[R-390] R-390 Ant trimmer question

[Roger Ruszkowski]

Jim,

Thank you for this post.
I did not know the coax would give us problems.
The coax is getting old.  We can think to check it if the
noise floor is coming up of we have the noise pops and can not
find a bad tube or other cause.
Roger AI4NI

Some things to consider:I've never seen 40db carrier readings on either ofmy 390a's except when AGC wasn't functioning fully. If the AGC lines are beingshunted somehow the radio "appears" to be more sensitive, i.e high carrier levelreadings.What can impact AGC action?  Any caps anywhere on the AGC circuitsin IF and RF decks.  If you lubricated the antenna trimmer shaft duringrestoration, the AGC will be affected because the phenolic shaft absorbs thelubricant.  The trimmer is on the AGC line and the phenolic shaft is supposed tobe an insulator.  Likewise if you sprayed the band switches with cleaner such asdeoxit, the chemical makes the wafers no longer be high impedance.  Theimpedance 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 withsomething like Big Bath.I agree with comments about silver mica caps in theRF 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 tendsto cause failures in those caps.  If failure is isolated to one band this issomething to look at.   Unfortunately many are across coils inside the cans soyou have to remove the cans, open them up, and replace the caps.  If the failureaffects all bands the same way, then it's something common to all bands.Nowthings get interesting when you think about the coax cables in the radio.  In myold SW, I was plagued with clicks, pops, etc.  Finally I discovered that thedielectric of diode load coaxes out of the IF module was breaking downinternally.  Had to replace them all with fresh coax.  Doesn't sound like whatyou're experiencing.Hope this helps.JimN4BE