but you must be brave to do this extreme repairs
No bravery needed, just have 2-3K$ cash handy and you will do fine. If A3 ADC bad - 1400$ for replacement. Repeat until it's stable

. It's no different than any other electronics repair, other than requirement of stable and verified calibration sources for DCV and OHM. For the first one you already on the correct way with LTZ module (you can calibrate 3458A to other calibrated 3458A using 7V LTZ output

. I even had luck to
transfer DCV internationally using A9 LTZ module, and got only 6ppm of error in the end.
I'm looking forward to see all you guys build some units and get them tested. I did not expect this thread to go this way,

.
As special case I'd be happy to offer calibration for your finished builds after it's all working and burned-in first 200 hours. Rules would be simple:
1. Build your KX-module, publish short worklog in this thread (few photos of assembled PCBA, photo of it running with DMM hooked up showing 7V)
2. Leave it running for 200 hours, so initial drifts get stabilized. Module should be enclosed in some box without vents.
3. Measure it again, record the temperature and voltage and put a label with values on the box
4. Ship the box to me (Taiwan). You pay shipping.
5. I'll power your box with +15V for 48 hours to have everything stabilize
6. Test 7V output voltage in temperature span +20....+30C, record the graph
7. Unit will be shipped back via EMS Express (~30-40$USD, 4-7 days to USA, bit more to EU). You pay shipping.
I don't have official Keysight/Tektronix/etc calibration certificates for my 3458A, but I'm confident it's +/-2ppm (likely <1ppm) on DCV.
All steps are required
