Electronics > Repair

Tektronix TDS754D - acquisition board test fail

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Rax:
Hi all,
I recently got this TDS754D with issues and am now trying to assess and hopefully repair it.

It'd fail, initially, the startup tests with an acquisition board failure (see first image below), but now it'd cursorily pass the startup tests, but fail its acquisition board checks if I run the full tests. So it seems very clear where the issues lie.

I enclose the log from the recent tests. March 17th is when I got this, so that's the area of interest for my own observations. Per these logs, it seems U207, U208, U214, and U216 seems to be repeat culprits, though there's some odd variability from test to test in what it returns in the failure log.

In rather general observations, CH1 seems the most able to display a waveform (in DPO mode), while the other three seem more cranky. In non-DPO mode, though, things seem pretty stable. DPO mode brings a ton of "snow" and garbage on the screen (see image included).

Here's some thoughts I put together:

* This unit's not impacted by the notorious caps failures in some of the related units. Additionally, I see no leaky caps (at least on the top side)
* I'm considering replacing the ICs mentioned above with 71024S12TYG. Has anyone gone through this? I have a Pace TT-65 which I think fits the bill (though I need to identify and purchase an appropriate tip for the job...)
I welcome input on where I should head next.

denimdragon:
Check your power rails. That power supply is due for some service. I use
to repair a lot of these old scopes. The 754D is one the best in that TDS5,6,7XX series.

Rax:

--- Quote from: denimdragon on March 27, 2024, 03:25:05 pm ---Check your power rails. That power supply is due for some service. I use
to repair a lot of these old scopes. The 754D is one the best in that TDS5,6,7XX series.

--- End quote ---

I also always start with the power supplies in a repair. "Unfortunately," I think in this case they check out. Per Table 6-5 at p.6-66, all rails under load seem to be in spec.

I wonder if others have found the errors logging system to reliably report faulty ICs. I'm trying to assess whether I should trust my error log - indicating likely culprits are U207, U208, U214, and U216 - enough to go ahead and replace them. From what I'm learning, others have needed to dig deeper than just that, in order to nail down the faulty parts. But maybe I'm reading too much (or not keenly enough) in prior casework I was able to find.

TERRA Operative:
I have a TDS794D that had bad RAM chips, it presented as the waveform on the display dropping to zero periodically as the faulty memory cells were stuck at 0.

I have a bunch of spare good chips that I pulled from salvage boards and have tested (I used a Retro Chip Tester Pro), so I can re-test a handful of them and send them your way if you need some.

Rax:

--- Quote from: TERRA Operative on March 28, 2024, 03:03:59 am ---I have a TDS794D that had bad RAM chips, it presented as the waveform on the display dropping to zero periodically as the faulty memory cells were stuck at 0.

I have a bunch of spare good chips that I pulled from salvage boards and have tested (I used a Retro Chip Tester Pro), so I can re-test a handful of them and send them your way if you need some.

--- End quote ---

I'm not convinced that's what I'm seeing here, as to me it seems my symptoms are pretty random; I don't see much of a pattern - though I should probably take a closer look. They're still my main culprits either way. How did you determine which ones were bad?

I appreciate your offer, and will keep it in mind. I think the 71024S12TYG are available in the US at the "big box part stores" - are those reasonable replacements? Not sure what others have tried with good success. Or why you felt you needed to reuse those ICs - other than already having them at hand.

But if you removed and maybe replaced some of these, I could use some direction on how to go about that. What worked, what didn't work so well? I think these are on an SOJ package, which I'm not so familiar with.

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