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.pdfOn 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?