[Collins] R-388 second mixer weirdness

[Gerald]

On Thu, 2005-12-15 at 13:10 -0500, Craig Roberts wrote:
> As you may recall, I complained early last week about an intermittent 
> 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 suggest, 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
Well this is proof that the micas can fail, but still not that they have
such a high failure rate that they should be shotgunned. Likely this one
would have changed from either prodding (with a fiber or orange wood
stick) or temperature cycling with hot air and freeze-mist.

-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer