Drifting bonanza continues...
Here's what done so far these days:
* Removed drifty U180 ASIC from original bodged up A3 PCB.
* Cleaned second A3 PCB with ASIC, replaced integrating cap to DK 399-7719-ND, installed new AD848
* Cleaned ASIC between legs with cotton swap and IPA
* Soldered in ASIC to A3
Tested stuff, ACDCV issues are gone now, meter pass selftest and calibrate okay.
1ppm jumps in readout were still there. As we tested this before, it was reference voltage jumping, so eyes to A9 PCBA (which I got from eBay, as some may remember).
* Added copper-foil hat around A9, thinking stray air-currents around A9 cause jumps.
* Proven useless, copper hats removed.
* Removed LTZ1000ACH
* Installed LTZ1000CH (1990) instead.
* Removed 95K resistor to get temperature setpoint back to +80°C
Since we talking sub-ppm now, I changed source at meters from HP 3245A 10V output to one of my
LTZ1000-references 7V output. Both K2001 and 3458 measuring same output. LTZ powered from K2400 12V ~30mA.
Now, no ppm jumps, only drift, at much slower rate
~0.2ppm/hour.
Funny part - LTZ1000CH non-A warms up much slower than original -A, this is visible by having 10V on meter and power it on from cold. Voltage creeps 3-4mV/second till it reference voltage stabilize after ~2 minutes.
If one try to run selftest or CAL 10V during this period - you will be greeted by
HARDWARE FAULT 204: REFERENCE CONTROL LOOP FAILURE or something like this
Once it warmed up, selftest and CAL works ok.
Could it be just leakage on ceramic surface of ASIC causing drift, all this time?