Hi,

I could use some help fixing a TDS640 digital Tek scope.

The baselines of all four channels show an offset'ed sawtooth
(different amplitudes and offsets for each channel) with a period of
4.5ms rather than a flat 0. When applying an input signal, it gets
overlaid over the sawtooth but otherwise looks fine.

I was following Teks troubleshooting guide and located the problem to
the acquisition board. The input attenuator is believed to be OK (input
to the AD converters look fine). All the low voltages (+/-15V, +/-5V)
are OK and within spec (checked with another scope, no ripples).

What I noticed is that over time when the scope warms up the amplitude
of the sawtooth decreases until it becomes zero, but there's still an
offset that won't go away. I was using some freeze spray to try to
isolate the problem further and ended up pin-pointing it to the section
around the AD converters. When I cold spray that area, the sawtooth
comes back and disappears again after a while. There's a bunch of
OpAmps, resisors, caps and diodes in that area.

Any ideas what to look at first?

Thanks in advance for any suggestions
....juerg