(Another) Tektronix TDS540 repair

[masterx81]

Hi!
Years ago i have bought a failed tektronix TD540 scope that i'm tring to repair.
The unit failed due to the notorius capacitor leakage problem.
It's a long time project, as i've bought it years ago, then stopped as i had no more time.
In the last weeks i've put back my hands on, as i want it working.
When i've bought it i wasn't able to do a nvram backup (not remember why), so in any case i have to do at least a field calibration (i'm searching the needed isa pcii iia board). At that time, i've had only a acq. board failing the self test. So by now i'm using an nvram backup found on ko4bb archive.
Weeks ago, when i've put back my hand on it, failed to boot, due to some flashrom errors. So i've tried to do a backup of the flashrom content (desolder and resolder flashrom ics), then an update to the last firmware. The operation failed (i suspect due to a wrong firmware image), and also the backup was not good as reading one chip i have not noticed one little short between 2 pins. While programming the flash ic's, i've noticed that one of them was difficult to verify, so i've had the suspect that my problms were all due to a failed flash ic. So, thanks to an user here on the forum (chronos42), has provided me a new flashrom board with fw 2.09e.
Put it on, and the scope booted normally.
As the acqboard was failing with a really bad error (nothing read by the ram, returned always a lot of 0's, i think a failure with the mux), instead of trying to fix it, i've found another cheap not working TDS540 (missing the firmface board). So i've bought it, and after the cap replacement and a good wash to all the boards, i've tried to power it on with the working firmface board, and absolutely no boot.
So, i've tried to swap boards, and with my old logic board, "new" acq. board, "new" nvram i've booted it, but failed the logic board at the NvLibrariansDiag test. I not know if it's due to a mismatch between the nvram version and th fw version, or if it's caused by a NVram corruption. So, put the nvram with the backup found on ko4bb, and the logic board booted normally.
Now, the "new" acquisition and attenuation board are filing the self test, but with more reasonable errors. I remember that i'm running the calibration values of another unknown scope.... I not know if this can cause an acq. board failing the self test.
The errors that i get are:

ctlDiag ........................ ***FAIL***
..error details:
ERRORID: 149 diagnostic test failure ctlConfidencDiag
** 2.50e-9 <= exp <= 1.20e-8  actual= 0.00e-1 main TI failed

and

dacRangeDiag ................... ***FAIL***
..error details:
ERRORID: 149 diagnostic test failure Dac Range Test
** DAC system failure

The last one seem a failure with the DAC system.
The ctlConfidencDiag i really not know what is.
The scope show the trace on all the channels, with a little bias on CH1.
I've tried to do a probe calibration, and failed with the following error:

ERRORID: 108 Calibration failed Ch1 Probe Gain Error: -31.2%

Then i've tried to do an SPC on CH1, and i've got this errors:

ERRORID: 193 Characterized values out of bounds A/D biasing cal
ERRORID: 193 Characterized values out of bounds Vertical
t50f99ec: Finished characterizing Triggers
ERRORID: 108 Calibration failed Repet Cal Failed

And now, i need to search what is failing...
As already stated, i get correct waveform on all channels.
The scope can trigger, manually, as autosetting the waveform not works.
On CH1, depending on the attenuation setting, i have a varying offset, the other channels seem fine.
Now i play a bit more for trying if i find if at least spc and probe cal works on other channels, and study a bit what the DAC are used for, and if the is a way to chack it, or at least check traces around it for find is something was corroded.

Someone in the meantime have some ideas about my errors?
Thanks!


---EDIT---

Perfect! I have found an interrupted trace around C1641, and now the selftest stops a bit earlier at

vertRegDiag .................... ***FAIL***
..error details:
ERRORID: 149 diagnostic test failure ch3VertShiftRegDiag
** exp data= 1 actual= 0 for bit # 0 for addr: 0x72c021e

And the ch3 trace isn't showed anymore...
Where is the CH3 vertical Shift Register? Maybe it's not related to the trce that i've fixed, but it's nice that fix something, and something other fail...

[shakalnokturn]

Have you downloaded the schematics?

[masterx81]

Yes, i have them and i was studying a bit.
Actually i've related the "main TI failed" error to the Time Interpolator section, and allowed me to find an interrupted trace near c1641. But now the ch3VertShiftRegDiag it's a bit more complicated. I think that the vertical shift register is related to the ad converter. But i've not touched anything in that section trying to repair the Time interpolator section.
Need also to check the service manual.
Also searching for VertShiftRegDiag on the net doesn't find anything...

[masterx81]

 
Maybe i've found what caused the ch3VertShiftRegDiag error...
I've not noticed that one of the back connector (that i never connect) has moved under the acq board, so with much probability has done some short circuits. 
Now i get a sram error at address 0x73c0004, that is a ch4 address.

Using the attached note from the service manual, i not really get how to find the ic number.
4 is 0100, the last 3 bits are 100, so always 4, add to the base u address (U101?) + 4 + 1? So u106?
And how it address U101?

[masterx81]

Really, i'm starting to hate it.
As i've got that memory error at address 0x73c0004 :

digDiagD1Conf .................. ***FAIL***
..error details:
ERRORID: 149 diagnostic test failure acqdataconf
** addr = 0x73c0004  exp data = 0x7fff  actual = 0x80ff
*** WAS TESTING DIG D ***

digDiagConf .................... ***FAIL***
..error details:
ERRORID: 149 diagnostic test failure acqdataconf
** addr = 0x73c0004  exp data = 0x7fff  actual = 0x80ff
*** WAS TESTING DIG D ***

vertAttenStrobeDiag ............ pass
vertAttenShiftReg .............. pass
dacRangeDiag ................... ***FAIL***
..error details:
ERRORID: 149 diagnostic test failure Dac Range Test
** DAC system failure

I've tried to replace the whole 8 sram of the ch4. Now the error is at address 0x73c0000

digDiagD1Conf .................. ***FAIL***
..error details:
ERRORID: 149 diagnostic test failure acqdataconf
** addr = 0x73c0000  exp data = 0x7fff  actual = 0x80f0
*** WAS TESTING DIG D ***

digDiagConf .................... ***FAIL***
..error details:
ERRORID: 149 diagnostic test failure acqdataconf
** addr = 0x73c0000  exp data = 0x7fff  actual = 0x80f0
*** WAS TESTING DIG D ***

vertAttenStrobeDiag ............ pass
vertAttenShiftReg .............. pass
dacRangeDiag ................... ***FAIL***
..error details:
ERRORID: 149 diagnostic test failure Dac Range Test
** DAC system failure

[masterx81]

With all 8 CH4 SRAM removed, it returns:

** addr = 0x73c0000  exp data = 0x7fff  actual = 0x8000

So, wasn't a sram ic failure. As the SRAM ic's are connected directly to the demux ic, by now i think that the short have damaged it.
I'm doing more damage to it than the corrosive action of the electrolyte

---EDIT---

So, new demux, same error:

Code: [Select]

** addr = 0x73c0000  exp data = 0x7fff  actual = 0x8000
Always without ram ic's

---EDIT---

I'm really a bit confused. if it's not the demux, it's not the ram, it's not the pcb (I can't notice any short, and a short can't add a bit to the countings). From the schematics ram and demux are directly connected. What can cause the ch4 to read 0x80ff (1000 0000 1111 1111) instead of 0x7fff (0111 1111 1111 1111)?
The only thing that remain is that the adc is passing the wrong value to the demux. Already checked the lines and resistors from adc to demux, and are all ok.
I can suppose that the first bit (the one "stuck") is the sign, but why when the sign goes to 1, the remaining 7 bits of the byte switch to 0?
And more important,this is an 8 bit scope, why the are 16 lines between ADC and demux (CH4A0~7 and CH4B0~7)? And why the word read is 16 bit? I'm missing something about the architecture.