Tektronix TDS754D - acquisition board test fail

[Rax]

BTW - not to jinx it, but I feel stronger and stronger that replacing those four SRAMs was called for, because there seems to be just one error now and those RAMs don't pop in there at all anymore.

[TERRA Operative]

Yeah, looks like you had a few errors to deal with, at least you are making progress!

I'll have a search and think about your current error. My feeling is to either track down that address to a RAM chip and replace it as well (if not already done) or figure out what is controlling that RAM chip and stare intently at that.
I'd expect that if it was the chip passing the data to the RAM chips, it would be a periodic error across multiple chips rather than just one RAM chip in that case due to how the RAM is written to with bank switching etc.

One way to find the right RAM chip is to ground/apply 5V (have to double check which one) to the WE (Write Enable) pin one chip at a time and see what errors are thrown on self test due to that chip not being enabled to be written to.
If you get new errors on the same address range as your latest error, you found the chip.

[Rax]

After running SPC, I got:

"SUN MAY 19 20:07:47 2024  ERROR: Internal adjustment range exceeded, ( 3 @   7464): DIGc gain = -0.400113, bal = 1.113079"

Bad news?... 

[TERRA Operative]

oooeerrrr.... Either bad cal (unlikely) or something is bad on that channel. Maybe as simple as a bad relay on the hybrid. It's not too hard to swap hybrids between channels, just need to be extra careful not to crack the ceramic PCB.
Relays aren't toooo difficult to replace, I have a video on Youtube about it.
youtube.com/watch?v=b0P1sJBomDQ

Is that the only error after clearing via GPIB?
Can you run SPC maybe 5 or 6 times to exercise the relays, then clear all errors, self test and SPC once more and list the entire error log here?

[Rax]

something is bad on that channel.

Would that be CH3? The error body is a bit cryptic to me.

Is that the only error after clearing via GPIB?

Those two errors above, yes. The second error occurs if I run SPC.

Can you run SPC maybe 5 or 6 times to exercise the relays, then clear all errors, self test and SPC once more and list the entire error log here?

Been doing a few of those, and the error seems pretty persistent.

I should also say, I don't feel there's anything very specifically wrong going on with CH3 - if that's the one found faulty by SPC. If anything, CH4 seems a bit choppier and there's a bit more freezing and vertical bars "noise." But they all work pretty well, their measurements are accurate as much as my other meters think. It just fails tests. Oh, and DPO mode doesn't work very well - there's a good amount of "video noise" - color pixelation ("snow") on the screen when that's enabled.

[TERRA Operative]

I think the firmware sees the chanel numbering as 0 indexed, i.e. 0, 1, 2, 3 corresponds to physical channels 1, 2, 3, 4. So I would look for problems on channel 4

Try swapping the channel 1 and channel 4 hybrid (don't forget the thermal paste on the chip on the hybrid module!) and see what happens.
I keep saying it and will say it again.... Be careful with the hybrids, and make sure they are perfectly seated in the metal holder before screwing it to the PCB!

Easy tests are easy, so may as well rule out bad relays or bad hybrid module.

[Rax]

Update on this, as only now I managed to work my way down the projects backlog and get back to this).

Putting this back on the bench, I got the error above, but also a new error originating in U208. Suspecting I may have some soldering issues - I absolutely don't love those SOJ packages one bit - I tested continuity pin to pin and indeed I had a short between two of them.

Once I fixed that, it seems now to pass all tests, startup or the full range from the Utilities (before this last fix, it would fail both). Given the original issue was reported to occur after extended running (hours), I'll hold back on popping that cork, but I'm hopeful this is it.

I think this failure mode occurs due to the heat from U200 nearby, which cooks those SRAMs in time.

[Rax]

An effusive thanks to TERRA Operative here for his sending me the SRAM ICs (used, but tested) to repair this. Much appreciated! Members like him make this a great place to hangout.

[Rax]

SPC also completed successfully. This always failed before.

[TERRA Operative]

Nice.
I reckon run it overnight and give it a bunch of SPC tests too.

[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.

[Tantratron]

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.

Hello, I look at the 2nd picture which you posted but maybe i'm wrong, what makes you think it is a memory issue from the acquesition board ?
Could it be so graphic video processor generation or some video plane in failure (i.e. grid and text OK but signal KO) ?

[Rax]

Hello, I look at the 2nd picture which you posted but maybe i'm wrong, what makes you think it is a memory issue from the acquesition board ?
Could it be so graphic video processor generation or some video plane in failure (i.e. grid and text OK but signal KO) ?

That may be possible, but I'm focusing on the acquisition board because:

• Running all tests indicates an acquisition module failure. This has always been the case (other modules have been passing the tests).
• The error code indicates a memory address fault, pointing  to U208. See the first screenshot.

Not sure if the diagnostic utility can point to a different module/part than what's actually causing the failure, but I assume I need to fix that before tackling anything else. Besides, after replacing the four SRAMs in the original errors (again, pointing specifically to the acquisition board and namely U207, U208, U214, U216), I had a fully functional system able to pass SPC. I'm not sure why it failed again after that, but determining the cause of that would probably mean identifying the smoking gun here.

[TERRA Operative]

I would be inclined to give U208 the flick for a replacement part.
Rule it out before potentially chasing your tail.