I was under the impression, that the instrument maybe stuck itself into this failure mode, by improper ACAL (?) calibration constants.
Does not look like that. It did run SELFTEST fine about 5 times I did, then passed complete ACAL on all functions, then I did CAL against 10V from K2400 SMU and against 10K resistor from ESI DB52.
Turn meter off, turn meter on, measured 10V from K2400, values matches, all okie, run SELFTEST couple more times, turn off unit.
Next day - power it back on, measured 10V, all ok, connected 1Meg VPG resistor and leave it sampling NPLC 10, all settings default. Was fine for at least 30 minutes, then I left room, come back in 5 hours - measurements halted, ERR message 114. Since then it's all the same, throwing these two errors on anything >100us conversion time on all ranges/modes in DCV,DCI,Ohms, no matter what other settings I fiddle with. Sometimes can do 3-5 samples at NPLC 10, sometimes cannot do even 1 count on NPLC 0.01. Usually locks up less the a second.
I assume, that you stored the content of the CAL nvRAM (2kx8).
And you may also have the old DALLAS parts, which you may erase to FF in all cells on your ext. programmer, and then insert into the instrument.
(Store the new nvRAM in a safe place)
Tried all that, no effect. New NVRAMs, old NVRAMs, erased, or in original state, all same. Just tried once again, same. I'd assume digital outguard domain working OK at this point.
Also, please remove these 4x 32kx8 additional RAM, U123-U126, as you used nvRAMs also, which may confuse the instrument on booting.
Maybe, you might also erase the other both 32kx8 nvRAMs to delete all possible stored states.
No effect, just did this, and tried this week ago. Regarding CAL ROM, I tried that (just poped in old NVRAM chip, which still have original data), no difference.
If you then start the instrument, you will have a virgin state, causing many CAL and ACAL errors. The FW will initialize all calibration constant to a default, and an initial ACAL will be necessary.
On 2nd boot up, there should be no error any more, provided, the old nvRAMs still work. (Otherwise you'd have to use the new nvRAMs for that)
Old SRAMs dead, but as all above, I don't think there is a problem. Yea, I even tried to run without CAL NVRAM, believe me. On this matter meter tells you to change jumper JP132 and power cycle
.
Maybe, this may cure the problem.
Nope, no magic pill on A5 board
.
Even removed all ROM chips and checked them to make sure all data verified 100% match to firmware binaries. (I had unit running open cover those two days when it covered, was thinking maybe bit or two got corrupted from sunlight UV. Nope all were 100% match.)
In case there's no success, you might first measure precisely the +/-12V and +5V ADC references, TP160, 165, 151, for correct ratio (1.69x) relative to LTZ value, and for stability.
(If there's a noteworthy deviation, that may also cause your error).
Now we have something.
ZR_HI to ZR_LO (entrance of zener voltage from A1, at A3 connector): +7.07411, stable up to 5th digit.
ZR_LO to TP160 (+12VREF) : +11.93936 V, stable to 5th digit. Ratio to LTZ is 1.687754
ZR_LO to TP165 (-12VREF) : -11.92274 V, stable to 5th digit. Ratio to LTZ is -1.685404
ZR_LO to TP151 (+5VREF) : +4.960405 V, stable to 5th digit. Ratio to LTZ is 0.701205
Also probed with scope, did not see any spurious large-scale noise.
I'm not sure, if the Time Interpolator is used for normal DCV operation (to my opinion, only in Digitizing Mode, like ACV Sub- and Random Sampled)
But who knows.. Inside there's another EL2018, U405, maybe you check this IC also.
Replaced U405 as well, forgot to mention, sorry. I have now both U142 and U405 with collet sockets, tried bunch EL2018, none show any difference.
These convergence errors have not yet been understood well in the past (for other instruments).. but this might have to do with the matching of the different slope resistors , as described in hph4/1989, page 12. It's necessary, that the absolute values of these internal resistors fit each other, not extremely precisely, 0.1% only.
They cannot be accessed externally, that's the problem.. But maybe you try carefully to measure the 50k between pin 6 and pin 11/13 of U180, and maybe the minor slope resistors, pins 18,19,21,23.
50K network read as 50.2095 kOhm. Added pair of 22M RN7C resistors between pin 6 and pin 28 - got 49.982K, still same behaviour.
I tried to measure minor resistors, got wrong values, but that's likely to opamps in loop. Only way is to desolder U180 and measure separately.
And you also did not leave any fingerprints there, I assume, which may cause parasitic currents at the A/D.
Yep, leakage are bad in low-current charge balanced circuit, but cleaned multiple times, IPA, distilled medical grade water, IPA again, no any visible difference.
Give all that a try before giving up ..
Nobody is giving up so easily, just more radical, less enjoyable approach would be taken
I also reflowed pins on U180, no effect (just heated up joints to solder point, without removing chip)
I also tried voltage margining on all supply rails (+5, +18, -18) and voltage references (-12, +12, +5) one at a time, injecting little current with SMU into feedback nets, did not see difference, so it's not like something is bit off tolerance, talking about voltage domain. Problem likely lies in current domain or in timing/sequencing.