Author Topic: Tektronix TDS754D - acquisition board test fail  (Read 11007 times)

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Offline RaxTopic starter

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Re: Tektronix TDS754D - acquisition board test fail
« Reply #25 on: May 20, 2024, 02:30:09 am »
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.
 

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Re: Tektronix TDS754D - acquisition board test fail
« Reply #26 on: May 20, 2024, 02:56:10 am »
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.
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Offline RaxTopic starter

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Re: Tektronix TDS754D - acquisition board test fail
« Reply #27 on: May 20, 2024, 03:24:19 am »
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?...  :-\
 

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Re: Tektronix TDS754D - acquisition board test fail
« Reply #28 on: May 20, 2024, 05:43:45 am »
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?
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Offline RaxTopic starter

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Re: Tektronix TDS754D - acquisition board test fail
« Reply #29 on: May 21, 2024, 11:22:05 am »
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.
 

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Re: Tektronix TDS754D - acquisition board test fail
« Reply #30 on: May 21, 2024, 12:11:20 pm »
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.
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Offline RaxTopic starter

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Re: Tektronix TDS754D - acquisition board test fail
« Reply #31 on: January 11, 2025, 04:15:05 pm »
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.
 

Offline RaxTopic starter

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Re: Tektronix TDS754D - acquisition board test fail
« Reply #32 on: January 11, 2025, 04:16:50 pm »
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.
 
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Offline RaxTopic starter

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Re: Tektronix TDS754D - acquisition board test fail
« Reply #33 on: January 11, 2025, 07:31:54 pm »
SPC also completed successfully. This always failed before.
 

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Re: Tektronix TDS754D - acquisition board test fail
« Reply #34 on: January 12, 2025, 04:30:53 am »
Nice.
I reckon run it overnight and give it a bunch of SPC tests too.
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Offline RaxTopic starter

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Re: Tektronix TDS754D - acquisition board test fail
« Reply #35 on: January 12, 2025, 05:31:21 am »
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.
 

Offline RaxTopic starter

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Re: Tektronix TDS754D - acquisition board test fail
« Reply #36 on: January 19, 2025, 01:47:29 pm »
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.
 

Offline Tantratron

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Re: Tektronix TDS754D - acquisition board test fail
« Reply #37 on: January 20, 2025, 02:50:10 pm »
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) ?
 

Offline RaxTopic starter

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Re: Tektronix TDS754D - acquisition board test fail
« Reply #38 on: January 20, 2025, 04:36:17 pm »
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.
 

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Re: Tektronix TDS754D - acquisition board test fail
« Reply #39 on: January 20, 2025, 06:08:40 pm »
I would be inclined to give U208 the flick for a replacement part.
Rule it out before potentially chasing your tail.
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Re: Tektronix TDS754D - acquisition board test fail
« Reply #40 on: February 09, 2025, 02:17:42 am »
Hi,

I was having the memory error on my 754D, U206 and U208 were specified in the log in different test runs. I did initially replace U206 and U208 without success. All the 308pins on the QFP controller had good solder joints. While probing for solder bridges after installing the new chips, I managed to catch two consecutive pins shorted.
 
Turns out this short was NOT a solder bridge, it was a bad SRAM chip with the pins (25-26) internally shorted! Not any one already replaced. I had to remove 7 chips from the gang before finding the bad one.
 
-> Note that the 8 gang is split 4 on top side and 4 on bottom side.
 
It is logical that if two address or data bits are shorted (I did not check de sram chip spec), the diagnostic will fail on the first chip affected.

No more error! Yay.

Cheers!
 
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Re: Tektronix TDS754D - acquisition board test fail
« Reply #41 on: February 09, 2025, 07:12:11 am »
Oh nice catch! I'll keep that in mind for the next repair I do.
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Offline RaxTopic starter

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Re: Tektronix TDS754D - acquisition board test fail
« Reply #42 on: February 09, 2025, 07:41:06 am »
Hi,

I was having the memory error on my 754D, U206 and U208 were specified in the log in different test runs. I did initially replace U206 and U208 without success. All the 308pins on the QFP controller had good solder joints. While probing for solder bridges after installing the new chips, I managed to catch two consecutive pins shorted.
 
Turns out this short was NOT a solder bridge, it was a bad SRAM chip with the pins (25-26) internally shorted! Not any one already replaced. I had to remove 7 chips from the gang before finding the bad one.
 
-> Note that the 8 gang is split 4 on top side and 4 on bottom side.
 
It is logical that if two address or data bits are shorted (I did not check de sram chip spec), the diagnostic will fail on the first chip affected.

No more error! Yay.

Cheers!

Well, that's very interesting, because that confirms my latest observations and the direction I was heading into.

Given I was still getting errors pointing to U208, I removed my replacement, and I was still getting what seemed to be a short between two (neighboring pins) addresses. Here are my notes from about a couple of weeks ago (I haven't had time to go back in there since):

"I checked all of the address lines (say A0 through the four chips on the top of the board, so I picked one chip's pin 12 and checked the other three pin 12s, and so on for all address buses/nodes) and they seem to check out. Then, looking for shorts - "address to address," so A16 to A14, A14 to A12, A12 to A7, etc, neighboring pin to neighboring pin starting at pin 2 - I didn't find any other measurement that'd "beep," other than pin 2 to pin 3, or A16 to A14.

Beeping threw me off, as I expected it to be a solder bridge, but I now think I was dealing with something completely different. It seems to me one chip has some odd condition between its pin 2 and pin 3 (maybe leaky/near shorted ESD protection diodes?)."

With this last sentence, I meant that, upon closer inspection (resistance measurement vs. continuity), I wasn't actually seeing a true short between A16 and A14, but about 10 ohms. Which to me ruled out an actual solder bridge (too large a resistance). So I take your observations as a confirmation that these 10ohms are the sign of an internal fault with one of the chips. Unfortunately, as you note, there's eight chips sharing those address buses, so any one of them could have this condition. I just hope it's not going to be the eight!  ::)
 

Offline RaxTopic starter

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Re: Tektronix TDS754D - acquisition board test fail
« Reply #43 on: February 09, 2025, 07:45:30 am »
I should also add that my current understanding of this is, depending on which data bits get addressed to U208, and specifically at A14 and A16, any one of the (other) chips with issues between pins 2 and 3 will likely cause this fault. So even if the SRAM flagged is U208, this is just indicative of memory allocation being assigned to U208, even if the "quasi-short" occurs at any of the U201-U207 chips. Not doing us any favors!
« Last Edit: February 09, 2025, 08:00:13 am by Rax »
 

Offline RaxTopic starter

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Re: Tektronix TDS754D - acquisition board test fail
« Reply #44 on: February 09, 2025, 07:53:48 am »
What's been suggested to me, and to me makes a great deal of good sense, is to execute a high resolution resistance measurement between pin 2 and pin 3 of U201 through U208, and to try to determine which one chip exhibits the lowest resistance.

Certainly, the difference would be extremely minute, and I never really went ahead with this (other than a couple of tests), as simply contact resistance/oxidization, probe pressure, etc could generate false readings and make this pursuit particularly finicky and unreliable.
 

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Re: Tektronix TDS754D - acquisition board test fail
« Reply #45 on: February 09, 2025, 08:49:16 am »
You need a heat gun and a chip tester. Pull them all off in one go, test them, and throw away the busted one. :D
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Offline RaxTopic starter

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Re: Tektronix TDS754D - acquisition board test fail
« Reply #46 on: February 09, 2025, 09:01:25 am »
You need a heat gun and a chip tester. Pull them all off in one go, test them, and throw away the busted one. :D

Probably a good idea. I am a huge fan of honing into the culprit and busting it pinpointedly, but as sexy as that path is, definitely not always possible.

I do have a Pace unit with SOJ tips, so removing the SRAMs is far easier than resoldering ;) I'd just like to find a path forward not involving a lot of rework (testing in circuit somehow). But maybe that's not feasible.

Regardless, I'm encouraged by getting solid confirmation on the path I was turning to.
 

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Re: Tektronix TDS754D - acquisition board test fail
« Reply #47 on: Yesterday at 01:23:12 am »
@Rax:
I may be wrong but I don't think you could reliably locate the shorted chip by R measurement. If you want to go all-in getto with it, you could inject 0,5V@15A and check the heating sram chip. You flood the chips with iso and check the one evaporating the fastest. That's a technique used by the people repairing apple motherboards.

I suggest just removing the chips with hot air and a lot of flux (amtech). I also have a pace station but for this I use the cheapest hot-air station Amazon.ca (64$C) sells. I have it 5+ years, thinking it would die on me but no, still works.

Be calm, careful and zen, no matter what, it will always be the last chip that you remove that will be the bad one!  ;)  I re-solder the good ones (and new ones) with no added solder, just a lot of flux. Re-soldering the chips requires less heating but careful positioning (the SOJ don't auto-allign as good as BGA, QFN, etc..). You have to be careful heating but the PCBs on these scope are good quality. I know the limit when the flux goes all-in vapor mode.

For me, the key is inspection so I have multiple stereo microscopes. I have one with 20cm viewing distance so I can look at the chip/pads at an angle (not over the bench).

Regards,
 

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Re: Tektronix TDS754D - acquisition board test fail
« Reply #48 on: Yesterday at 02:57:55 am »
-> Just to be clear, I was joking about injecting voltage thru the shorted signals. This can work for shorted power rails but I would not try it for signals.
 

Offline Tantratron

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Re: Tektronix TDS754D - acquisition board test fail
« Reply #49 on: Yesterday at 05:43:17 am »
Just my 2 cents here since we know via GPIB to flash, read, write, access many memories in the CPU board (i.e. tekfwtools, NVRAM, flash). Would it be more efficient and less risky to have a dedicated application to write and read the acquisation SRAM chips. The idea would be to have a C compiled application which would provide an educated guess to identify by remote testing (GPIB) which of the SRAM is failed.
 


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