Tektronix TDS754D - acquisition board test fail

[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]

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.

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]

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.

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.

[TERRA Operative]

I knew which banks were bad as only inputs 1 and 4 were affected (Note, the 4 banks of RAM and Acquisition chips DON'T line up with the physical order of input jacks!)
Then from what errors I had, I took some educated guesses as to what actual RAM chips needed to be replaced.
A bit of hot air and a fine soldering tip and I swapped out the chips one by one until I had no more errors or distortion of the displayed trace.

I had some salvaged boards from scrapped scopes so the chips were free, so I tested them and kept all the good ones to use which allowed me to just swap out chips freely until the scope worked.

I can also pull other chips if you need them too. I still have the scrap boards here.


It looks like the RAM chips are about $3.50 each from Digikey/Mouser, but if you want a bunch I won't ask nearly that! Postage from me to you will be pretty ok for a small parcel I think, so PM me and we can compare which way is cheaper if you want.


[EDIT]
I found my notes. The error I had was "ERROR: diagnostic test failure, dsyRastModeV0Walk, raster fail @ 1,1 read: 8, expect: f" and I've attached the display I was getting with no input.
You can see the spikes from the bad RAM. (I said things were stuck at 0, the error message has the correct details, but the RAM was bad regardless).

[shakalnokturn]

RAM seems to be a common failure although I've never encountered it myself.
I don't know if the solders to the RAM MUXed fail, try applying pressure to MUXes and RAM while it's working to see if that affects the displayed traces.

[Rax]

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.

Maybe I was glazed over that too quickly. DCV levels seem fine, but the ripple on the 25V rail doesn't seem so OK to me - see enclosed scope screenshot. This is done with my scope (HP 54522A) over a 10x probe, but my Fluke 189 agrees with this about 160mV level of ripple. If this is not such a smoking gun as it seems to me, I welcome input.

There's some healthy level of disassembly, so I won't go ahead with this tonight after a "week-tail-end" day at work, but I plan taking apart the LV PS board and taking a closer look at what may be off on it. Before doing that, I'll probably also look at the "no load" ripple levels on the various rails (in case some module is spilling this crap on the 25V rail). I am using the "component level" SM from the 520B, which is supposed to have some good resemblance with this scope.

The ripple on the other rails is in the mV ACV range (for instance, about 8mV AC on the 5V rails).

[TERRA Operative]

These PSU's are known for bad caps occasionally, so you may be making a shopping list for new ones soon...
There's also the possibility you have a version of the PSU that absolutely needs at least one cap replaced regardless.

[Rax]

Check your power rails. That power supply is due for some service. I use
to repair a lot of these old scopes.

These PSU's are known for bad caps occasionally, so you may be making a shopping list for new ones soon...
There's also the possibility you have a version of the PSU that absolutely needs at least one cap replaced regardless.

Thank you both for this direction. Is the PS quite as dead on at the core of the whole unit as I'm finding it? I think there's a minimum of two maybe three major other modules that have to come out whichever way I go about this (and I am reading the manual, which has about four different sections on pre-pre and prerequisite to disassembly, and will follow its steps).

Any good tips?
(aside from "stop complaining and get to it!!!"  )

[Rax]

I did exactly what my last statement above indicates and here's a pic of my actual PS board.

[Rax]

On further work on this, I seem to have a PS board (nearly?-)identical with the TDS520B. Yes, that one - the closest scope to this still supported with component-level SM.

So, if I were to believe that, the 25V may - possibly - be a 1V lifted version of the 24V rail on the 520B? The current rating seems to match. I'll examine the corresponding parts.

[Rax]

Further report - replacing C21 (for which I used a UC KYB 1000uF/50V) and C102 (Nichi HE) - the two electrolytics on that rail - hasn't improved anything. The ripple on the +25V is still about 160mV, and the acquisition tests still fail.

I think I'll call it a day and think further where I am with this in the morning.