TDS754C SPC failure / Characterized values out of bounds Ch2 trigger gain

Hello,

I recently bought a TDS754C as a broken equipment from a local 2nd hand online dealer, with high voltage power supply broken. I replaced the CRT display with an LCD module which I was quite pleased with. However, after running full diagnostics and doing some further tests, I noticed CH2 is giving errors and SPC consistently fails. I obtained a serial debug convertor and connected a debug console connection. I found this a lot more practical than GPIB for diagnostics purpose, though its everyday use is extremely limited, and probably dangerous.

Anyways, obviously the issue is with the acquisition board and I found some relevant threads online on this matter. Since, the old forum.tek.com is being decommissioned, I'm not able to register to the old forum user database. 

Based on this thread below, I'm receiving exact identical symptoms for only CH2:
https://forum.tek.com/viewtopic.php?f=568&t=142841&sid=49e96f00853d7dc8ea3c0b335afff5f1&start=20

Logs wise I'm receiving the same errors, as indicated only for CH2
When I feed a DC or AC signal into CH1, CH3, CH4 I have everything as it should be, trigger works, measurements are more or less precise (50ohm/1M impedance switching works fine too)
But, when the same signal is applied to CH2, the amplitude of the signal changes based on the offset location of the signal on screen.
When vertical position of the signal is at the bottom of the screen, amplitude is much lower than the input signal and when vertical position is at top it's same or more than applied signal.
I went through schematics, and somehow I'm inclined to think this is a specific issue with the attenuator on CH2 only and others seem to be fine. Though errors include ;


CH2 (at 20 MHz) varigain DAC failed at    0.001 volts per div by   0.7076
CH2 (at 20 MHz) varigain DAC failed at  0.00101 volts per div by   0.6937
CH2 (at 20 MHz) varigain DAC failed at    0.002 volts per div by   0.7056
CH2 (at 20 MHz) varigain DAC failed at  0.00199 volts per div by 0.008398
CH2 (at 20 MHz) varigain DAC failed at  0.00202 volts per div by   0.6917
CH2 (at 20 MHz) varigain DAC failed at    0.005 volts per div by   0.6999
CH2 (at 20 MHz) varigain DAC failed at  0.00505 volts per div by   0.6861
CH2 (at 20 MHz) varigain DAC failed at     0.01 volts per div by   0.6983
CH2 (at 20 MHz) varigain DAC failed at  0.00995 volts per div by 0.004552
CH2 (at 20 MHz) varigain DAC failed at   0.0101 volts per div by   0.6845
CH2 (at 20 MHz) varigain DAC failed at     0.02 volts per div by   0.7004
CH2 (at 20 MHz) varigain DAC failed at   0.0199 volts per div by 0.003729
CH2 (at 20 MHz) varigain DAC failed at   0.0202 volts per div by   0.6865
CH2 (at 20 MHz) varigain DAC failed at     0.05 volts per div by   0.6931
CH2 (at 20 MHz) varigain DAC failed at   0.0505 volts per div by   0.6793
CH2 (at 20 MHz) varigain DAC failed at   0.0995 volts per div by 0.001121
CH2 (at 20 MHz) varigain DAC failed at    0.101 volts per div by   0.6814
CH2 (at 20 MHz) varigain DAC failed at      0.2 volts per div by   0.6973
CH2 (at 20 MHz) varigain DAC failed at    0.199 volts per div by 0.002166
CH2 (at 20 MHz) varigain DAC failed at    0.202 volts per div by   0.6835
CH2 (at 20 MHz) varigain DAC failed at      0.5 volts per div by     0.69
CH2 (at 20 MHz) varigain DAC failed at    0.505 volts per div by   0.6763
CH2 (at 20 MHz) varigain DAC failed at     1.01 volts per div by   0.6847
CH2 (at 20 MHz) varigain DAC failed at        2 volts per div by   0.7006
CH2 (at 20 MHz) varigain DAC failed at     1.99 volts per div by 0.003816
CH2 (at 20 MHz) varigain DAC failed at     2.02 volts per div by   0.6867
CH2 (at 20 MHz) varigain DAC failed at        5 volts per div by   0.6933
CH2 (at 20 MHz) varigain DAC failed at     5.05 volts per div by   0.6795
CH2 (at 20 MHz) varigain DAC failed at     9.95 volts per div by 0.001207
CH2 (at 250 MHz) varigain DAC failed at    0.001 volts per div by     1.26
CH2 (at 250 MHz) varigain DAC failed at  0.00101 volts per div by     1.24
CH2 (at 250 MHz) varigain DAC failed at    0.002 volts per div by    1.257
CH2 (at 250 MHz) varigain DAC failed at  0.00199 volts per div by   0.2858
CH2 (at 250 MHz) varigain DAC failed at  0.00202 volts per div by    1.238
CH2 (at 250 MHz) varigain DAC failed at    0.005 volts per div by    1.249
CH2 (at 250 MHz) varigain DAC failed at  0.00498 volts per div by  0.08721
CH2 (at 250 MHz) varigain DAC failed at  0.00505 volts per div by     1.23
CH2 (at 250 MHz) varigain DAC failed at     0.01 volts per div by    1.247
CH2 (at 250 MHz) varigain DAC failed at  0.00995 volts per div by   0.2805
CH2 (at 250 MHz) varigain DAC failed at   0.0101 volts per div by    1.228
CH2 (at 250 MHz) varigain DAC failed at     0.02 volts per div by     1.25
CH2 (at 250 MHz) varigain DAC failed at   0.0199 volts per div by   0.2793
CH2 (at 250 MHz) varigain DAC failed at   0.0202 volts per div by     1.23
CH2 (at 250 MHz) varigain DAC failed at     0.05 volts per div by     1.24
CH2 (at 250 MHz) varigain DAC failed at   0.0498 volts per div by  0.08428
CH2 (at 250 MHz) varigain DAC failed at   0.0505 volts per div by     1.22
CH2 (at 250 MHz) varigain DAC failed at      0.1 volts per div by   0.2708
CH2 (at 250 MHz) varigain DAC failed at   0.0995 volts per div by   0.2757
CH2 (at 250 MHz) varigain DAC failed at    0.101 volts per div by    1.223
CH2 (at 250 MHz) varigain DAC failed at      0.2 volts per div by    1.245
CH2 (at 250 MHz) varigain DAC failed at    0.199 volts per div by   0.2772
CH2 (at 250 MHz) varigain DAC failed at    0.202 volts per div by    1.226
CH2 (at 250 MHz) varigain DAC failed at      0.5 volts per div by    1.235
CH2 (at 250 MHz) varigain DAC failed at    0.498 volts per div by  0.08253
CH2 (at 250 MHz) varigain DAC failed at    0.505 volts per div by    1.216
CH2 (at 250 MHz) varigain DAC failed at        1 volts per div by   0.2687
CH2 (at 250 MHz) varigain DAC failed at    0.995 volts per div by   0.2735
CH2 (at 250 MHz) varigain DAC failed at     1.01 volts per div by    1.228
CH2 (at 250 MHz) varigain DAC failed at        2 volts per div by     1.25
CH2 (at 250 MHz) varigain DAC failed at     1.99 volts per div by   0.2795
CH2 (at 250 MHz) varigain DAC failed at     2.02 volts per div by    1.231
CH2 (at 250 MHz) varigain DAC failed at        5 volts per div by     1.24
CH2 (at 250 MHz) varigain DAC failed at     4.98 volts per div by  0.08437
CH2 (at 250 MHz) varigain DAC failed at     5.05 volts per div by    1.221
CH2 (at 250 MHz) varigain DAC failed at       10 volts per div by    0.271
CH2 (at 250 MHz) varigain DAC failed at     9.95 volts per div by   0.2758
CH2 (at 250 MHz) varigain DAC failed at       10 volts per div by    0.271
CH2 (at Full) varigain DAC failed at    0.001 volts per div by    1.259
CH2 (at Full) varigain DAC failed at  0.00101 volts per div by     1.24
CH2 (at Full) varigain DAC failed at    0.002 volts per div by    1.257
CH2 (at Full) varigain DAC failed at  0.00199 volts per div by   0.2857
CH2 (at Full) varigain DAC failed at  0.00202 volts per div by    1.237
CH2 (at Full) varigain DAC failed at    0.005 volts per div by    1.249
CH2 (at Full) varigain DAC failed at  0.00498 volts per div by  0.08708
CH2 (at Full) varigain DAC failed at  0.00505 volts per div by    1.229
CH2 (at Full) varigain DAC failed at     0.01 volts per div by    1.246
CH2 (at Full) varigain DAC failed at  0.00995 volts per div by   0.2803
CH2 (at Full) varigain DAC failed at   0.0101 volts per div by    1.227
CH2 (at Full) varigain DAC failed at     0.02 volts per div by    1.249
CH2 (at Full) varigain DAC failed at   0.0199 volts per div by   0.2792
CH2 (at Full) varigain DAC failed at   0.0202 volts per div by     1.23
CH2 (at Full) varigain DAC failed at     0.05 volts per div by    1.239
CH2 (at Full) varigain DAC failed at   0.0498 volts per div by  0.08415
CH2 (at Full) varigain DAC failed at   0.0505 volts per div by     1.22
CH2 (at Full) varigain DAC failed at      0.1 volts per div by   0.2707
CH2 (at Full) varigain DAC failed at   0.0995 volts per div by   0.2755
CH2 (at Full) varigain DAC failed at    0.101 volts per div by    1.223
CH2 (at Full) varigain DAC failed at      0.2 volts per div by    1.245
CH2 (at Full) varigain DAC failed at    0.199 volts per div by    0.277
CH2 (at Full) varigain DAC failed at    0.202 volts per div by    1.226
CH2 (at Full) varigain DAC failed at      0.5 volts per div by    1.235
CH2 (at Full) varigain DAC failed at    0.498 volts per div by  0.08241
CH2 (at Full) varigain DAC failed at    0.505 volts per div by    1.216
CH2 (at Full) varigain DAC failed at        1 volts per div by   0.2685
CH2 (at Full) varigain DAC failed at    0.995 volts per div by   0.2734
CH2 (at Full) varigain DAC failed at     1.01 volts per div by    1.227
CH2 (at Full) varigain DAC failed at        2 volts per div by     1.25
CH2 (at Full) varigain DAC failed at     1.99 volts per div by   0.2793
CH2 (at Full) varigain DAC failed at     2.02 volts per div by     1.23
CH2 (at Full) varigain DAC failed at        5 volts per div by    1.239
CH2 (at Full) varigain DAC failed at     4.98 volts per div by  0.08424
CH2 (at Full) varigain DAC failed at     5.05 volts per div by     1.22
CH2 (at Full) varigain DAC failed at       10 volts per div by   0.2708
CH2 (at Full) varigain DAC failed at     9.95 volts per div by   0.2757
CH2 (at Full) varigain DAC failed at       10 volts per div by   0.2708

ERRORID: 209 Characterized values out of bounds Ch2 trigger gain
ERRORID: 209 Characterized values out of bounds Ch2 trigger gain
ERRORID: 110 Calibration failed Triggers
I hope the issue is not DAC related. Though, attenuator IC might be faulty either (due to some high voltage being applied previously). I cannot pinpoint. Can you please assist me to troubleshoot this and point me to the right direction please?

Kind regards,

Berkin

Hi Strick,

Thank you so much for your response. I have an Agilent GPIB-usb cable, I can try the procedure to exercise the relays. Is it possible for you to share your script? 
i was considering that dismantling the attenuator section might be needed, prepared all the equipment already. But didn't want to do this without an explicit advise. This will be fun :)

I checked the relays and compared the "clicks" between channels. CH2 seems to be clicking same the count as other channels. However this doesn't say anything about the oxidation on relay contacts of course.

Thanks

​​​​​Berkin

sure,
# Note: The first two commands, shown below behind the hash symbols, only need to
# be executed once.
#
# Also, the entries shown assume your 'scope is set at GPIB Address 1 and your GPIB
# card is address 0. Alter the address values appropriately (second digit in the ibdev
# command) if you're using a different setup.
#
#ibfind gpib0
#ibdev 0 1 0 13 1 0
#
ibwrt "CH1:VOLTs 100E-03"
ibwrt "CH2:VOLTs 100E-03"
ibwrt "CH3:VOLTs 100E-03"
ibwrt "CH4:VOLTs 100E-03"
ibwrt "CH1:VOLTS 100E-02"
ibwrt "CH2:VOLTS 100E-02"
ibwrt "CH3:VOLTS 100E-02"
ibwrt "CH4:VOLTS 100E-02"
ibwrt "CH1:VOLTS 100E-01"
ibwrt "CH2:VOLTS 100E-01"
ibwrt "CH3:VOLTS 100E-01"
ibwrt "CH4:VOLTS 100E-01"
ibwrt "CH1:VOLTS 100E-02"
ibwrt "CH2:VOLTS 100E-02"
ibwrt "CH3:VOLTS 100E-02"
ibwrt "CH4:VOLTS 100E-02"

you save the above to a file (e.g. exer.txt) and then hook up a gpib and run run in a dos or dos window the ibic command: ibic <exer.txt  >con.  I usually cheat and run the ibdev command by itself and just invoke ibic < exer.txt > con without the ibfind and ibdev commands.  also, if you only want to exercise one or two channels, get rid of the others to make it faster to run.  When run several times, the gpib commend on my computer seems to get confused and will stop and give an error. just reboot and keep going.

the main thing you are doing is changing the voltage to exercise the relays from 100 to 200 mv and 1 to 2 volts. 

Good luck!   Strick

Oh, and if you do decide to open up the attenuator, send me a PM and I'll give important instructions to help you from damaging something...

Strick