34970A DMM Module Repair  - Page 1
Forum section: electronics / repair

author: CrazyTiger
author_rank: Regular Contributor
timestamp: September 06, 2022, 01:33:59 am
content: I am in the midst of repairing a 34970A with multiple issues.The unit started with selftest errors as below:604 Self-test: A/D slope convergence609 Self-test: DC gain x1610 Self-test: DC gain x10611 Self-test: DC gain x100612 Self-test: Ohms 500 nA source613 Self-test: Ohms 5 uA source614 Self-test: DC 300V zero615 Self-test: Ohms 10 uA source616 Self-test: DC current sense617 Self-test: Ohms 100 uA source619 Self-test: Ohms 1 mA source621 Self-test: AC rms full scaleReplacing allAD711AD706 took away all the error, though the actual culprit was oneAD711AD706 (U400), might as well change all I suppose.Upon changing self-test passed. Measurements were super out of tolerance. Zero adjustment did not go thru stating no signal, That directly pointed me to possibility of defect on the input relays.Had to wait an extra week after the relay that Element14 delivered came broken in half..Replacing relays, Zero Adjustment passed. Yet I am noticing in all functionality measurements are slowly drifting up, another weird error i have encountered is if I am to perform -10V adjustment, the meter gives me error 731 Cal: A/D turnover correction out of range, in all positive ranges adjustment is going thru.Next up will be measuring the supply voltages to see if they are stable or not, maybe its drifting causing this issue.Any other idea is much appreciated.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 06, 2022, 06:59:26 am
content: The 604 error and thus an ADC problem could explain all other errors. Without the ADC correctly working, the other self tests may easily fail as well.Normally there should not be very much turn over corrent needed for the ADC. For the turn over error I see mainly 3 components:1) A problem with some offset at the input, so the turn over error is at the zero point. This error enters twice and adds up. The test from positive to negative side needs a careful offset adjustment.Excessive input current could be a cause for additional unexpected offset.2) A problem with the amplifier, especially the output side cross over error of the OP27. This error would be more like a step at low voltage. So one would alread see it with some 0.1 or 1 V in the 10 V range.3) An INL problem if the ADC, especially a square law contribution.A point to check would be the 5 V supply to the 74HC4053 switch chip. Too low a supply here could cause an INL problem, especially a square law contribution and thus turn over error. As the AD711 at the ADC seems to have been bad, there is a slight chance that the HC4053 could have been damaged as well (e.g. from some odd transients in the supply turn on/turn off sequence).Aof cause check the general supplies. AFAIR there are quite a few extra zeners to reduce the supply to some chips that may also fail (e.g. after a latch up).

author: Dr. Frank
author_rank: Super Contributor
timestamp: September 06, 2022, 07:27:51 am
content: I repaired my 34970A last year, with partly same errors:https://www.eevblog.com/forum/repair/hp-34401a-error-612-613-615-617-618-619-621/msg3571485/#msg3571485U201, an AD706 was defect (as often) for Ohm mode.Frank

author: CrazyTiger
author_rank: Regular Contributor
timestamp: September 07, 2022, 06:17:05 am
content: I made a mistake on my initial post, the replaced part was AD706 @ U400. U201 have been replaced as well.Regarding the 5V line, the voltage to the IC is measuring 5.012V, and stable. Will scope the voltage lines for noise later.