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

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" wrote in message
ps.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

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
htremovethistp://www.geocities.com/SiliconValley/Monitor/4710/

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
htremovethistp://www.geocities.com/SiliconValley/Monitor/4710/

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
htremovethistp://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
htremovethistp://www.geocities.com/SiliconValley/Monitor/4710/

I have to agree with mike: The service manuals which can be downloaded
from the usual places (www.tektronix.com, www.teknetelectronics.com)
have NO detailed schematics. The go down to moduel level only. So, I'd
recommend you try to inspect the board and when you are nesr a D/A
converter (AFAIK they used devices from AnaloDevices, marked AD) then
try to find the cps nearby.

To Juerg: I know this is not a perfect way to diagnose. But what
options do you have? The instrument is out of long term support from
Tektronix. No schematic avail for the public.
I guess then you have to do it the hard way. When you are located in
Europe, I might be able to help via phone.


hth,
Andreas

That's what I figured from searching the web...

Anyways, I took a closer look today and found the D/A converter (AD667)
for the bias voltages and a bunch (5) of analog 8:1 multiplexers (don't
remember the part number)... And yes, as Mike suspected, the frequency
of the sawtooth is the same as the refresh rate of the D/A. The
sawtooth is also visible on the output of a couple of the multiplexers
as well as on the input and output of a couple of the OpAmps (TL074C).
But I haven't found any holding caps yet, that also show the sawtooth
across them.

I'll keep hunting around. Maybe I just replace all the electrolytic
caps in that area...

Thanks for your help, guys
....juerg





mike wrote:
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
htremovethistp://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
htremovethistp://www.geocities.com/SiliconValley/Monitor/4710/

Juerg wrote:
That's what I figured from searching the web...

Anyways, I took a closer look today and found the D/A converter (AD667)
for the bias voltages and a bunch (5) of analog 8:1 multiplexers (don't
remember the part number)... And yes, as Mike suspected, the frequency
of the sawtooth is the same as the refresh rate of the D/A. The
sawtooth is also visible on the output of a couple of the multiplexers
as well as on the input and output of a couple of the OpAmps (TL074C).
But I haven't found any holding caps yet, that also show the sawtooth
across them.

I'll keep hunting around. Maybe I just replace all the electrolytic
caps in that area...

Thanks for your help, guys
....juerg



Look for quad op-amps. TL074 in the 540.
They're configured as simple voltage followers.
The holding caps are typically 0.1uf SMT on the input.
Sometimes there's a series resistor.
The caps probably aren't bad, unless they got cracked.
There's a lot of stuff on the backside of the board.
Make sure to check for VCC on the op-amps.

Replacing the electrolytics is good, but won't fix it.
If they leaked, the electrolyte is already on the board.

Do the math. It takes very little board leakage to turn your
reference voltages into triangles.

You have to CLEAN the board. First one I fixed, symptoms got better
every time I cleaned it. About the fourth time, it started working.
Clean the whole board, both sides. I found electrolyte droplets far
from any caps. If you try to spot-clean you'll just move the gunk around.

Use something like Simple Green and a stiff toothbrush. Clean the CRAP
out of it. You have to get the invisible coating out from under the
chips. I used an Xacto knife to scrape gunk from between IC pins.
Sprayed cleaner HARD in attempt to force it under chips.

Cleaned it again with 99% Isopropyl Alcohol.
Dried with a hair dryer then let it sit overnight on the heater vent.

There's a reason for the two cleanings. Some stuff dissolves in the
cleaner that won't in alcohol and vice versa.

Took FOUR cycles of this before I got it all working. At the start, I
was very timid about scrubbing the board. By the end, my scrubbing was
brutal.

Tape up any switches on the board and try to keep the cleaners out of
'em. Alcohol probably won't hurt the switches, but all the gunk
dissolved in it might.

Are we having fun yet?
mike

OH, about replacing caps...I have never seen your scope, this relates to
the TDS540. YMMV. If one is leaking, they probably all are.
Change them all. There's over a hundred on a 540. Don't forget the
front panel board. You don't want that gunk getting into the encoders.

If they're the round silver caps like mine, the connections will be
corroded. Go over them one connection at a time to burn off the gunk
and get down to clean solder. Then use TWO soldering irons. Apply the
heat and twist slightly with the irons to rotate the caps. When it gets
hot enough, they spin right off. This technique applies shear force to
the pad and is much less likely to lift the pad.
Clean it while the caps are off.

Mine had two values of caps. I did some voltage checks and decided that
47uF 25V caps would work everywhere. Again, this was a 540 YMMV.
Good news was that I had a bunch of 'em. First board, I put a .1uF 1206
smt then put the 47uF leaded cap on top of that. By the fourth board,
I left off the .1s. Didn't seem to matter.
If you lift a pad, the .1 gives you something stable to solder to.

Start on caps where you can get at the trace on both ends. If you lift
a pad, you'll be able to fix it. Save the blind ones till you've done a
few dozen.

Now, we're having fun...


--
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
htremovethistp://www.geocities.com/SiliconValley/Monitor/4710/