[R-390] AVC Voltages

[DJED1 at aol.com]

Thanks to all for the comments on my AVC problem. I disconnected the RF  
chassis from the radio and found that the AVC line has a resistance to ground of  
1.75 M, so something is leaky.  The IF chassis shows more than 40 M, so  it's 
OK.  Got some other strange results though.  I put a DC voltage  on the AVC 
line and measured voltages at the test points in the RF  chassis.  Voltages were 
all in the range of 10.5 V, versus 7 to 9 V with  the same voltage on the AVC 
terminals.  I don't understand why the AVC  voltage shoud behave any 
different than a DC supply voltage, and the 5 V drop in  AVC voltage can't be 
accounted for by a 1.75 M leakage current through a  cap.  
Then I remembered the problem of the antenna trimmer- it's hot with AVC  
voltage, and leaks when it gets dirty.  I dug up some alcohol and cleaned  all 
around the gears, and watched the resistance plummet to about 0.5 M.   Must have 
been some water in the alcohol.  I finally got it all dried out  and got the 
resistance back to where it started, but no better. The  strangest thing I 
found was that if I shunted the antenna trimmer to ground by  putting my fingers 
on it, the resistance at the RF amp test point increased,  rather than 
decreased??  Maybe I need an exorcism. 
Given that any further investigation requires pulling the RF chassis, I've  
decided to defer that to next winter when I think I'll recap the whole radio.  
For now, I'll have to live with some minor errors in the meter readings.
Ed