one of two built in tones (by switched action) of either sonic tones of 1kc/s or 1.4kc/s, conventional AF tones for MCW purposes. By the time the 75C/75D were rolled-out, those modulating sonic tones had become supersonic tones of 21kc/s and 31kc/s.




The Type 75 is a combined transmitter and receiver designed for VHF. The VHF valves and circuits for both transmitter and receiver are housed in aluminium transmitter-receiver units fitted aloft, generally in the maintop and foretop, but in ships of the "NELSON" class, a third unit is fitted on the superimposed turret. These units are each fitted with two 4-foot rod aerials, one for transmission and one for reception. Usually, however, the receiver in the maintop position uses the lightning conductor as an aerial instead of a 4-foot rod. The transmitter-receiver units are each connected by two 3-core cables to the control unit which is fitted in the Auxiliary W/T Office.

The control unit contains switches for putting either transmitter or receiver into use, controls for sensitivity of the receivers and a main switch for connecting all power supplies. Two alternative lines (nets) of communication, either Divisional of Sub-Divisional Nets, are available on one wavelength, the change over being made by a switch in the control unit. Here, for wavelength read Frequency. What this means is that if the Frequency is, say, 60 Mc/s, then the Divisional Gunnery Net uses 60Mc/s with the 21 kc/s modulating frequency and the Sub-Divisional Gunnery Net uses 60 Mc/s with the 31 kc/s modulating frequency. Remote control is provided in the gunnery TS (transmitting station) and remote control W/T Office by switches in the control unit. Tuning of the transmitters and receivers must be performed aloft at the unit and when completed the unit is locked up, all operation being carried out at the control unit in the Office. Local signalling between the control unit and the transmitter-receiver units during tuning is carried out by means of small lamps fitted in the unit and worked off the ship's 20-volt supply.

Under Photographs, Schematics, Details etc picture number 1 shows the transmitter switched over to the 21 kc/s modulation frequency. The valves/tuned circuits on the right are the frequency source, valves 20 and 22 used for 21 kc/s and valves 21 or 23 used for 31 kc/s, the change over being made at the control unit sited in the Auxiliary W/T office. Valves 14 and 15 are the Modulator valves and valves 8 and 9 the RF stages for the VHF transmitter. The HT for the RF valves comes direct from the ship's 220 V ring main. For modulation, an alternative voltage at the appropriate supersonic frequency (21 or 31 kc/s) which comes from the control unit, is superimposed on the HT of valves 8 & 9. You will note that the outputs of valves 20/22 (for 21 kc/s) and 21/23 (for 31 kc/s) is via a "special double tune circuit". This is to eliminate harmonics of the modulation frequency in use from getting to the modulator circuit valves (14 and 15).


Tuning Arrangements
Common Aerial Working


Photographs and Schematics.


In the handbook for the Type 75 of 1934, a confidential document (CB1882) downgraded to an official document (OU6369) - but not to a book of reference (BR) - its 1929 stated range had been increased from 2 to 5 miles to 5 to 10 miles. The set is a combined transmitter and receiver installed into Battleships only, for their fire control, the 75C into CCS (central control system) ships and the 75D into CWS (central W/T system) ships. There was also a 75X which was an experimental set.

The elimination of harmonics from the transmitter is of paramount importance when alternative lines (nets) are required using supersonic frequencies in conjunction with quench receivers, which is what the 75 receiver is. This is easy to explain for our purposes. Basically, a quench receiver does the same job (ultimately) as does the now familiar heterodyning receiver (BFO) - it mixes RF (supersonic frequencies) to get, as one of the by-products, an AF tone representing the original Morse code key input at the transmitting end. Unlike a normal AM circuit on MCW where the transmitter is modulated by a sonic frequency (usually 1 kc/s) all the receiver has to do is to mix the carrier with a sideband to demodulate and produce the original 1 kc/s AF tone, here, the resultant tone would not be AF (sonic) but supersonic, and at 21 kc/s (or 31 kc/s) not audible. Any receivers job is to produce AF so in this case, we need an extra stage to convert from supersonic 21 kc/s (or 31 kc/s) down to sonic 1 kc/s. This is where the quench (or quenching) comes in, and it should come as no surprise to find that the quenching circuit (valve and tuned circuit) oscillates at either 20 kc/s or 30 kc/s given the position of the switch at the control panel. However, there is a major problem here which hampered the design processes in the early 1930's - the valve in the receiver, which also acted as the detector stage, radiated and gave off undesirable harmonics: (this problem was the reason why, some years later, and as we shall see, the receiver for the Type TCS was actually disconnected - not used - because the receiver radiated, and those radiations wandered up and onto the aerial (the send-receive relay set the aerial to the receiver when the TCS transmitter was not being used) giving the ships position away for D/F purposes, and involuntarily transmitter when Radio Silence was imposed). The radiation was also used by the BBC to find TV Licence dodgers at a much later date.

The problem of the receiver creating self radiated harmonics was not a real problem to demodulating the incoming radio signal always providing that the incoming signal itself did not have harmonics.

If the detector stage was set to the Divisional Net which was always the 21 kc/s circuit, the quench frequency (within the RX) would be a fundamental of 20 kc/s; 2nd harmonic 40 kc/s; 3rd 60 kc/s; 4th 80 kc/s etc. If set to the Sub-Divisional Net, always the 31 kc/s circuit, the fundamental would be 30 kc/s and then 60, 90, 120 kc/s etc, and as stated, these in themselves would not impede communications. However, if the 75 transmitter had not had an elaborated system to make sure that none of the harmonics of either the 21 kc/s or the 31 kc/s modulating supersonic tones were transmitted, it would have made selectivity at the receiver between the two nets very difficult and the two nets would have been lost.

To see the potential problem of not suppressing fully the harmonics of the 75 TX, let us take as an example only, a system where we are using the transmitter-receiver in the foretop set to 60 Mc/s on the Divisional Net (21 kc/s) and the transmitter-receiver in the maintop set to 60 Mc/s on the Sub-Divisional Net (31kc/s). Both the transmitter and the receiver are tuned to 60 Mc/s. When a transmission is made on the Divisional Net using 21 kc/s, the following frequencies leave the transmitter aerial:-

(a) 60.021 Mc/s (fundamental);
(b) 60.042 Mc/s (2nd harmonic);
(c) 60.063 Mc/s (3rd harmonic);
(d) 60.084 Mc/s 4th harmonic etc.

The receiver would produce internal frequencies of (e) 60.020 Mc/s (fundamental) and mix with (a) to produce an AF tone of 1kc/s; (f) 60.040 Mc/s (2nd harmonic) and mix with (b) to produce an AF tone of 2kc/s; (g) 60.060 Mc/s (3rd harmonic) and mix with (c) to produce an AF tone of 3kc/s; (h) 60.080 Mc/s (4th harmonic) and mix with (d) to produce an AF tone of 4kc/s etc, where the desired combination is of course (e) mixing with (a). For successful communications on the Divisional net.

The same set of AF tones would be produced using the Sub-Divisional Net on 31 kc/s using harmonics of that frequency for the TX and harmonics of 30 kc/s for the RX harmonics for example (a) 60.031 Mc/s and (e) 60.030 Mc/s etc.

In a receiver with an effective AF stage acting as a low pass filter to 6kc/s, the desired 1kc/s on the Divisional Net is heard by the operator using the foretop set (21-20) but the 11kc/s tone produced by the Sub-Divisional Net in the maintop (31-20) will not. However with transmitter harmonics, the resultant received pattern would come to the operator as a 1kc/s note but also as a 2,3,4,5,6 kc/s note this latter train as interference. Moreover, the 3rd harmonic (for example) of the foretop TX 60.063 Mc/s would mix with the 2nd harmonic of the maintop RX 60.060 M/cs producing an AF tone of 3kc/s being received alongside the 3kc/s tone produced as shown above. Thus both the Divisional Net and the Sub-Divisional Net would grossly interfere with each other. This is why the harmonics of the Type 75 had to be suppressed at al costs for successful communications for two nets on the same frequency.

It is believed that much experimentations went on before the successes of the Type 75 were achieved, and these experiments included the previously mentioned Type 72,73 and 74 of which little is known.

The TX and RX are fitted into a light aluminium box having all the necessary controls at the front. An aluminium screen divides the transmitter, on the left hand side of the box, from the receiver, on the right hand side. This box is slung on steel springs inside another strong watertight aluminium box, leather covered hair cushions being fitted between the two boxes to damp vibrations of the inner box. This springing system is necessitated by the fact that these units are fitted in positions exposed to gunblast and concussion due to shell-fire. There is another small box fitted in the outer box (below that containing the transmitter and receiver) which contains the local signalling lamp and key, a stowage position for the tuning handle, a key for working the transmitter from this position whilst tuning and a pair of terminals for connecting earphones during tuning, these terminals being normally shorted by a link. A hinged watertight lid fitted with a padlock covers the entire front of the outer case. Openings with screw covers are provided for observation of the aerial ammeter and for final adjustment of the transmitter and aerial tuning. A small section at the bottom of the watertight lid opens on independent hinges to provide access to the lower box without uncovering the upper portion, so that local communications with the Auxiliary W/T Office, listening-in and keying may take place whilst the final tuning is proceeding. A small heating element is fitted between the inner and outer boxes to prevent, as far as possible, the accumulation of moisture. The current for the heater is obtained from the 20V-supply and can be switched over from the heater to the signalling lamp in the control unit.

The management/organisation and tuning of this equipment was quite cumbersome. Under Further Reading see file Tuning Arrangements.

Under Photographs, Schematics, Details etc the second photograph shows the front cover of a Royal Air Force Publication AP116E-020-1 which was also Admiralty's BR1389. Note that the Naval Wireless Set Type 87Q (which is covered separately in the transmitter scuttle matrix) was in fact, this set, the Type 75C.

Under Further Reading look at file Common Aerial Working for details of the CAW with the 75C. The details in this file are taken from AP116E-0201-1/BR1389.