[SOLVED] Agilent 34410A repair - Page 1
Forum section: electronics / repair

author: Cyclotron
author_rank: Unknown
timestamp: January 12, 2025, 11:52:17 pm
content: In case someone has seen this and knows what's gone wrong. I picked up a 34410A on ebay noted for being broken.I figure its an opportunity for me to log a repair and maybe get some help from the geniuses around here as I go along.Like several things I've picked up in the past, this unit came with the input voltage set wrong. The unit was claimed to have come from a US manufacturer but the voltage was set to 100. I changed it to 120 and figured that might be all that was wrong.It isn't.When the unit starts up, the DCV shows a gigantic number, and no measurements can be taken on any setting, DC, AC, or OHM. Sometimes, a relay will switch back and forth continuously. I've been able to run a self-test and that completely fills the SCPI queue.I am making observations right now and taking notes. I may need to hook it up to a computer to see what all errors are there in a self test since it goes beyond the SCPI queue.Below is the list I have so far and images from the various modes I've put the meter in.602 A/D Feedback Test603 Fine A/D Test604 Fine A/D Linearity605 A/D & FE Measure Zero606 Input Amplifier x100 Zero Test607 Input Amplifier x10 Zero Test608 Input Amplifier x1 Zero Test609 Input Leakage Test610 Input Amplifier x10 Gain Test611 Input Amplifier x1 Gain Test612 Ohms 500nA Current Source613 DC High Voltage Divider Test614 Ohms 5μA Current Source Test615 Ohms 10μA Current Source616 Ohms 100μA to 200 Ohm ShuntI think I have the correct schematic and the service manual. Time to start figuring out where to start.

author: coromonadalix
author_rank: Super Contributor
timestamp: January 13, 2025, 10:38:46 am
content: 1st i would check all analog voltages sectioni would check around AD706 U300the 6xx errors are often related to ohmssimilarities with 34401a errorshttps://www.eevblog.com/forum/repair/agilent-34401a-error-606/

author: Cyclotron
author_rank: Unknown
timestamp: January 13, 2025, 03:22:27 pm
content: Agreed, first the voltages. Attached are my observations from probing the test points depicted on the diagram.As far as I can tell, all voltages are reasonable and stable.Measurements referenced from negative output of CR802

author: Cyclotron
author_rank: Unknown
timestamp: January 13, 2025, 03:54:22 pm
content: Observations from U300 in the OHMS Current Source circuitry.Measurements are referenced from the negative output of CR802 and again as a reference to pin 4, which appears as AGND on the schematic.I am suspect of the output on pin 7 of U300. I'm trying to upload this content as a walkthrough of my diagnostics, so I haven't analyzed what the output should be on that pin.But it looks like it's swung completely to the high.

author: Cyclotron
author_rank: Unknown
timestamp: January 13, 2025, 04:36:14 pm
content: Quote from: Cyclotron on January 13, 2025, 03:54:22 pmI have another 34410A and I compared the measurements on U300. They appear to be the same.Pin 7 is swung to 17volts there too.

author: floobydust
author_rank: Super Contributor
timestamp: January 13, 2025, 06:19:50 pm
content: I would instead focus on a simple function like DCV. Short the DMM input 0V and see what the input amplifier is doing, DC_OUT at U303 is around 0V or stuck? Similar to DCA, you'd expect 0V at rest. If not then work back.

author: Cyclotron
author_rank: Unknown
timestamp: January 13, 2025, 08:28:45 pm
content: Quote from: floobydust on January 13, 2025, 06:19:50 pmI was able to trace the input. U303 pin 6 out is around 0 when shorted as described. I have also put in 10 volts and traced that back to U506, and compared to another working unit, the results are similar.When probing U506, I mistakenly shorted two pins, and the system indicated a comms failure of that ADC, but the error disappeared when restarted.This can be a fair indication that communications from the FPGA to U506 work since it detected that failure.This path appears to be ok for U506. I will trace more paths and see if I find anything interesting.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 13, 2025, 11:26:39 pm
content: There seems to be problems with the ADC or main amplifier. Such a problem can also cause many of the other error messages like gain and ohms tests.

author: Cyclotron
author_rank: Unknown
timestamp: January 14, 2025, 12:39:11 am
content: Quote from: Kleinstein on January 13, 2025, 11:26:39 pmInteresting. When I accidentally shorted two pins on the ADC U506 the meter started behaving differently. It still fails the same self tests but the display and measurement behaviors changed. Its almost like it is measuring things but those measurements are wildly off.I'm going to come up with a plan to test the ADC and main amplifier. I appreciate the thoughts.

author: Cyclotron
author_rank: Unknown
timestamp: January 14, 2025, 04:13:15 pm
content: Today, I took a fresh look and discovered solder damage from my probes on the ADC pads. That damage chipped some solder, and it was shorting across pins, I believe. The attached picture is after I picked the bits out, and you can see them lying on the board. My ham-fisted probing isn't compatible with modern solder.I've cleaned the debris, resoldered the pads, cleaned the board, and started over. However, it is back to the original symptoms, so I'll start again. Given the OLVD indication everywhere, I suspect a short somewhere that's causing power to feed into the AMPs. At least that's the tack I'm going to take. I'm also going to measure reference voltages and document more observations. I'm hoping something will pop out as abnormal.Thanks.

author: floobydust
author_rank: Super Contributor
timestamp: January 14, 2025, 06:52:07 pm
content: Try not to wreck it, get probes that can do this work. Or don't probe at the IC pin but instead at a part that connects to that pin- these are larger and a bit away.There are local rails for the ADC, +/-15VADC, NEG_FINE_BUFF, and the +/-9VREF to look at.There is an overload detect circuit opto U201 to the FPGA but I think slim chance this is false-activating.

author: Cyclotron
author_rank: Unknown
timestamp: January 14, 2025, 07:33:06 pm
content: @Floobydust, thanks. It was definitely rushed and ham-fisted. I changed to one of my other probe sets and will avoid contact with the solder if possible.I decided to walk through the DAM SWITCH and discovered anomalies there.I'm comparing my readings to a known working 34410A. The image attached shows the readings in red for the failed unit and green for the working unit.The source of the anomalous voltage out of the DAM SWITCH comes from U606-A, an op-amp. That's how far I've gotten between meetings.

author: Cyclotron
author_rank: Unknown
timestamp: January 14, 2025, 09:33:42 pm
content: Quote from: Cyclotron on January 14, 2025, 07:33:06 pmAs I study the diagram, I'm not sure my initial thought about OpAmp is correct. Q601 could be allowing that voltage on the line.I am not sure why the Hex inverter has the voltages it does, either. This could be all red herring. I will take some time to think about this and maybe trace some other paths and compare.Thoughts welcome of course.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 14, 2025, 11:05:19 pm
content: The values at the DAM circuit part are different. However they still look plausible for a different PWM ratio from the FPGA side. The voltages more say that things look good in this area.This could be from a non zero voltage reaching the ADC or another reason to cause an offset.

author: Cyclotron
author_rank: Unknown
timestamp: January 15, 2025, 05:48:25 pm
content: For the next test, I input a well-regulated 2volts DC on both working and nonworking meters simultaneously and observed the recorded readings in the attached image.These appear to be fairly well in order. Only the readings across VBS_AMP+ and VBS_AMP- appear significantly different.I don't know what is going on with the differences there, but I will move on to other tests and come back to this once I understand more about what that circuitry does.At the least, this test from the front panel through to the low DC inputs seems to validate those paths.Not on the diagram, I did test the voltage dividers going into U302-C and those readings agree with what the diagram suggest I should expect.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 15, 2025, 06:15:19 pm
content: With 2 V at the input, the amplifier should not see 1.6 V, but more 2 V or so in non AZ mode and more like 1 V in AZ mode (alternate between 0 and 2 V).A assume the small differences from input to output are from drift of the test source. Normally the difference should be smaller.

author: Cyclotron
author_rank: Unknown
timestamp: January 15, 2025, 06:26:35 pm
content: Quote from: Kleinstein on January 15, 2025, 06:15:19 pmI will run the tests with the 2v DC input with the meters independently. The 2v DC comes from a PDVS 2.0 mini, and it's been pretty accurate.I just realized that running the tests simultaneously with the input shared could cause a small short in the failed meter to impact the results on the good one.I'm not understanding "AZ mode." Is that Auto-Zero or Auto-range? I want to be sure I understand. I've had both meters set to the 10v DC range.Thank you very much for your responses; it really kept me thinking and avoiding some assumptions like I just did on this past test.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 15, 2025, 08:05:35 pm
content: I used the AZ abreviation for the auto zero mode. The easier test is in the non auto zero mode, as the input signal should be connected to the amplifier all the time. In auto zero mode the amplifier input is switched between the external input and zero.

author: Cyclotron
author_rank: Unknown
timestamp: January 15, 2025, 08:56:12 pm
content: U302-B gets the inbound DC signal on pin 5. I've moved to using one of my scopes as I the multimeter wasn't telling me the whole story easily.The 2volts is being well represented on pin 5.Then U302-B sends a PWM signal from pin 14 to pin 3 on U305A opamp. That signal is then sent over to U302b and you see the output of that opamp on pin 6.This all seems to be a means to change the scaling of the voltage being analyzed.Based on these measurements, I'd say both the working and failed are behaving the same up to this point.I'm going to go back and probe further along with my scope to see where that takes me.

author: Cyclotron
author_rank: Unknown
timestamp: January 16, 2025, 05:30:21 pm
content: Today I started by working from the ADCs backwards and found an anomaly.The AIN on both the fine and course ADC chips has a saw tooth signal on it when measured on the working meter. This corresponds with data link side singnals.The failed meter appears to be pegged at 13 volts on those pins and no sign of a saw tooth signal. I'm trying to trace the source of the the saw tooth signal (I'm guessing it emanates U507 but it could be U503).Based on my tests, I believe the 13v is coming from U503 on the non-working unit. There's also a 500khz signal on pins 2 and 3 on U503 on the failed unit that I don't see on working unit.So, I'm guessing a failed component leaking somewhere.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 16, 2025, 06:03:11 pm
content: The saw-tooth signal is at the integrator output.The integrator is made from Q504+U503 for the fast part and U502 for the precision path. The integrator input (pin 2 of U502) should be very close to GND. The output of Q502 could also be interesting - for the working unit this should be around some 3 V, depending the threshold of the JFET.For the non working unit it's likely way off, more at one extreme, depending on what is wrong.The question is why the integrator saturates to +13 V. It can still be the DAM signal, the integrator itself or the feedback via the coarse ADC not working.A point to look at is the DAM signal: with +13 V at the integrator output this should be most of the time positive to bring the integrator closer to zero.Though likely also not working, it could be worth also looking at the ADC in AC mode with a shorted input. This would use a different input path.

author: Cyclotron
author_rank: Unknown
timestamp: January 16, 2025, 06:50:56 pm
content: I've been moving backwards and found an odd signal on the upstream inputs from the resistor pack area. There's this sawtooth signal up there and a 1.028 volts that I am looking for the source on.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 16, 2025, 07:14:31 pm
content: The saw tooth signal is from the residual modulation in the DAM signal. The testesd point is the integrator input, that is no longer working as a virtual ground.From the voltages it looks like U502 is bad. With the voltages at the inputs as shown the output should be more like -13 V, but from what reaches U503 it looks like it is more like -13 V. To be sure one could check the voltage at its ouput (pin 6). If this voltage is postive U502 is very likely bad.

author: Cyclotron
author_rank: Unknown
timestamp: January 16, 2025, 07:26:48 pm
content: Quote from: Kleinstein on January 16, 2025, 07:14:31 pmI was coming here to say that I believe u502 must be bad. That's the only way I can imagine the voltage on pin 3 of that opamp. I looked at pin 6 on it and it has +7.663 DC on it.I'll source a replacement and see how it behaves after that.Thanks to all. Special thanks to you @Kleinstein

author: Kleinstein
author_rank: Super Contributor
timestamp: January 16, 2025, 09:02:32 pm
content: An output voltage of only 7.6 V is well less as the fully saturated. With the input waveform this suggests that the signal is not fixed, but with quite some AC part. This would suggest that the amplifier still works, at least partially.If parts are ordered, I would still include an OP27, but chances are there are other defects. I would also get a replacement for Q504 (not that expensive anyway), Q502 and Q503 and maybe check a few more signals with the scope. Something unusual is responsible for the relatively steep downward slope at the integrator input. It could be something else than way too high bias current from U502. Gate leakage at Q502 is a possible issue. A way too negative signal from the AC signal path could also be a source. Q503 is only blocking the positive sign.I don't think U500 has enough power through the 47 pF capacitor to really make a difference and there should not be much AC at it's output.

author: floobydust
author_rank: Super Contributor
timestamp: January 17, 2025, 02:05:31 am
content: Why is pin 3 up so much? Either U502 input is damaged or Q507 is for the 0.2mAI would also scope the FET_CTRL signals to rule out a shorted mosfet like Q507 or Q502, Q503 feeding out from their gate and pushing the virtual ground point up.I'm not sure how to confirm JFET Q504 is not leaking from its gate either.I think you'd have to lift U502 pin 2 and with a say 100k load resistor to AGND there see what waveform you get at the virtual ground node.Or just replace several parts, whichever is easier.

author: Cyclotron
author_rank: Unknown
timestamp: January 17, 2025, 02:19:33 am
content: Thanks for the input. I have ordered some parts. I want to order Q502 and Q503 but have no idea yet what they are.I've left the lab for the day but tomorrow will physically examine them see if I can determine what they are.I'm surprised I don't have some of this in my parts bin but I think I used some of it on the last HP PSU I repaired.You folks have been spectacular! Thank you for helping a hobbiest out. I really enjoy these puzzles. In this case, I can only imagine that someone wanted a new meter. Though the meter was filled with dust and some looked like it may be metalic. I understand the unit came from a factory but I don't understand why it was set to 100v on the back panel. I get the idea that it was messed with to get rid of it.If you know what Q502/Q503 is let me know, I'll get some on order.

author: floobydust
author_rank: Super Contributor
timestamp: January 17, 2025, 03:22:11 am
content: I have this info:Q503 2N7002, 1855-0734Q502 BSS84P, 1855-0768Q504 MMBF4392, 1855-0752Q507 2N7002, 1855-0734Q508 2N7002, 1855-0734U502 OP-27GS, 1827-0926The unit might have been in a test fixture or someone put it aside to take home, as a dud unit.

author: Cyclotron
author_rank: Unknown
timestamp: January 21, 2025, 07:48:44 pm
content: I got my parts in today and replaced U502 (OP-27GSZ), and the meter is now passing the Self-Test. I will run it through more testing, but that appears to have been the root cause and fix for the problem.