[R-390] AVC voltages revisited

[Jim M.]

I have noted that if there is oil or conductive film residue on some tube 
sockets, on some RF switch wafers (that carry AGC), or especially the 
insulated antenna trimmer shaft, then the AGC control may tend to be 
degraded in the RF deck.  Over spraying RF deck switches or luibricating the 
antenna trimmer shaft can cause this.  The residue can be removed by "Big 
Bath" or equivalent moisture displacement spray.  Leaky AGC line bypass caps 
can also be suspect.  It's a high impedance circuit so it doesnt take much. 
Jim N4BE


----- Original Message ----- 
From: <DJED1 at aol.com>
To: <r-390 at mailman.qth.net>
Sent: Thursday, May 18, 2006 8:01 PM
Subject: [R-390] AVC voltages revisited


> Just a tidbit for anyone else who is investigating the AVC line:   As I
> mentioned before, I disconnected the cable to the RF module and made some
> resistance measurements, which I thought indicated a leaky cap.  As I was 
> listening
> to the radio last night, I was browsing through the Y2K manual (a  great 
> job).
> I was surprised to see that the RF amp schematic was different  than in my
> old TM, and the differences accounted for the irregularities in 
> resistance
> measurements.  Specifically, the newer manual shows a 1.5M  resistor from 
> the AVC
> line to ground in the RF module, while the old TM shows no  path to 
> ground.
> Since the new schematic agrees exactly with my resistance  measurements, I
> assume I'm all OK with the capacitors, and don't need to pull  the RF 
> deck.
> Ed
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