| Electronics > Repair |
| [SOLVED] HP 34401a - Error 612, 613, 615, 617, 618, 619, 621 |
| << < (17/22) > >> |
| ua4yhz:
Today I changed Q201 (MMBF4392), Q202 ( MMBF5461) and U201 (AD706). Tests-pass. 2W mode worked. 4W mode worked. |
| bitseeker:
Good progress. :-+ |
| calin:
Well, i have to thank everyone .. or brag about it :) Thanks to this thread I revived my "gamble" 34401A that I scored on ebay for 180$ (shipped !!!) :-+ . At arrival it was not looking very bad, power supplies all dandy but it was showing the usual errors 612, 613, 618 and 619. After I replaced the famous U201 and Q201 bah still no luck .. same failures. I started to go component by component and after super careful inspection of the mainboard (under microscope) i found the culprit .. R122 which had left the magic smoke escape. Most likely it was fried by some voltage spike applied at input since that seems to be on the DCV measurement path if I read the schematic correct. Anyways after that no more errors, all self tests pass with success. I tested as much as i could and everything seems to be working correct as far as I can tell. It even looks like still being in calibration even if it seems that was last calibrated in 1995. I need to search for a cal lab to get it calibrated. I kind of remember that I saw somewhere on the forum someone saying that there is a cal lab in Phoenix, AZ (my town) which does not charges an arm and a leg for a calibration. Does anyone know the name of the company ? or find that post somewhere ? :) Again, thanks all for the good info on this thread !!! EDIT: I found them !!! https://4gte.com/products/75-calibration-special/ The thread is here - https://www.eevblog.com/forum/metrology/$75-calibration-with-adjust-and-data-for-a-variety-of-multimeterscountersetc/ |
| Dr. Frank:
Hello, I bought this defect 34970A with the same error codes. It has the same DMM circuit inside, like the 34401A, but on a separate board. In the picture you see the infamous U201, AD706, which also was defective in my case. The measurement on the constant current circuit revealed that the 2nd OpAmp, pin 5, 6, 7 broke down as usual. I've written down the voltages before and after '=>' repair . What I was not aware of before is the fact that the bipolar input transistors break down in such a manner, that they source fault currents from the positive supply (+18V) into the +/- inputs. The -input therefore drives an additional current via pin 40 backwards into the FET switches of ASIC U101, which raises the voltage on the Source of j-FET Q202 to these 12.6V. One can measure this excess current of several mA by attaching a current meter in its mA range from the Source of Q202 (pin 1) to AGND, when selecting a low test current of 5µA or 500nA in the 1 or 10MOhm ranges. After repair, the DAQ 34970A operates correctly again. It's from 2000, has still its first factory calibration (CAL COUNT = 32), and deviates by about +30ppm in all DCV and Ohm ranges, pretty good for that time period. What's really nice, compared to the 34401A, that the Firmware features temperature measurement, and OFFSET COMPENSATION for the Ohm mode, which I'm missing on my old 34401A from 1993. Frank |
| bitseeker:
Thanks for including the analysis schematic, Frank. My 34970 just needs a new battery, but I'll know where to come back to if it misbehaves beyond that. |
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