Update: Keithley 175 Overvoltage during Calibration

[zaphi]

Hi,
I made a stupid mistake  I step through the Calibration of DC 1,9V to 950V and repeated this in AC :-( of course this was not a good idea.
I got some strange noise out of the Keithley and the Fluke Calibrator get an overload error.
now I am getting only strange readings.

any idea which component I should check fist?
a Schematic can be found her
https://www.utwente.nl/en/tnw/slt/documentation/Equipment/GeneralEquipment/keithley175.pdf

hope for some suggestions
best regards form Germany
Tobias

[coromonadalix]

i would check page 63    every input components  from the volt ohms amp  section

[zaphi]

did some further investigations.
After opening the case and inspecting the components i did not find any burn marks or visual damaged parts :-(
If i power up the unit in mega ohms mode with open circuit it starts at 15 mega ohm and clim up till arround 20 mega ohms...
shorting the terminals result in arround 2 ohms Looks like Switch range and Auto ranging is working.

i read about troubleshooting mode in the manual but i can not anderstand it fully. Entering troublshooting works fin but what should i check?

[zaphi]

just a quick follow up about my Keithley 175
after a lot testing and bit of contact spray into the push buttons i get a reading responding to my bench power suply but way off...
after studying the shcematic and probing around i think U103 the op-amp for the -2v reference is not working any more.
after desoldering it nothing changed the scope is still way off but the same as bevor. so i ordered a new one will give a follow up.

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[zaphi]

finaly i traced down U103 is working fin. the - 7V rail was draged down bei U105 a TLC271CP after changing the -2V refferenc is rock solid again. But now i have a reading off -1.2V in the 20v range open circuit. If i supply 19V i get a reading of 17.8 the REL function is working fin. Any ideas how i can get rid of this offset?
Changed so far:
all CD4053
LM393N
TL061CP