[Collins] RE: 75A-3 AVC problem

[Laird Tom N]

Garey,

Thanks for the info. One always runs the risk of looking stupid, I read
the section on Detector and AVC, but didn't look any further for the
info, HI... If -41 is right, then I have an issue somewhere else, cuz
that's what I read on mine.

Tom WC9M 
Moline, IL.

-----Original Message-----
From: Garey Barrell [mailto:k4oah at mindspring.com] 
Sent: Wednesday, July 06, 2005 10:42 AM
To: Laird Tom N
Subject: Re: 75A-3 AVC problem

Tom -

I no longer have a 75A-3. According to the manual, Pin 3 of V-9 should
read -41V in AM Mode with RF Gain at MAX.

Looking at the circuit, it's a little different than I'm used to seeing.

First thing is there is NO AVC in CW mode, only a variable bias
controlled by the RF Gain control. From the schematic, the bias supply
(C-94) should read about -50V. There is a voltage divider consisting of
R-56 (15k), the RF Gain control R-57 (10k) and R-56 (560) to ground. So
in CW mode, the voltage at the top of R-57 should vary from -1 to -21V
as the control is rotated, and this is the voltage applied to the AVC
bus.

In AM, the plate (Pin 1) of V-9 should read about -1.7V, and this is the
"no signal" AVC level. This voltage should vary from about -1.7V to
-20V.  The plate voltage of V-9 is derived from the top of the RF Gain
control through R-55 (220k), so the maximum plate voltage (AVC voltage)
is set by the RF Gain control.  Pin 3 voltage is derived from a fairly
stiff divider off the bias supply, so should not vary much, which is
what you are seeing.

Garey - K4OAH

Drake C-Line Service Manual
<http://www.k4oah.com>