Electronics > Repair

Agilent 34410A repair

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Cyclotron:
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.

coromonadalix:
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/

Cyclotron:
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

Cyclotron:
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.

Cyclotron:

--- Quote from: Cyclotron 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.

--- End quote ---

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.

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