[R-390] Below 8mhz problem

[Barry Hauser]

Dan wrote:
<snipped>

>This was my personal 390A I've had for years, a nice '67 EAC, and this
particular
> solder junction just went south for no particular reason! Anyone else had
> this happen?

Yes -- me too -- and there was a thread on this way back.  Cited were long
term electrolytic reaction -- between dissimilar metals, and/or the result
of impurities, such as those from trace amounts of rosin that didn't boil
off when the joint was originally made. They said the resulting bad joint
could form a sort of resistor, or even a capacitor or semiconductor -- of
sorts.

>I've often heard of a couple of guys that "reheat all the
> joints in the RF deck using silver solder" during their refurbishment
> efforts. I've always thought that to be snake oil.

Probably most of the joints have another 40-trouble free years to go,
however it's the two or three like the one you found to worry about.
Re-doing each joint can head that off -- if you're careful, or else you'll
make more of 'em trying.  Someone also provided a practical metallurgical
reason for using silver solder -- something like you use it when soldering
on silver plated termnals to avoid de-plating action.  For different
situations, you'd use some other solder.  I hope he who knows what I'm
writing about here jumps on and clarifies.

BTW - the really slow-fail solder joints often don't have the tell-tale
characteristics of a cold joint -- dullness, ring-mark, etc.

Barry