Electronics > Repair

Tektronix TDS754D - acquisition board test fail

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Rax:
It worked flawlessly for part of the afternoon, on and off. Then tonight I fired it up again, and it gives me the artifacts on screen (see below) and essentially the same error regarding U208 as before (see below), just with a slightly different address.

Back to the drawing board. I feel I need to check U208 thoroughly for input/address and the rest of the pins. Maybe replace it with another one I have here, just in case this one's taken a dive.

Rax:
Some progress last night.

I'm now not sure how I fixed the short or what exactly is going on, I've found a "short" between what I think are another two pins of U208 (but always on the address pins). I think last time it was 3 and 4 or maybe 4 and 5. It's possible that was indeed a solder bridge.

But this time around my meter beeped a short between 2 and 3 (A14 and A16). Per 520B component level SM, the address buses are shared between U201 through U208, and then separately U209 through U216. So my "pin 2 and 3" short could be with my recently replaced U208 (where the utility flags it), but could also be originating at U207.

I removed U208, and I have the same "short." I call it "short" because in closer examination resistance there is almost 10 ohms, which I can't imagine is caused by a solid short, even at a relatively remote point from where I measure it (so through traces). Besides, it's 10 ohms at the U207 pins as well as on the currently free pads of U208 (it being lifted).

Any ideas welcome.

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