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From: Juerg on 2 Mar 2005 18:23
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

From: Phil Bowser on 2 Mar 2005 21:14
Without any personal experience on your particular model, I would say
that the only thermally intermittent components I run into on a daily basis
in consumer goods ('scopes shouldn't be any different) that gets better as
the set gets warmer is electrolytic caps. Get an ESR meter and go around
the 'lytics in that area carefully, and you'll find 'em.


"Juerg" <juergh(a)gmail.com> wrote in message
news:1109805818.807884.6300(a)g14g2000cwa.googlegroups.com...
> 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
>


From: mike on 2 Mar 2005 22:18
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

--
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
ht<removethis>tp://www.geocities.com/SiliconValley/Monitor/4710/

From: Juerg on 3 Mar 2005 01:41
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
>
> --
> 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
> ht<removethis>tp://www.geocities.com/SiliconValley/Monitor/4710/

From: mike on 3 Mar 2005 13:36
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
>>
>>--
>>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
>>ht<removethis>tp://www.geocities.com/SiliconValley/Monitor/4710/
>
>



--
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
ht<removethis>tp://www.geocities.com/SiliconValley/Monitor/4710/

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