HP/Agilent 4156C failing self test on power on

[chilternview]

I'm seeing one or more SMUs fail self test at power on. The error code is 10x24 where x is the SMU number, apparently this error is 'SUM failed I set and I measure calibration'

The error goes away after the unit has been powered on a while, but I'm concerned it may get worse. Has anyone seen this error message, and found a solution? Unfortunately the service manual does not have any schematics. I'm guessing that if just the current calibration checks fail (and not the voltage ones) it may be something like leakage in the range resistors?

[chilternview]

So this problem with errors 10x24 and also 10x27 is caused by input leakage in current measure mode.

I found that carefully cleaning the HRSMU board with a clean soft brush and also a hot air gun set to low temp (100C) to blow away  any dust around the OPA128LM and associated components (they are under a screen, under another screen for the analog section of the HRSMU) got rid of the problem. Handle with plastic gloves on to avoid getting any sweat etc on the board. Even the slightest touch in the wrong place can give rise to leakage.