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Hi all,

I recently acquired a TDS794D and am having an issue with Channel 2 and a slight issue on Channel 1.
It was originally failing the boot self-test with a "ERROR: diagnostic test failure, dsyRastModeV0Walk, raster fail @ 1,1 read: 8, expect: f" error which I assume is probably a video RAM or other video error, but I had a spare Processor board from a TDS754D with other faults that I swapped in and flashed the TDS794D firmware on, all working ok now.

Next is Channel 2 and Channel 1. I am getting spurious signals on these channels, Channel 2 in all settings, and Channel 1 in only one setting.
Linked image is Channel 2 with no signal input. If I connect a signal, the signal is shown with these spikes overlaid.



I'm hoping this is a RAM issue as I have a bunch of RAM chips I have pulled from scrap acquisition boards.
Has anyone seen this fault before and knows the likely fix?


Thanks!
Jared

Running the full self test from the utility menu should reveal whether any memory chips are bad. Looks like it could be faulty ram. I've replaced a lot of the 1Mb parts over the years, for some reason they have a high failure rate.
Jay

I ran the self test and got this error:

"ERROR: diagnostic test failure, digDataFormatDiag, ERROR!!BYTE mode, in demux 198 ,i= 20 memBaseAdr 0x730002a= data = ffffb400, expectedData[i] = 3400"

Looks like bad RAM I think... I suspect that ffff is what's making the spikes on the display, pegging the data out at maximum for that value. So next step is to try to figure out what chip contains address 0x730002a so I can avoid shotgun replacing these RAM chips..

If you run the self test enough times, it may give you a reference designation of the at-fault chip IE U413.
Keep in mind that the DMUX chips for each channel are not consecutive. From the edge of the board where all the high density connectors are, it goes CH3, CH4, CH1, CH2. So you'll be working the banks nearest to the 4 coax cables from the rear panel. I've had some of these scopes with multiple bad 1Mb chips! I usually replace 4 at a time then test again.
Here are some experiments I did on a 2M enabled scope on CH2:

CH2: shorting *WE on each bank:
component side of board.
U405: "WED JUL 13 12:51:08 2016 ERROR: diagnostic test failure, digDataFormatDiag, ERROR!!BYTE mode, in demux 198 ,i= 4 memBaseAdr 0x730000a= data = ffffb500, expectedData[i] = aa00";""
U413: "WED JUL 13 09:28:51 2016 ERROR: diagnostic test failure, digDataFormatDiag, ERROR!!BYTE mode, in demux 198 ,i= 12 memBaseAdr 0x730001a= data = ffffa500, expectedData[i] = ff00";""

Now solder side of board:
U409: "WED JUL 13 09:42:06 2016 ERROR: diagnostic test failure, digDataFormatDiag, ERROR!!BYTE mode, in demux 198 ,i= 8 memBaseAdr 0x7300012= data = 1a00, expectedData[i] = ff00";""
U401: "WED JUL 13 09:39:35 2016 ERROR: diagnostic test failure, digDataFormatDiag, ERROR!!BYTE mode, in demux 198 ,i= 0 memBaseAdr 0x7300002= data = ffffc400, expectedData[i] = 0";""

CH2 experiments:
NOTE: References to DAT[x] are the pinouts from the chip datasheet.
The Data lines on the top two banks are in parallel.
Tied DAT3 (U413 –17) on comp. Side of board to +5:
"WED JUL 13 12:35:20 2016 ERROR: diagnostic test failure, digDataFormatDiag, ERROR!!BYTE mode, in demux 198 ,i= 20 memBaseAdr 0x730002a= data = 3c00, expectedData[i] = 3400";""
Tied DAT3 (U414 –17) on comp. Side of board to +5:
"WED JUL 13 12:39:41 2016 ERROR: diagnostic test failure, digDataFormatDiag, ERROR!!BYTE mode, in demux 198 ,i= 21 memBaseAdr 0x730002c= data = 1a00, expectedData[i] = 1200";""
Tied DAT3 (U415 –17) on comp. Side of board to +5:
"WED JUL 13 12:41:23 2016 ERROR: diagnostic test failure, digDataFormatDiag, ERROR!!BYTE mode, in demux 198 ,i= 6 memBaseAdr 0x730000e= data = 5d00, expectedData[i] = 5500";""
Tied DAT3 (U416 –17) on comp. Side of board to +5:
"WED JUL 13 12:45:34 2016 ERROR: diagnostic test failure, digDataFormatDiag, ERROR!!BYTE mode, in demux 198 ,i= 7 memBaseAdr 0x7300010= data = 5d00, expectedData[i] = 5500";""

Great, thanks for the info!
I replaced U413 and now have an error on address 0x730000a, so I think there are a number of bad RAM chips to chase down...

Can you provide more info from your experiments? If you could list all the info you have as you did previously, it will save me having to run through and do it myself.. :)


Thanks!
Jared

And after replacing U405, I now have a fully working unit. Hooray! :)

Thanks Jay for the assistance.


Jared

That's great! Glad to have been of help.
Jay

Do you mind sharing the results of your RAM exploration for all RAM chips on all channels? It would be useful info to have for future reference. :)

Thanks!
Jared

I'm reviving this thread, because I seem to have a very similar issue and am not sure how to progress.

This is a 754D that had a plethora of errors, most of which got solved by replacing SRAMs serving U200 (so, if the sequencing described by Jay applies to this acquisition board, that'd be CH4, but it seems to impact all channels apparently). The root cause was (possibly alongside other types of failures of the SRAMs) a low-resistance condition between pins 2 and 3 of those SRAMs (corresponding to A16 and A14, all addresses being in parallel across the eight ICs) of about 10 ohms. I had to remove seven of the eight chips before this condition was eliminated (one original SRAM left on that channel). Once I did that, all errors pointing to specific U201 through U216 went away.

I've also reflowed all pins for U200 to eliminate cold joints being as a possible cause.

I have on error left: "diagnostic test failure, digDataFormatDiag, ERROR!!BYTE mode, in demux 200 ,i= 4 memBaseAdr 0x738000a= data = fffffa00, expectedData[i] = aa00" This seems to generate some spurious peaks, just like Jared seemed to experience (I'll enclose a picture shortly).

Not sure what else to look for to narrow down the issue left.
Radu.

I added a couple of pictures here:
/g/TekScopes/album?id=301426. Please note:

1. CH 1 seems to be able to display the square wave from the
test/adjustment terminals.
2. CH2 and CH3 display a flatline, but no square wave (not quite sure
why).
3. CH4 - the one with the main issues, by all signs - doesn't display a
waveform at all.

Radu.

On Sun, Mar 23, 2025 at 9:37?AM Radu Bogdan Dicher via groups.io <vondicher=
[email protected]> wrote:

I'm reviving this thread, because I seem to have a very similar issue and
am not sure how to progress.

This is a 754D that had a plethora of errors, most of which got solved by
replacing SRAMs serving U200 (so, if the sequencing described by Jay
applies to this acquisition board, that'd be CH4, but it seems to impact
all channels apparently). The root cause was (possibly alongside other
types of failures of the SRAMs) a low-resistance condition between pins 2
and 3 of those SRAMs (corresponding to A16 and A14, all addresses being in
parallel across the eight ICs) of about 10 ohms. I had to remove seven of
the eight chips before this condition was eliminated (one original SRAM
left on that channel). Once I did that, all errors pointing to specific
U201 through U216 went away.

I've also reflowed all pins for U200 to eliminate cold joints being as a
possible cause.

I have on error left: "diagnostic test failure, digDataFormatDiag,
ERROR!!BYTE mode, in demux 200 ,i= 4 memBaseAdr 0x738000a= data =
fffffa00, expectedData[i] = aa00" This seems to generate some spurious
peaks, just like Jared seemed to experience (I'll enclose a picture
shortly).

Not sure what else to look for to narrow down the issue left.
Radu.