Author Topic: Agilent 34410A repair  (Read 1565 times)

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Offline CyclotronTopic starter

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Agilent 34410A repair
« on: January 12, 2025, 11:52:17 pm »
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 Test
603 Fine A/D Test
604 Fine A/D Linearity
605 A/D & FE Measure Zero
606 Input Amplifier x100 Zero Test
607 Input Amplifier x10 Zero Test
608 Input Amplifier x1 Zero Test
609 Input Leakage Test
610 Input Amplifier x10 Gain Test
611 Input Amplifier x1 Gain Test
612 Ohms 500nA Current Source
613 DC High Voltage Divider Test
614 Ohms 5μA Current Source Test
615 Ohms 10μA Current Source
616 Ohms 100μA to 200 Ohm Shunt

I think I have the correct schematic and the service manual.  Time to start figuring out where to start.
« Last Edit: January 13, 2025, 12:10:49 am by Cyclotron »
 

Offline coromonadalix

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Re: Agilent 34410A repair
« Reply #1 on: January 13, 2025, 10:38:46 am »
1st i would check all analog voltages section

i would check around  AD706   U300

the 6xx  errors are often related to ohms


similarities with 34401a  errors
https://www.eevblog.com/forum/repair/agilent-34401a-error-606/
 
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Offline CyclotronTopic starter

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Re: Agilent 34410A repair
« Reply #2 on: January 13, 2025, 03:22:27 pm »
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
« Last Edit: January 13, 2025, 03:40:50 pm by Cyclotron »
 

Offline CyclotronTopic starter

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Re: Agilent 34410A repair
« Reply #3 on: January 13, 2025, 03:54:22 pm »
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.

 

Offline CyclotronTopic starter

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Re: Agilent 34410A repair
« Reply #4 on: January 13, 2025, 04:36:14 pm »
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.

I have another 34410A and I compared the measurements on U300.  They appear to be the same.
Pin 7 is swung to 17volts there too.
 

Offline floobydust

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Re: Agilent 34410A repair
« Reply #5 on: January 13, 2025, 06:19:50 pm »
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.
 
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Offline CyclotronTopic starter

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Re: Agilent 34410A repair
« Reply #6 on: January 13, 2025, 08:28:45 pm »
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.

I 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.
 

Offline Kleinstein

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Re: Agilent 34410A repair
« Reply #7 on: January 13, 2025, 11:26:39 pm »
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.
 
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Offline CyclotronTopic starter

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Re: Agilent 34410A repair
« Reply #8 on: January 14, 2025, 12:39:11 am »
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.

Interesting. 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.
 

Offline CyclotronTopic starter

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Re: Agilent 34410A repair
« Reply #9 on: January 14, 2025, 04:13:15 pm »
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.

 

Offline floobydust

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Re: Agilent 34410A repair
« Reply #10 on: January 14, 2025, 06:52:07 pm »
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.
 

Offline CyclotronTopic starter

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Re: Agilent 34410A repair
« Reply #11 on: January 14, 2025, 07:33:06 pm »
@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.
 

Offline CyclotronTopic starter

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Re: Agilent 34410A repair
« Reply #12 on: January 14, 2025, 09:33:42 pm »

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.

As 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.
 

Offline Kleinstein

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Re: Agilent 34410A repair
« Reply #13 on: January 14, 2025, 11:05:19 pm »
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.
 

Offline CyclotronTopic starter

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Re: Agilent 34410A repair
« Reply #14 on: January 15, 2025, 05:48:25 pm »
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.
 

Offline Kleinstein

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Re: Agilent 34410A repair
« Reply #15 on: January 15, 2025, 06:15:19 pm »
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.
 

Offline CyclotronTopic starter

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Re: Agilent 34410A repair
« Reply #16 on: January 15, 2025, 06:26:35 pm »
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).

I assume the small differences from input to output are from the drift of the test source. Usually, the difference should be smaller.

I 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.

 

Offline Kleinstein

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Re: Agilent 34410A repair
« Reply #17 on: January 15, 2025, 08:05:35 pm »
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.
 

Offline CyclotronTopic starter

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Re: Agilent 34410A repair
« Reply #18 on: January 15, 2025, 08:56:12 pm »
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.
 

Offline CyclotronTopic starter

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Re: Agilent 34410A repair
« Reply #19 on: January 16, 2025, 05:30:21 pm »
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.
 

Offline Kleinstein

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Re: Agilent 34410A repair
« Reply #20 on: January 16, 2025, 06:03:11 pm »
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.
 
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Offline CyclotronTopic starter

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Re: Agilent 34410A repair
« Reply #21 on: January 16, 2025, 06:50:56 pm »
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.
 

Offline Kleinstein

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Re: Agilent 34410A repair
« Reply #22 on: January 16, 2025, 07:14:31 pm »
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.
 
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Offline CyclotronTopic starter

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Re: Agilent 34410A repair
« Reply #23 on: January 16, 2025, 07:26:48 pm »
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.

I 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
 

Offline Kleinstein

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Re: Agilent 34410A repair
« Reply #24 on: January 16, 2025, 09:02:32 pm »
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.
 


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