HP 34401a Repair - Fails ST 606, 609, 610, 612, 613, 615, 617, 618 - Help?

[abnormalalien]

Hi all,

I've been reading many of your posts about fixing various HP 34401A issues. Our lab has one assembled circa 1999. Unfortunately, if I'm being quite honest, I don't really understand a lot of what y'all are talking about (my fault here I'm only a bit more experienced in electronics than your garden variety squirrel). Sorry this is a bit long but I did a lot of testing on my own so I wouldn't come off as a kid not trying their homework before asking for help!

About a year ago, this DMM was a functional data acquisition machine in the lab but suddenly stopped functioning...I ran each self-test individually and received the following fails: 606, 609, 610, 612, 613, 615, 617, 618. I then tested functionality: VDC measurement with a supply checked via Fluke116. From 2V to 56V, the HP consistently reads a voltage 2.6x smaller than the Fluke reports. VAC supplied by a lockin amplifier and compared to a working Keysight/Agilent at 17.76Hz over the range 0.02V to 2.0V: the HP measures VAC x2.6 larger than the supplied value. 2W R measurement does not read overload when nothing is attached. Measured values are never close to real values (2.33ohms for a 2700 resistor for example). Similar result for 4W resistance. Of possible importance is that above a certain value for the real resistances, around the hundreds or kilo-ohm range, the measured value has similar numerics while the order is incorrect (9.45ohms for 1000 ohms, 934ohms for 1M-ohm, etc). This leads me to believe there's a scaling problem like the HP is calculating the resistance based on some input current which happens to be different than what it should be (sort of agrees with descriptions of self-test). For DCI measurement, I used a Lakeshore DCI supply (verified by working Keysight/Agilent) and the HP measured values 2.6x larger than the supply current. (I see a trend but don't know what it means!  )

Next step, watched a guy on YouTube who tested the supply currents to the resistance measurements. For the 100 and 1k ranges, the supply should be 1mA, etc. My HP: 0.31uA supply for the 100 and 1k, 8nA for all other ranges. That's probably why my R measurements aren't working. According to the video, a common culprit is the U201 chip. My understanding is that this is a chip with 2 op amps inside? I'm not sure I really understand op amps or what these particular op amps are doing but I followed his steps to check the op amp voltages but his U201's behavior and mine diverged so I didn't know what to do with that. He said that op amp B in his failed (pins 5-7) because 5 and 6 were not the same and 7 was not gnd. Should op amp A operate under the same principle? If so I think both op amps A and B inside U201 for my HP are dead: using pin 4 as gnd, pin 8 should be 18V. I think that's the power to the op amp? It's 18.76V so that's probably ok. The +- input pins 2 and 3 should be 7V but are 4.14V and 3.38V. The output on pin 1 is 0.65V (should be 0?). For the 2nd op amp, the +- input pins 5 and 6 are 14.68V and 15.3V. The output pin 7 is either 12.6V when 4W is selected and 16.6V when 2W is selected.

On here, I've seen many mentions that the power rails should be checked so that's the last thing I've tried: sort of followed the schematic (a dept expert helped with this part) to regulators U552 and U551. With the white/grey wire as ground, pins 2 and 4 on U552 read -25.87V which I think is where the -26V input is. Then pin 1 is -16.91V; I don't understand that but ok. Pin 3 gives -18.17V which as far as I can tell goes through one other component (JM552, a resistor maybe though I don't know what JM stands for?) and comes out as -18V. Close enough for me. Similarly for U551, I again have no clue why 17.51V on pin 1 but input is 26.83V and output is 18.77V so sure I guess the +18V line works. I wanted to follow the same logic for the +5V line but couldn't find it inside the actual HP so instead I tried measuring a few of the little capacitors hanging out everywhere. Some of them had 18V and I definitely saw a few with +5V so I guess that line's cool too?

Any guidance you guys can give me would be much appreciated! I feel like I'm starting to understand some stuff (I now realize I kind of knew the 18V power line was good because it was at the power input of my op amp, didn't get that before ) but am still a bit overwhelmed/under-educated about what to try next.

Thanks!

[coromonadalix]

see this  may help you : https://www.eevblog.com/forum/repair/hp-34401a-error-612-613-615-617-618-619-621/

[abnormalalien]

Hi coromonadalix,

Thank you, this has been one of many that I've been reading closely.

[peertux]

Hi,

I had an error 611 with one of my 34401A's in the past, it was opamp U103B that was faulty with mine. When I look at your error's it also looks like a problem in the source area to me. have you measured  U153B and the transistors behind it?

Edit:
While you're at it also measure the voltage behind cr103A and look if it is clean.

[abnormalalien]

Thank you for the suggestions and sorry if I'm a bit ignorant here.

I understand how to check the voltages on the op amps, though I don't know what voltages to expect from them except the obvious supply voltages. For U103, the voltages measured are (for 1,2,4,6,7,8) 0.4V, 0.15V, -18.15V, 0.000V, -6.43V, 14.4V. So, the -18V supply seems fine and the +18 is measuring 4V less than it should. Not sure what that means or if it matters? Applying my (possibly faulty) logic of op amps, maybe it's good that pin 6 reads 0 since 5 is grounded and the op amp function is to make 5 and 6 the same? There's some output voltage for pin 7, don't really know if it's the right output. Similarly, for U153: (pins 1,2,3,4,5,6,7) the measured voltages are -4.8V, -5.9V, -5.9V, -18.15V, -5.9V, -5.9V, -4.8V. If I had to place a bet, I'd guess U153 is ok?

What exactly do you mean by measure the transistors behind it? I have a vague notion of what a transistor is which makes me think you're asking for another voltage but again I don't know what I'm looking for or at. Do you mean check the voltages on Q150-Q153? Those are the ones that come after U153 on the schematic. Same goes for the cr103...what do you mean by clean?

Thanks again!

[peertux]

Have look at https://www.eevblog.com/forum/repair/hp-34401a-error-612-613-615-617-618-619-621/ reply#6 , measure your values against Dr. Franks and post them in that topic. Big change you get some more help.