[R-390] Saturday's Line & Local Report

[N4BE_Jim]

I would also suspect the wire harness.  Check continuity between the pot pins at the front panel and their ends in the various modules.  Are the pots grounded to front panel or through the harness?  

One end of gain controls are typically grounded.  If the grounds are lifted, the pot has little or no affect.

I once had an old SW 390a that would produce an awful popping noise on strong signals.  After much hair pulling, it turned out to be the little shielded cables, particularly the one carrying diode load signal to the rear apron. Who would have thought?  The dielectric was breaking down under "high" diode load voltage (if you consider 10 volts to be high).  Took a good part of a day to thread new miniax through the harness.

Another thing to look at is AGC not being able to control some stages.  It is conceivable that AGC is controlling early stages ok, but due to a bad bypass or agc cap in later stages, the IF is being over driven.  That can make for high audio.

Another fun anomaly is oil or contact cleaner on the phenolic antenna tune shaft.  Looks like a low impedance and will kill your AGC.  Same for contact spray or lubricant used carelessly on band switches.  High impedance tube circuits are such fun!

Jim N4BE

Sent from my iPad