[R-390] R-390 Ant trimmer question

[N4BE_Jim--- via R-390]

Some things to consider:

I've never seen 40db carrier readings on either of my 390a's except when AGC wasn't functioning fully. If the AGC lines are being shunted somehow the radio "appears" to be more sensitive, i.e high carrier level readings.

What can impact AGC action?  Any caps anywhere on the AGC circuits in IF and RF decks.  If you lubricated the antenna trimmer shaft during restoration, the AGC will be affected because the phenolic shaft absorbs the lubricant.  The trimmer is on the AGC line and the phenolic shaft is supposed to be an insulator.  Likewise if you sprayed the band switches with cleaner such as deoxit, the chemical makes the wafers no longer be high impedance.  The impedance of the wafers could be reduced just enough to affect agc or tuning.  Same for tube sockets.  These may dry out eventually, or just spray them with something like Big Bath.

I agree with comments about silver mica caps in the RF stages.  I recently had to recap the RF and tuned IF stage coils in a SW.  Any cap that directly touches B+ will be suspect because the high voltage tends to cause failures in those caps.  If failure is isolated to one band this is something to look at.   Unfortunately many are across coils inside the cans so you have to remove the cans, open them up, and replace the caps.  If the failure affects all bands the same way, then it's something common to all bands.

Now things get interesting when you think about the coax cables in the radio.  In my old SW, I was plagued with clicks, pops, etc.  Finally I discovered that the dielectric of diode load coaxes out of the IF module was breaking down internally.  Had to replace them all with fresh coax.  Doesn't sound like what you're experiencing.

Hope this helps.

Jim N4BE