After using the Keithley 236 SMU a lot in the last months without any problems, it suddenly showed the message "IOU Cal Checksum Error" and "IOU EEROM Failed" after power on. Additionally, while booting, it strangely showed a GPIB address different to the one set in the menu. It also was not possible to get any response via the GPIB interface on either of the addresses.
As always there was no visible damage to any part. The earth referenced digital section is powered from a single 5V rail provided by an LM323, which was perfectly fine.
EEPROM data loss
The error messages point to a failure of U33, which is a 2816A 16Kb parallel EEPROM. Its purpose is to store the calibration constants and seemingly the GPIB address too. The inputs and outputs of the EEPROM looked fine one the oscilloscope, so it probably did not fail but suffered from memory corruption. Anyway I couldn't simply recalibrate the unit, as the GPIB interface did not work, so I had to remove the chip from the board. Unfortanely it is not socketed like the firmware ROMs, so I had to de-solder it.
It was possible to read it out with the TL866II and wipe it successfully, so the chip seems to be fine. After soldering in a socket and putting it back, the unit showed only the "IOU Cal Checksum Error". Interestingly I could set the GPIB address and the GPIB Interface worked again, but obviously the calibration constants were lost.
Anyway I replaced the old 2816A with a "newer" NOS Xicor X2816A and performed a first voltage calibration. This also removed the Cal Checksum error.
DS1210 failure
I then noticed that the user settings, which are stored on battery backed RAM controlled by a DS1210 IC, were not saved after rebooting. The battery was nearly new and the voltage didn't drop under 3.15V under load, so I checked the pins and noticed, that there was no output, so the DS1210 seem to have failed. After replacing it with a "new" one from eBay it worked again.
So what could be the root cause for these problems? Probably there has been a voltage transient, which took down the memory controller and disturbed the floating gates of the 2816A.