[Collins] 75S-1 Problem (update)

[Jim Brannigan]

According to the manual, the AGC threshold is 1.5 microvolts.
Try checking the stage gains against the manual.

The CRA site has a very good article on the PTO rebuild.

Jim

> Well,
> I re-aligned it with the swamping network and checked the IF gain with  a
> known/calibrated 50 micro volt source. This time I know for sure that the
> S-Meter reads 25 db over S-9 (65 db) at 50 micro volts with the IF gain
set pot
> adjusted as low as it will go. There appears to be no ALC action at 1
micro volt
> and no distortion of strong signals (excluding my neighbor across
street).
> Nothing really stood out with voltage and resistance checks. I guess  I'll
have
> to make  more through voltage/resistance checks and try tube  swaps.




 While I
> was trying to find the source of my outlandish S-Meter readings,  a couple
of
> other problems cropped up. When the  calibrator is oscillating or I put
the TX
> in synch mode, the  audio starts to crackle (maybe a noisy tube?). The
second
> problem is  the PTO. Every so often, it suddenly jumps, at random, 1.5 to
2
> kc's up or  down. Otherwise it is rock stable. I replaced the 1N34A
USB/LSB
> shift diode,  hoping that might be the problem. No joy. Next step is to
scrounge
> some 1% or 2%  silver mica's, some good quality .02 bypass caps and
shotgun
> the PTO (I'm  getting tired of pulling it out). I suspect it might be the
200 pf
> "postage  stamp" mica, most likely suspect, I have seen plenty of them go
bad.
>
> Thanks for reading my long winded dissertation and 73! DE N5MCJ,  Jerry