quote:

---

> 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
>

quote:

---

> 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
>

quote:

---

> Juerg wrote:
of[vbcol=seagreen]
to[vbcol=seagreen]
(input[vbcol=seagreen]
+/-5V)[vbcol=seagreen]
amplitude[vbcol=seagreen]
an[vbcol=seagreen]
section[vbcol=seagreen]
>
> All my experience is with the TDS540, so my comments may or may not

quote:

---

> 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

quote:

---

> will appear on the acquired signal.
>
> I'd compare the triangle on the waveform to the refresh rate of the

quote:

---

> hold circuits. Another simple thing to do is dig out your data

quote:

---

> and check the power supply voltages on all the op-amps and

quote:

---

> chips. I had several +15V supply pins to op-amps go open.
>
> ON the 540 series, a major cause of this leakage was failed

quote:

---

> caps that leaked caustic goop onto the board. It can be too little

quote:

---

> see and still cause problems. Another problem I've seen is corrosion

quote:

---

> between op-amp and multiplexer pins facilitated by this electrolyte.
> Sometimes it gets down tiny blind via holes and eats out the via.

quote:

---

> 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/

quote:

---

> 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.

quote:

---

>
> ....juerg
>
> mike wrote:
>
> of
>
> to
>
> (input
>
> +/-5V)
>
> amplitude
>
> an
>
> section
>
>
> be
>
>
>
>
> D/A
>
>
> sheets
>
>
> multiplexer
>
>
> electrolytic
>
>
> to
>
>
>
>
> You
>
>
>

quote:

---

> Juerg wrote:
the[vbcol=seagreen]
>
> TDS schematics are unpublished and likely not found anywhere.
> Anybody who had them would be sitting on a repair bonanza and

quote:

---

> to give them up.
> mike
>
sawtooth[vbcol=seagreen]
gets[vbcol=seagreen]
triangle[vbcol=seagreen]
corrosion[vbcol=seagreen]
electrolyte.[vbcol=seagreen]
>
>
>
> --
> 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/

quote:

---

> 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
>
>

quote:

---

> Juerg wrote:
>
> 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...
>
>

quote:

---

>
> If you really want to clean it,run it through your dishwasher(alone,no
> dishes!) using Calgonite,then dry thoroughly,repeat;thoroughly.
> We used to run 2236 DMM boards thru a DW to cure a Hi-Z leakage problem.
> Then 3 days in the drying oven.
>

quote:

---

>
> If you really want to clean it,run it through your dishwasher(alone,no
> dishes!) using Calgonite,then dry thoroughly,repeat;thoroughly.
> We used to run 2236 DMM boards thru a DW to cure a Hi-Z leakage problem.
> Then 3 days in the drying oven.
>
> --
> Jim Yanik

quote:

---

>

quote:

---

> Jim Yanik wrote:
>
> Jim, have you seen the small powder coat oven at Harbor Freight? The
> temperature range is too high, but it looks like a small environmental
> chamber. For the $399 price it wouldn't be hard to change or modify the
> controller to use for circuit boards, if needed.

quote:

---

>
>< http://www.harborfreight.com/cpi/ct...temnumber=46300
>
>

quote:

---

>
> You could make a large plywood box,put a bank or two of 100W light bulbs in
> it,and a couple of fans drawing air out of the box,for a lot less,IMO.
> And size it large enough to fit a 7000 series scope mainframe.
> Gets too hot inside,unscrew a light bulb or two.
>
> Not fancy,but workable.
>
> --
> Jim Yanik
> jyanik
> at
> kua.net

quote:

---

> Jim, have you seen the small powder coat oven at Harbor Freight? The
>temperature range is too high, but it looks like a small environmental
>chamber. For the $399 price it wouldn't be hard to change or modify the
>controller to use for circuit boards, if needed.

quote:

---

> On Wed, 09 Mar 2005 07:47:37 GMT "Michael A. Terrell"
><mike.terrell@earthlink.net> wrote:
>

I think Mike is considering commercial uses,not home/hobby applications.
For that(commercial apps) it probably would be a good idea.
I'd want something larger,that I could put a whole instrument in after a
wash.
[vbcol=seagreen]
>
> You don't have to get fancy if you just need to do a single part (or a
> few parts.) I've done wonders with a steel wastebasket and a small
> light bulb. You adjust the temp by trying different size bulbs, and
> you close off the wastebasket opening with alum foil, just to limit
> air circulation, which would reduce the temp. Make sure the bulb is
> placed so that it can't come in direct contact with the parts you're
> drying out.
>
> It doesn't have to be hot, just warm enough to make the vapor pressure
> of the air low (ie, reduce the relative humidity.) 125-150F is plenty
> warm.

quote:

---

>
> On Wed, 09 Mar 2005 07:47:37 GMT "Michael A. Terrell"
> <mike.terrell@earthlink.net> wrote:
>
>
> You don't have to get fancy if you just need to do a single part (or a
> few parts.) I've done wonders with a steel wastebasket and a small
> light bulb. You adjust the temp by trying different size bulbs, and
> you close off the wastebasket opening with alum foil, just to limit
> air circulation, which would reduce the temp. Make sure the bulb is
> placed so that it can't come in direct contact with the parts you're
> drying out.
>
> It doesn't have to be hot, just warm enough to make the vapor pressure
> of the air low (ie, reduce the relative humidity.) 125-150F is plenty
> warm.
>
> Jim Adney

quote:

---

>
> Jim Adney <jadney@vwtype3.org> wrote in
> news:5rjv21tgdc4csoj5f71jlfok03eg6onc4d@4ax.com:
>
>
> I think Mike is considering commercial uses,not home/hobby applications.
> For that(commercial apps) it probably would be a good idea.
> I'd want something larger,that I could put a whole instrument in after a
> wash.
>
>
> Actually,you do NOT want it any hotter than about 150degF, plastic parts
> like cam drums begin to be affected.
>
> And the muffin fans providing a slight negative pressure helps draw out the
> moisture,increasing evaporation.
>
> (BTW,I've also used the lightbulb/enclosure trick for curing epoxy.)
>
> Jim Yanik

quote:

---

> Juerg wrote:
(AD667)[vbcol=seagreen]
(don't[vbcol=seagreen]
frequency[vbcol=seagreen]
multiplexers[vbcol=seagreen]
(TL074C).[vbcol=seagreen]
sawtooth[vbcol=seagreen]
>
> 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

quote:

---

>
> Use something like Simple Green and a stiff toothbrush. Clean the

quote:

---

> 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,

quote:

---

> was very timid about scrubbing the board. By the end, my scrubbing

quote:

---

> brutal.
>
> Tape up any switches on the board and try to keep the cleaners out of

quote:

---

> '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

quote:

---

> 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

quote:

---

>
> 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

quote:

---

> heat and twist slightly with the irons to rotate the caps. When it

quote:

---

> hot enough, they spin right off. This technique applies shear force

quote:

---

> 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

quote:

---

> 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

quote:

---

> smt then put the 47uF leaded cap on top of that. By the fourth

quote:

---

> 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

quote:

---

> a pad, you'll be able to fix it. Save the blind ones till you've

quote:

---

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

quote:

---

> OK, here's an update on my progress:
> I cleaned the acquisition board using all methods mentioned in the
> thread (toothbrush with simple green and isopropyl alcohol followed by
> a dishwasher cycle). I reassembled the scope and - oh wonder - the
> sawtooth/triangle is gone but all the channels still have different
> offsets (even when the input is grounded). Could that still be an after
> effect of leaky caps?
> I'll start looking at the bias voltage generation/convertion to figure
> where the error gets introduced.
> NOW I'm having fun :-)
>
> ...juerg
>
>
>
> mike wrote:
>
>
> (AD667)
>
>
> (don't
>
>
> frequency
>
>
> multiplexers
>
>
> (TL074C).
>
>
> sawtooth
>
>
> around.
>
>
> CRAP
>
>
> I
>
>
> was
>
>
>
>
> to
>
>
> encoders.
>
>
> the
>
>
> gets
>
>
> to
>
>
> that
>
>
> 1206
>
>
> board,
>
>
> lift
>
>
> done a
>
>
>

quote:

---

> I managed to get rid of the offsets by running the internal signal path
> calibration. The scope is now fully functional and works like a charm
> :-)
> thanks for all the help!
>