Collins 431-D /FRT-24 Speech Amp

[w5dud]

Hello, anyone have any experience with this amp, I have one and the audio is  not good, looks to be designed as a carbon mic amp, the lowpass  filters have been bypassed and I have changed all of the coupling caps with orange drops,still can only get 50% mod, all votages are within tolerance, and tubes are new, Thanks Dudley Parkin, W5DUD

[k7yoo]

Is this the external speech amp that I sent with the transmitter? I never used it. I always fed the audio from my little Behringer right into the microphone input of the modulator deck. That external amp was primarily for remote use. You are correct in that the 431 was actually designed for a carbon microphone. All I did was feed the 600 ohm line to the input. I can't remember for sure but I think I took the carbon mike bias off of the mike jack. It had buckets of audio--right to baseline if you got too frisky with it!
Skip

[w5dud]

Hello skip, feeding audio from a symetrics 528, to 600 ohm line , audio is bad, only can get 50% at best, have tested all voltages and replaced all of the tubes with new, still no luck, did find some bad caps and replaced,I will keep digging,

[k7yoo]

Check the modulation transformer connections--forget the cable numbering--look at where the HV comes through the secondary of the mod transformer (modulated B+ to the 4-1000). Check the jumper from the B+ (unmodulated side) to the CT of the mod Xfrmr primary for feeding the 4-400's. I recently rebuilt a Techrad T-350XM and had exactly the same problem. 50% audio and terrible sounding. I retraced all of the wiring and found that the B+ to the 805 modulator tubes was coming from the wrong spot. The shorting relay across the mod Xfrmr is open isn't it?
Call me--you still have my number I hope.
Skip

[w5dud]

Thanks I will check again, I did disconnect the CW shorting wires,good point, helped a friend check his21-E and found the reactor and cap wired wrong, dud

[w5dud]

Problem solved, Mod Transformer was no good, repalced with a new one for a V-3 and lots of audio, also rewired sec wiring ass, with mod to final amp,thanks Dudley/W5DUD