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.
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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TDS schematics are unpublished and likely not found anywhere.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
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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...
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...
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mike wrote in news: snipped-for-privacy@netscape.net:
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, 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.
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.
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----------------------------------------------- Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA
Jim Adney wrote in news: snipped-for-privacy@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.)
I was thinking about baking large SMD ICs before soldering them in a reflow oven, or with a hot air rework station. If you don't, that can cause cracks in the moulded body and destroy the chip. I saw a couple hundred $80 FIR chips destroyed by assembly because they ignored the warning when the set up production of a new circuit board.
We also baked chips used by rework, then put a descant pack with the chip in a modified carrier and heat sealed it in a moisture proof bag.
A few boards were built with a large chip turned 90 degrees (Or more) The boards were backed prior to rework to lift and rotate the part.
Repair of some equipment will move in this direction before you know it.
Yes, I was talking about professional use. I've looked at the oven in the Ocala store. It would handle most circuit boards and small equipment. I didn't have a tape measure with me to get the inside dimensions. I would like one for my small shop to do multi layer circuit board repairs, but those days are likely behind me.
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!
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