[R-390] AGC voltage issue

[Ryan Scott]

Craig, Roger, Larry, Charles,

Thanks so much for your help so far.  I took note of the AGC
troubleshooting post by Charles.  With the Good IF Deck, I get about -7V
AGC with a strong AM BCST station.  The Resistance readings appeared funny
- I read -2.7M ohms.  Hmmm.  Had some residual voltage on the AGC line that
was messing up my reading (I'm using a Fluke 87 DVM)..  Once I removed C551
from the circuit the readings were in the ball park.  This module is good
but will get a C551 replacement - Craig stated this could be bad, see below.

The Bad (Amelco) IF Deck has -0.182V AGC that does not move at all for
anything.  The resistance readings are in the ball park as well.  I'm
suspecting low gain in the circuits of V501-4 and will use Roger's advice
to troubleshoot that.   C551 in this IF deck seemed more normal, it only
had 0.05V across it.

So this C551 on my good EAC IF Deck:  When I removed it from the circuit it
has 0.6V across it.  When you short it, the voltage goes away.  When you
remove the short, the voltage is back.  Reading is about 10M ohm (one way,
less the other)  So I'm thinking this cap is BAD.  Does anyone think it's
good?  I mean this residual voltage that won't go away.... I have a sencore
LC502? that should be able to test it at it's rated voltage but I have only
done the test with a DMM thus far.

I remember seeing on the messages sometime before about replacements for
C551..  But I have to ask, is a 500V cap necessary?  I have (2) 1.0uF @400V
in my junk box.  The B+ is only ~210V and the AGC voltage is much less.

Thanks and Regards,
Ryan
N7QJ

On Sun, Sep 14, 2014 at 9:17 PM, Larry H <dinlarh at att.net> wrote:

> Hi Ryan,  Yes, you're right on the pos voltage there.  If the IF sig is
> very low, then the agc delay bias R544 can make it a little pos (about
> .2v), but not what you're seeing.  However, since the agc and audio are
> still low I think I'd focus on the gain of IF 1, 2 and 3.  Good luck.
>
> Regards, Larry
>
>
>   On Sunday, September 14, 2014 8:09 PM, Roger Ruszkowski <
> flowertime01 at wmconnect.com> wrote:
>
>
>
> Scott,
>
> Cold solder joint in an IF transformer bumped around by all the other work.
> You can pull the can tops and re solder the coil wires on the assembly
> corner post strut wires.
>
> Put a good IF deck back in the test receiver.
> Use an isolation cap between the signal generator and the If deck.
> Pull a tube.
> Insert 455 to the plate pin of the tube socket.
> Determine how much signal is needed to drive the diode load to -7 volts DC.
> Write the tube number and signal level down for reference.
> Do every tube in the signal chain of the IF deck.
>
> Now put the problem deck back in.
> Wait for it to fail.
> From the back to the front:
> Pull a tube
> Insert a signal
> Decide if the stage being driven is good or bad.
> Remember you are going into the plate output of the prior stage.
> So you get the coupling cap into the next stage and the next tube stage.
> Isolate the problem to one tube stage and analyze.
>
> It could be a bad new cap  or resistor or solder joint.
> But you need to work it down to a stage so you only have a few parts to
> fret about.
>
> Roger AI4NI
>
>
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-- 
Ryan P. Scott
n7qj at arrl.net
503-880-1214