[Collins] 75A-3 AVC problem

[Gerald]

On Tue, 2005-07-05 at 09:58 -0500, Laird Tom N wrote:
>  I have a 75A-3 with a "pegged" S-meter. I have checked all the tubes. I
> measured the AVC voltage
> at V-9 (AVC amp pin 3) and it measures -38 volts DC. I think it should
> be around -11 volts????
> 
> Any specific ideas as where to look for the problem. I have checked most
> the components around that
> circuit and they seem to be in spec. I also changed the 20uf cathode
> 'lytic of V-9, still no RX.
> 
> Thanks in advance,
> 
> Tom Laird WC9M
> Moline, IL.
> 
I don't have an A-3 schematic available so I'm just guessing. Seems like
its time to search for the source of the high AVC voltage. Could there
be an oscillation in the IF or the AVC IF amplifier? Seems some
receivers of that era used an extra AVC IF power amplifier to drive the
AVC rectifier harder.

Does the AVC voltage go down when the detector or the AVC detector tubes
are pulled? If so its coming from something out of the IF. A search with
a LF receiver or a scope might show the problem, likely an open screen
bypass capacitor.

Does the radio use negative bias to the AVC line as RF gain control?
Some Collins radios do that. Could it be that the RF gain pot is open
putting more bias on the AVC line? 

I'm fresh out of ideas without some measurements.
-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer