SB-102 S meter woes

[Craig Buck]

I have a negative reading S meter.  It starts out OK but goes negative after
a minute or two.  The RF gain control works and will make the meter go
positive.  The AVC appears to be working but don't hold me to it - I am not
sure what is normal having just gotten this rig.  I have replaced the tubes
per the Manual's troubleshooting chart namely the first and second IF
amplifier and the AVC/product detector.

I squirted cold stuff on the components around those tubes with no effect.  I
have checked the resistors cold but they seem OK.  I am not sure how to check
them in the circuit when it is "hot."  The two diodes in the meter circuit
appear OK cold -- high resistance in one direction, lower resistance the
other way.  The plate voltage on the product detector seems low 80 vs 130 in
the diagram.

I have a Series 15512 but my manual is a reproduction from 1970 and I don't
know which is newer.  There are two resistors that are different from what is
shown in the manual.  I have a 33k 1 watt resistor on the screen of the First
IF amplifier vs. 22K in the manual.  I also have a 510 ohm resistor between
the meter zeroing pot and ground vs a 330 in the manual.  This tube feels
hotter than the companion 6AU6 which is the second IF amp.  The meter zero
adjustment has no effect once the meter goes negative.  The voltage across
the S meter is about -2.5.

I notice the receive audio makes CW notes sound "rough" and suspect the 20uf
electrolytic on the grid of the audio amp may need replacing.

That's a lot of information but I am out of ideas folks.  Anyone able to help?

Radio K4IA
Craig Buck
Fredericksburg, Virginia USA
QRP ARCI #2550  FISTS #6702 CC 788
K1 #470
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