RCA ET4335E

As this is going to be a lengthy project, it will be worthwhile to dedicate a separate page to the restoration of the ET4335E. Now first of all, to make sure there are no misunderstandings, RCA never manufactured or sold an ET4335E. I have not found any official documentation that proves that this transmitter was manufactured by RCA. This particular transmitter seems to be a hybrid between an ET4335LF (low frequency transmitter) and the ET4336H (high frequency transmitter). Some details of the ET4335LF can be seen at Radio equipment. As the operators at the Luven station were quite industrious, they seem to have taken parts of an ET4335LF transmitter and parts of an ET4336H transmitter to build the ET4335E. Why? Well, I can only guess. From the notes I have, it would seem that they had difficulty in expanding the equipment at the station by normal means, i.e. they had to obtain the equipment without attracting undue attention. Switzerland did not qualify for the USA lend-lease system as they were neutral and not at war with Germany. So, the operators used every opportunity that presented itself to add equipment. Maybe some details will emerge at some point.

The RCA ET4335LF chassis can be seen in Figure 1. It needs a good clean, especially where the tarry substance had leaked out on the chassis floor. It is painted a light grey, I will adhere to the colour scheme when I repaint it. I plan to place the chassis on a wooden tray, equipped with four large castor wheels, unless I can find the appropriate single nut castor wheels; these can be bolted directly onto the rubber-bonded metal mounts affixed to the bottom of the frame.

Some historical facts about the transmitters involved in the restoration of this ET4335E:

ET4335LF transmitter: Designed in 1942, CW (A1) and AM (A3) modes. Oscillator 807, RF amplifier 2 x 813. Output is 250 watts on AM and 350 watts on CW. The transmitter covers the frequency range of 150 to 600 kHz. It is crystal controlled, with four crystals selectable by a front panel switch. The modulator uses two 805 tubes, these are driven by an external audio amplifier (MI-7181).

ET4332B transmitter: Precursor to the ET4336 range of transmitters. Frequency coverage is 2 - 2o MHz. Designed in 1941, CW (A1) and AM (A3) modes. Oscillator 807, RF amplifier 2 x 813. Output is 250 watts on AM and 350 watts on CW. Crystal controlled, provision for one crystal only. Under the USA lend-lease programme, the British Empire received 1603 ET4332 transmitters and 200 ET4332-A transmitters. The first version, the ET4332A, was designed and built in 1939.

ET4336 transmitter: Based on the ET4332B, but more compact. Designed in 1943. Several versions were built, the main difference being the frequency control section. The earliest version had a single crystal that plugged into the front panel, later versions had a metal plate that could be removed to fit a variable oscillator. The version with the variable frequency oscillator was known as the ET4336K. Large numbers were produced and used in WW2. As part of the USA lend-lease programme, the British Empire received 1415 ET4336 transmitters and 2258 ET4336-B transmitters. Figures 2 to 6 are different views of the RCA MI-19467 VFO of the ET4336 transmitter as used in the ET4336E. It is very well built.

The filament choke of the 807 tube was found to be open circuit. This filament choke also performs the task to drop the 10 V from the ET4335 power supply section to 6.3 V. The filament wire is made from fairly hard copper, cotton covered, and needs to be handled carefully. Softer, multi-strand copper wire was soldered to the filament wire, soft rubber sleeving was placed over the joints, and epoxied onto the corner section of the coil former.

Upon testing, the filament choke (L404) performed as designed. I applied 200 V DC as well, and the VFO came to life. A 7 hour test of the stability of the VFO indicated that it was stable to within 100 Hz from cold switch-on. At a later stage I will make a more detailed test. This initial test did show very clearly that the VFO is a high quality device and certainly stable enough to use on the HF bands in either CW or AM modes.

During May 2022 I spent some time on restoring the RF antenna tuning section. This section was in a sad state and required a number of missing high voltage capacitors, and I was doubtful that after 79 years or thereabouts, when this transmitter was originally constructed, that I would be able to find the original capacitors. However, I did, and I found these in the shed. I found a battered old wooden box stashed away underneath a table, and inside are a number of spare parts for the RCA transmitter, meters, capacitors, transformers, tubes etc.