[Collins] R-388 second mixer weirdness

[Gerald]

On Mon, 2005-12-05 at 12:49 -0500, Craig Roberts wrote:
> Here's a baffling problem that's been plaguing me for months.  It's one 
> of those "every time I think it's fixed, it comes back" weirdnesses:
> 
> After my R-388 has been playing for awhile (anywhere from a couple of 
> hours to a couple of days), the sensitivity drops off dramatically.  
> Replacing the second mixer tube (V-106 -- a 6BE6) solves the problem.  
> The strange thing is, the tube doesn't have to be a new one.  It can be 
> a used one, a JAN tube, a civilian tube, or even a tube that's been used 
> in the same position before -- as long as it's different than the one 
> currently employed.  This sounds heat related, but V-106 doesn't get 
> unduly warm and the problem may not show up for many, many hours and -- 
> apparently -- the tube's not being destroyed.  Voltages seem okay.
> 
> Upon the advice of an old Collins factory tech, I replaced all the 100 
> pf micas in the set -- but still... 
> 
> Anyone else run into this?
> 
> Thanks and 73,
> 
> Craig

I  haven't seen exactly this but what I would check would be the tube
socket and solder connections to the pins. I'd clean the socket and tube
pins with DeoxIT, and then I'd examine the soldering of all connections
to all pins. I'm suspicious that the bottom (first installed) wire was
made mechanically sound but didn't get soldered and now is getting
intermittent. The motion of removing and replacing the tube renews the
contact for a while but tube heat and age corrode it again. That is a
long term result of the old scheme requiring a mechanically sound
connection before soldering. It can work for 30 years without solder but
then gets intermittent. An if its under another wire it can be hard to
find. A spot of active liquid flux and plenty heat is often the cure but
don't melt adjacent parts and wire insulation in the process.
-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer