Author Topic: Agilent 34410A repair  (Read 1624 times)

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Offline floobydust

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Re: Agilent 34410A repair
« Reply #25 on: January 17, 2025, 02:05:31 am »
Why is pin 3 up so much? Either U502 input is damaged or Q507 is for the 0.2mA
I 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.
 

Offline CyclotronTopic starter

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Re: Agilent 34410A repair
« Reply #26 on: January 17, 2025, 02:19:33 am »
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.



 

Offline floobydust

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Re: Agilent 34410A repair
« Reply #27 on: January 17, 2025, 03:22:11 am »
I have this info:
Q503 2N7002, 1855-0734
Q502 BSS84P, 1855-0768
Q504 MMBF4392, 1855-0752
Q507 2N7002, 1855-0734
Q508 2N7002, 1855-0734
U502 OP-27GS, 1827-0926

The unit might have been in a test fixture or someone put it aside to take home, as a dud unit.
 

Offline CyclotronTopic starter

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Re: Agilent 34410A repair
« Reply #28 on: January 21, 2025, 07:48:44 pm »
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.
 
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