The Bendix transmitter TA 12.
This transmitter is originally an American made aircraft transmitter, but also used in the British Royal Airforce (RAF). Aircrafts like Mosquitos.
The TA 12 has four frequency bands, one for longwave and 3 for shortwave. The funny thing is that each frequency band, has it’s own VFO, and only one poweramplifier for all of them, but again 4 output tuning filters with variometer inductance, to match it to the antenna. It delivers about 40 – 50 Watts on phone and CW. Modulation art, A-G2 modulation. In the mid the earial current meter.
The four knobs at the left are the channel preselector tunings, for each wave band one. Left of the knobs are the channel numbers you preselected. In the mid under you can see the knob for preselecting one of the four wave bands.
At the right are the four knobs for tuning, by variometer inductance, the output filters for matching the transmitter to the antenna.
In the beginning, by lack of the original powersupply/modulator unit, type MT 28 BA , I used a separate modulator, homemade, with 2 valves 807 in the final. With Ronette crystal microphone. It was doing very well. Also a homemade mains powersupply is used.
But some time ago, I managed to buy one on EBAY, a MT28BA. See the pictures below.
Sideview with power connector.
Inside view with at the left the Audio PA amplifier valves, in the mid, the modulation transformer.
View components, at the right, the several fuses and the switch for switching from carbon mic to dynamic mic.
The unit had to be cleaned a bit and inspected. But nothing was wrong with it. The unit came also with home made cables and control box for working with the transmitter on AM, CW and MCW. For AM I used a T17 carbon microphone. Modulation deph 100 %. The noise level of the rotary transformer is not to heavy. It starts only at CW (contineously) and at AM (during transmitting).
Behind a small panel, you can chose by a switch, between a carbon mic or a dynamic mic. Note that at a type MT 28 B, this provision is not available.
Inside the transmitter.
The upper view of the transmitter.
The box at the right is the VFO compartment. At the left upper, one of the four variometer inductances, four each band one.
View at the HF power amplifier with relay for switching the antenna and HT to the amplifier. The 2 tubes are in parallel.
The under view of the transmitter. The motor for automatic channel switching at the left is missing. At the right, the VFO box again.
Side view with the four tubes of the seperate VFO parts. Each frequency band has it’s own VFO part, so also it’s own tube. Left up the power input connector.
The modulator unit is doing very well. In mode AM, I can make almost 100 % modulation depth. On CW, the tone is very stable. A pleasant way of working with it.