[R-390] AGC voltage issue

[Larry H]

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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