[Collins] R-388 second mixer weirdness

[Craig Roberts]

As you may recall, I complained early last week about an intermittant 
sensitivity dropoff in my R-388 that appeared to be related to V106, the 
6BE6 second mixer tube.  In each case, the problem could be solved -- 
though only temporarily -- by the substitution of another 6BE6.  The odd 
nature of the failure led several people to auggest, logically, that a 
dirty tube socket or "cool" solder joint might be at fault.  Though I 
had done so during earlier servicing of the radio, I cleaned the socket 
pins and reheated the solder joints.  After several hours of operation, 
however, the problem returned.

During my initial rebuild, I had replaced the 100 pf mica cap on pin 1 
of the affected tube. I had been warned by a veteran Collins company 
tech that many of these particular value micas installed during this 
radio's production run were faulty.  I had not replaced, however, the 
second mica on the same pin -- a 220 pf device.  This time I did, 
however, and the problem seems to have been cured permanently.  The 
radio has been running for over a week now with no faults.  Not only 
that, but the AVC action is noticeably improved.

Ya never know...

Thanks to all who replied to my initial plea for advice.

73,

Craig
W3CRR