[R-390] AGC Line Again

[ROSS HOCHSTRASSER]

Hi Dave, I just pulled out the jumper and measured the resistance with my digital meter. With the + to ground, terminal 4 is infinite to ground. With - to ground it read 380 K ohms but kept rising up. Terminal 3 is 466K. I'm heading to the schematic now. THANKS!!!
> On February 17, 2020 at 9:31 PM dog <agfa at hughes.net> wrote:
> 
> 
> I had the dreaded IF filters going bad. As I remember, you can't tell 
> which one it is. What's happening is the internal resistance of the 
> filters is becoming something measurable in relation to the AGC circuit. 
> And no matter which position you have the BW in, that resistance to 
> ground is always there. Look at the schematic.
> 
> I repaired mine, but would not do it again. It worked but not like 
> before. I'd like to know what the alternative is?
> 
> I used a Fluke 189 in the nS mode to check them, after I took them out, 
> it only takes megohms of resistance to screw up the agc. It's a very 
> high Z circuit. Took me forever to find the problem.
> 
> Dave N3DT
> 
> On 2/17/2020 9:17 PM, ROSS HOCHSTRASSER wrote:
> > I am currently monitoring the AGC voltage at the jumper. With the radio cold, I was measuring -9.01 volts relative to ground.As the radio warmed up 15 minutes or so, the voltage has dropped to -5.06 volts, the signal is distorted and the s meter is pinned. The s meter started at 60db . It is tuned to WBZ which is about 15  miles from here.(very strong)  I am going to see if the AGC voltage drops any further. If it doesn't, I'll shut it off and check the resistance to ground with the jumper removed. Correct me if I'm wrong, but wouldn't the problem be on only one of the mech filters? I notice no change when switching between them.  Many Thanks to all of you for your help in beating this demon!!
> >> On February 17, 2020 at 2:51 PM Jacques Fortin <jacques.f at videotron.ca> wrote:
> >>
> >>
> >> Ross,
> >>
> >> Could it be the AGC line leaking to GND thru one of the Mechanical Filters ?
> >> With the IF module out of the radio and not powered, you should be able to
> >> apply up to -100V to the J512, pin 6 without "seeing" any current
> >> "delivered" to the AGC line.
> >> If ever you measure a leakage there, disconnect R507 at S503, pin 8 and
> >> re-test.
> >> If the leakage goes to zero, your problem is inside one (or many) of the
> >> Mechanical Filters.
> >> If ever it proves to be the case, there is a turnaround to this problem
> >> (without re-building the offending(s) Mechanical Filters, say..)
> >>
> >> 73, Jacques, VE2JFE
> >>
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