[Collins] Help on troubleshoodint a 51S-1

[Dr. Gerald N. Johnson]

On Mon, 2007-09-17 at 11:29 -0600, DW Holtman wrote:
> Hello,
> 
> Thanks to everyone who had suggestions, I basically tried all of them. Where
> 
>  After lots more troubleshooting, I found that the BFO was FMing. It looked
> like a classic 5 or 10 KHZ FM signal. I replaced C162 (along with every
> other lytic in the receiver) in the base of Q1 (SSB/CW Pre Amp) and the
> FMing stopped. It showed good stability on a frequency counter (it was
> bouncing around a couple of KHZ before) and visually on a scope. But, the
> raspy signal on SSB/CW is still present. I injected an audio signal on the
> base of Q1 and it produced clear audio. That pretty much takes me back to
> the balanced bridge diode mixer/product detector. All of the diodes read
> around 0.3 volts using the diode range on my Fluke meter, might have to
> change out the diodes. Is there anything better than 40 year old 1n34A's  to
> replace them with?

Schottky diodes, perhaps 1N5711. Are you testing with the fluorescent
bench light shining on the glass diodes? They are light sensitive. What
is the effect of backing of on the RF gain to keep the signal to the
detector small? AM based AVCs often don't work well on SSB or CW. Is
there still ripple on the BFO shifting diode? Is there a varactor for
BFO setting? Is the supply to it perfectly clean? Is there heater
cathode leakage in the BFO tube?
> 
> Any other suggestions on options besides changing out CR1,2,3 and 4? AM has
> nice clean audio.
> 
> Best,
> DW Holtman
> WB7SSN
> 

-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer