I'm working on a TDS744A digital oscilloscope that fails SPC (signal
path compensation) most of the time. It usually passes when it's
cold, but after warm up, it almost always fails with the error "Cal
meas clipped at max dig o". The self test always passes.

I haven't been able to find any obvious performance, or accuracy
problems on any channel, or range, and it seems to be stable during
warm up. Are there any known common causes of this error on the
TDS744, or other similar model? It seems to be a common complaint
about this model, but no one has mentioned a fix.

So far, I've checked it over for obvious problems. Cooling U1250 on
the acquisition board seems to increase the chances of a successful
SPC, but it's hard to be sure. It might be a coincidence, or just
compensating for another problem. It doesn't have the leaking
electrolytic problem of earlier models.


I'm also looking for the TDS600B and TDS700A adjustment software (both
are needed for calibration according to the service manual).