mike wrote in :
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...
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
jyanik
at
kua.net
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