Hi all,
I'd like to share last weekend's story...
I have a Keithley 196 6.5 digit Multimeter which suddenly stopped working in the 300 mVDC range. It showed 96 mV when the input terminals were shorted and overflow when any significant voltage was applied to the inputs. The other DC ranges and the Ohms ranges seemed to be fine.
I suspected a fault in the range switching or input amplifier, so I opened the thing, removed the shielding plate, and powered it up again. That was a mistake, because the PCBA had moved a bit on the metal poles in the middle, which then had shorted something on the board out.
Learned the hard way when and where to use a strip of Kapton tape...
The supply voltages were now all gone, +15 rail down to 2 V and alike. Transformer buzzing. Not easy to debug from there, especially as the mains fuse blew after a short while, too.
Long story short, the LM399 reference heater was fried, and that's all what was broken. I fortunately still had another scavenged LM399 reference in the empire of dirt, and the meter was back to the initial state, with the 300 mV range not working (and the others now way out of cal).
Luckily TiN has all the required documents on his site, including hi-res schematic scans, and the manual includes a description of the low-level test modes the Keithley 196 has built in. Thank you so much for that, TiN!
I spent many hours going through the schematics, understanding only little, and probing the analog board. Everything seemed good and working until I got to the input of the ADC section. The input voltage drops to 0 there, but I suppose that's how it is supposed to work, as it is the case for all ranges.
Stopped probing, connected my (way out of cal) Adret 103 A and a Keithley 2010, and did the front panel calibration for the 300 mV range. Works like a charm again now
Same for the AC ranges which were completely gone as well. Adjusted the lower ones with a frequency generator (and K2010), and they were bang on again.
It seems the X2816CP EEPROM does not have great data retention time, so the DMM just lost its calibration values. Has anyone here ever seen that happen?
That particular EEPROM is only available from eBay, and those chips are probably no younger than the one I have. Can I still trust any of these when I re-calibrate the unit? Is there any suitable drop-in replacement?
I'll re-cap the meter soon, then take it to my employer's cal lab, and hope for the best...