Didn't you walk right into the problem? It seems that by a yet unknown
cause the circuit sometimes runs into some state that makes it oscillate
or in some other way fries the resistors. This kind of intermitted faults
are the most difficult to solve as most of the times there seems to be no
problem. Even your observations with the scope may mislead you as
connecting the probe may change the circuit enough to change its behavior.

petrus bitbyter


Yes, there may indeed be some kind of intermittent problem. I will put them
all back up again on Monday, and see how they perform this time. As to my
'scope muddying the waters, it's generally pretty well behaved in that
respect. It's a high quality 100 MHz job, which is always used with a x 10
low capacitance probe. If that is having much of an effect on the circuit,
then it must for sure be a pretty poor design. To be honest, I still think
that the problem lies with me somehow. Something that I did differently
between the first and second times that I tried them, but I'm not sure what
....

Arfa