Repairing Tektronix scope TDS620 front panel PCB

[pathlost]

Hello all,
I have a TDS620 scope. It was working nicely untill it developed a boot error on the front panel board. I am very convinced it was due to not having replaced the electrolytic capacitors on time I hope now is not too late. I replaced the capacitors, but the problem remains. The PCB had some corrosion and I've found two damaged tracks near one capacitor that was leaking (C12). Coincidently, it is this capcitor that has no voltage across after boot up. But I think it is delayed and only becomes active if self test passes (pls confirm..)

After some more tests and verifications, I cannot find anymore damaged tracks or vias. Now, it seems pretty much about damaged components.

I have found the schematics on the web for the TDS520B which is very similar:
http://www.austexsoftware.com/documents/tektronix/service/070971003%20TDS520B%20Mod%20CM%20Digitizing%20Oscilloscope%20Component%20Service%20Manual.pdf

On the last pages you can find the Front Panel board schematics, page 5-126 onwards. The front panel is almost identical, only difference I could spot is the main board processor. The shematics depict "68HC05B4" and my board has a Motorola SC408068FN, but they seem pin-to-pin compatible. The Front panel is not very complicated and besides the processor the other chips are mostly 74HC logic chips, so quite cheap.

Back to problem, I am convinced - looking at the schematic - that the lack of voltage in C12 is not the cause but just a symptom. The cause is elsewhere.
What I know:

- C12 is zero volt across, but the signal that controls the voltage on it is always ZERO. It comes from U3 (R90) and depends on the processor.
- Screen keys DO NOT work, except power on.
- Front panel keys: some work, like AUTO-SET and others don't.
- POTs: I could only see the TRIGGER Level work

All these keys and pots are combined in the 74HC logic chips and some processor lines. I find strange the possibility of having several 74HC (U3, U4, U8, U9) damaged at once. The only common chip is the processor, but I also find it strange to be just partly damaged and caused by bad capacitors.

I did power on the scope once with the FAIL on this board already present...

Any clues? How often do we see Front panel processor or 74HC logic chips damaged?

[pathlost]

About the lack of voltage across C12.

- I've followed the schematics for TDS520 front panel and I'm convinced that this voltage only appears if the scope passes self test, or at least after one presses "clear menu". This voltage depends on micro line "RB7". I'm now convinced the front panel is repaired - it realy had some leaking capacitors and bronken tracks. All remaning problems are elsewhere.

BUT:

- I discovered the FLEX ribbon cable that connects the display front keys to the front panel PCB has 2 broken traces, which causes "clear menu" and other keys to not work.
- The self test is pointing to Acquisition "FAIL ++ Attn/ Acq Interface", but I also have recapped this board some years ago.
-

Questions:
1. what is the best way to repair a Flex ribbon cable?
2. what do you think about an Acquisition board that worked ok for 3 years after recap and now gets again the FAIL on selft test? I used good quality PANASONIC low ESR capacitors from Farnell. The scope seem to work ok, except for the keys on display.

[nctnico]

Perhaps you didn't clean the boards well enough so electrolyte has been eating away the PCB traces.

[pathlost]

yes, could be...
But I've found 2 capacitors on the front panel that were clearly under rated in the original design by Tektronix. They were C22 and C23, 33uF /10V parts and I read 15 V across them!

Could be that the acquisition board also has some under-rated parts? Has anyone found more than 5V on the 10V capacitors?

[nctnico]

Good question. I know about the front panel but I'm not sure whether the acquisition and CPU board also have capacitors with excess voltage across them. When replacing capacitors I used (IIRC) 25V types just to be safe.

[pathlost]

I've found somewhere reference to caps 1603 and 1693 on the acquisition board having also overvoltage. I didn't know about this issue when I replaced, or sure would have also selected 25V or at least 16V parts...

I'm thinking also in removing the acquisition and wash it again with I.P.A., but before I do that, just a basic question:
- I just have the FAIL error on the Attn/ Acq Interface, not actually on the Aquisition line on self test. What do you think this means? Could it be some failure on the attenuators mini board? That one does not have any electrylitc capacitors and I wonder what could have caused any problem, since the error appeared after many mothns of inactivity...

[nctnico]

I think you need to wash and check your boards again because it seems a trace has been interrupted. See if you can find stuff with comparable ingredients as what they sell under the name 'simple green' in the US. In the NL I buy 'St Marc' which is a very strong de-greaser soap. It works better for removing dirt from boards than IPA.

I never read anything about the attenuators causing the error messages so you better leave those alone.

[pathlost]

Hello,

Thanks for your replies, they are useful.
Yes, I think I'll follow your advice. The first time though, I also used a detergent before final cleaning with IsoProp. Alcohol. I think I used window cleaner, but I really can't remember.

About the problem, I powered on just one more time to see the problem that Attn/ Acq Interface Fail refers to. I've noticed that both CH2 and Aux CH2 have a kind of "moving DC offset". Since I don't have all keys in front panel operational, the test was limited, but applying the osc test/cal square wave to CH2 or Aux Ch2, I see the bottom of the square wave to remain more or less on same spot, when moving the vertical scale knob. Only the top changes. This cannot be, is lilke the DC offset changes when we change the vertical scale knob.  I wanted to turn off quickly, so no more details.

It would help to have a schematic one could follow.... does anyone have a clue of each IC area are suspect for this kind of problem?
BTW, maybe better to change the title, as the front panel problem is down to the flex cable and the more serious problem is on CH2/ Auc Ch 2 DC offset.

EDIT: I've just found simple green near me! I will buy it, any advices on how to use it?
 

[nctnico]

Look at the TDS520B component service manual. AFAIK Tektronix has used a similar scheme to create many outputs from one DAC by using CMOS switches and TL072-ish opamps as a buffer. The offset on one channel going bad indicates there are issues on the circuit board.

[pathlost]

ok, I managed to find "simple green" (the original) here in Portugal! Before trying to do any electrical repair, it is better to wash again the Acq. board. Who knows, might be some conducting dirt between pads..

So
a) I diluted some simple green in an aprox. ratio of 1:4 in tap water - what dilution ratio do people advise to use with simple green for this purpose?
b) I brushed during some 10 mins both sides of board, but mainly the top. Tried to push water with simple green beneath the ICs. I let the board quietly "swimming" during some more 10 mins.
c) I took the board out of the pool and washed with running water, the tap with most speed I could find. Tried to push the most out of the ICs and everything
d) I let the board dry a little to be free of most of the water
e) I then arranged a different pool with I.P.Alcohol and submerged the board in it. I brushed some more the surface of the board. I did thos step 2 times.

f) The board is now drying at normal temperature. I will let it dry also tommorrow and then will test to see if the error went away.

This was the Acquistion board. I think I will do the same with the processor board, although it does not report any errors.
However, I am in much doubt about the Front Panel board. This was the last one to have the capacitors replaced and some tracks are in bad shape. I just cleaned with IPA so far, and very locally. It would deserve some "simple green" cleaning for sure, but... It has mechanical parts!! All the POTs of the front panel are there.

Would it do more damage than good applying simple green to them? And even IPA, is it ok?
Has anyone had problems cleaning the front panel PCB?