[R-390] AGC

[quartz55]

Gordon asked about the AGC problems he is having.  It's been my understanding that people think the AGC in the R390A was always not very good and reduction of the RF gain could compensate. I think there may be another problem going on.  As the filters age, they have less resistance to ground through the deteriorating foam.  Look at the AGC line in the schematic where it's connected to R507 and that resistor is connected to the mechanical filters on the output side which is RF grounded by C512.  If you have low resistance in that circuit due to the filters, it certainly will affect the AGC action by dragging the voltage down.  All it takes is one of the filters to be non-compliant.

Dave
N3DT