Identify old AM transmitter

[tonytony]

Hi,
A long shot - but does anyone recognise this TX, its an old AM one I had many years ago, 200-500w I think, bottom rack was power supply, top unit was the TX, it wasnt a commercial spec unit, but it was very well built, the TX used KT88 output valves and a had a big collins modulation transformer, and there was an old round meter on the top the TX rack that was removed in the photo. There was an alluminium box underneath the chassis in the TX which was the VFO.
(Ignore the link receiver on top), I know it looks just like two 19" rack units - but maybe it will jog someones memory. I'd like to know the history of the unit. It was built by some ex BBC guy - apparently!

AM TX.jpg

Thanks

[Albert H]

I remember that rig really well! It was built by "Brig" - sadly, no longer with us. He was (like me) ex-BBC. It was built in his flat in Chalk Farm, and he borrowed my 'scope (his was knackered) to check the modulator.

The original oscillator in the box at the bottom was a crystal-controlled Clapp type, and there was a Vackar VFO as well, so that the frequency could be tweaked if necessary. At the time it was built, we were having a nightmare with crystals - as soon as you'd get one cut for (say) 1404kHz, some commercial station would start up on that frequency, requiring a move!

This rig was slightly unusual (for the time) in that it had "envelope feedback" - a diode probe looking at the output signal and passing a demodulated audio output back as negative feedback to the input of the modulator. This meant increasing the gain of the input stage of the modulator, but gave really hi-fi results on the air!

As I recall, it did about 250 Watts carrier and was capable of about 95% modulation, which meant almost 1kW peak! The supplies for the output and driver stages could be changed by selecting tappings on the power transformer, which would vary the output power. However, if the supply was raised much, the modulation depth went down. It was set up as a compromise.

I don't remember the mod transformer as being a "Collins" - I thought that it was a "Wotan", but I could be wrong: the memory's not what it once was!

[tonytony]

HI Albert,
Thanks - I was wondering of anyone would recognise it!
Is there anywhere else I can get some info ? Schematics or pics?

[Albert H]

Probably not. Unfortunately "Brig" died about eight years ago, and he usually just made up circuits as he went along!