Juerg wrote:
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.

TDS schematics are unpublished and likely not found anywhere.
Anybody who had them would be sitting on a repair bonanza and unlikely
to give them up.
mike


....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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--
Return address is VALID but some sites block emails
with links. Delete this sig when replying.
..
Wanted, PCMCIA SCSI Card for HP m820 CDRW.
FS 500MHz Tek DSOscilloscope TDS540 Make Offer
Wanted, 12.1" LCD for Gateway Solo 5300. Samsung LT121SU-121
Bunch of stuff For Sale and Wanted at the link below.
MAKE THE OBVIOUS CHANGES TO THE LINK
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