Tim, I think what you are describing is poorly formed solder joints that
could not withstand the forces of installing/removing transistors. No,
solder does not normally "crystalize", but could be grainy and weak due to inadequate
melt temperature when the soldering was done or excessive oxidation of metals prior to soldering. A process control excursion
at the factory, unusual for Tektronix.
from Tim P (UK)The posts re '465M junker' remind me of a post I sent a couple of years
ago. I had a 1S1 plug-in that was intermittent, sometimes OK, sometimes
just would not sample.
Did the usual things, re-seat transistors, clean switches, check sampling
bridge etc. Finally, I lifted the PCBs, thinking maybe a wire-clipping or
solder blob had got between the PCB and the chassis. Close examination with
a jewellers loupe and bright light showed a bad joint, where the solder
seemed to have parted from a transistor socket - there was a visible gap
between the solder fillet and the pin, not broken but as if the solder had
'shrunk'. Found a few other cases also. I guess solder ages like most
things, or sort-of-crystallises.