Hi Mike,

Thanks for the tips, I'll check them out. You don't happen to have
schematics for the TD540, do you? That might help me figuring out the
640.

....juerg

mike wrote:
Juerg wrote:
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


All my experience is with the TDS540, so my comments may or may not
be
relevant.
All the bias voltages are set up by a single D/A converter that gets
multiplexed onto hold caps followed by op-amps.
Leakage to any other part of the circuit or a defective op-amp will
cause the voltage to decay between refreshes. The resultant triangle

will appear on the acquired signal.

I'd compare the triangle on the waveform to the refresh rate of the
D/A
hold circuits. Another simple thing to do is dig out your data
sheets
and check the power supply voltages on all the op-amps and
multiplexer
chips. I had several +15V supply pins to op-amps go open.

ON the 540 series, a major cause of this leakage was failed
electrolytic
caps that leaked caustic goop onto the board. It can be too little
to
see and still cause problems. Another problem I've seen is corrosion

between op-amp and multiplexer pins facilitated by this electrolyte.
Sometimes it gets down tiny blind via holes and eats out the via.
You
can't get to the other side to test it.

Don't know if the 640 series is afflicted with leaky caps.

mike

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