[R-390] R390A- Intermittent sensitivity variation

[Eric Gauja]

Dear Group,
                        An update:   Thanks to all those who made helpful 
suggestions -  it seems that my problem was in fact caused by aged silver 
mica capacitors.  After checking the bandswitch operation and cleaning of 
contacts, the intermittent behaviour was still present.  After reading David 
Wise's comments, I looked more closely at the circuitry  around the 16-32MHz 
RF transformer stages, and noticed that of the transformers T206,  Z206-1 
and Z206-2, only  Z206-1 has mica capacitors connected to the B+ line.  My 
hunch was that it was more likely that the Z206-1 caps had problems with 
value change or breakdown with B+ voltage stress..  Since Z206-1 and Z206-2 
have identical capacitors and coils , I decided to swap over these 
transformers.  After realignment at 17.6 and 30.4 MHz,   sensitivity was 
restored , and no more intermittent behaviour!   I'll see what happens over 
time - for a proper fix, I will replace the mica capacitors in these 
transformers with newly manufactured silver micas or ceramic capacitors at a 
future date.   I'm also slowly working my way through past posts , and 
problems with the old silver mica capacitors are well documented. 
Heartfelt thanks to all involved in archiving and maintaining this 
information!

Best Wishes,
                       Eric.

-----Original Message----- 
From: David Wise
Sent: Tuesday, May 08, 2012 4:30 AM
To: Roger Ruszkowski ; R-390 at mailman.qth.net
Subject: Re: [R-390] R390A- Intermittent sensitivity variation

If "buzzing" doesn't reveal bandswitch trouble, look to the mica caps.

I could be remembering wrong, but I think there are fixed mica caps that are 
in service over the 16-32MHz octave and not used elsewhere.  One could be 
shifting in value or intermittently partially opening or leaking, which 
would mistune the affected coil or reduce the Q, respectively.  A little 
electrical "jostling", in the form of transients that occur during 
switching, can temporarily restore operation.  This happened to my receiver, 
although it was in the IF not RF.  Unfortunately, you have to lift one end 
of each suspect to check this.  Even though most of these caps see no DC 
voltage in operation, applying voltage, say with a TelOhMike-type cap 
analyzer, can drive this kind of problem into the open.

Dave Wise