Gentlemen,
I deleted all my previous analyses of this converter for earlier I misread the schematic regarding the 10 Meg range and have corrected it and just posted above the FINAL version of the operation and my suggestions.? ?I extensively edited other sections to reflect some of the comments from others in this thread.
This is a very strange ohms converter in the way it is implemented in this meter.? ?This was before my time for I joined HP Labs in 1978 and the date on the schematic is 1972.? ?I have an idea who may have designed this and it would be interesting to find an old HP Journal from 1972 era to see if this meter was featured and who the authors are from the design team.
Given what I have read from others and steps taken in trouble shooting, I would lift the gate of Q11 and the cathode of CR13 to completely isolate and float the protection circuit and then see if the converter works properly after that is done.? ?Your current problem could very well lie at the input terminals with excess current being pulled into the voltmeter front end by the clamping diodes CR10, CR11 through the unknown with no leakages in the converter itself.? Follow the ohms sense line into the 34740A display module where the sense line is clamped at +/- 5v.? ?Go to page 179 of the on-line manual:
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? http://hparchive.com/Manuals/HP-3470-Manual.pdf
Good luck.
George? ? ? ? ?Feb. 13, 2019? 3:39am
