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Advice please, used 34401a

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Hydron:
The older ones at least match the service manual schematics, newer ones replace the input buffer dual JFET + OP27 with a bootstrapped OPA130. Not sure what the performance implications are, the older version might be better (there are tempting replacements for the OPA130 with better specs, especially lower noise, though others have mentioned that the ADC is the limiting factor in a 34401A, so might be a futile change).

In a perfect world I'd pick a FW 07-05-02 or newer, as it has the new display board and hidden options to enable in the v7+ FW. Also would do a self test and enable 6 digit mode to check that the last digit isn't way brighter than the rest (can be the case if it's been run in the default 5 digit mode most of it's life - I'm using the store/recall hidden FW function to make mine boot into 6 digit volts mode by default).

Definitely happy with my 2007 vintage Agilent meter, especially after changing the VFD for a bright new clone. ~3 seconds to boot (settings recall adds about a second), almost totally silent (just a little transformer hum) and with a lovely VFD display, it's a great complement to my DMM6500.

indeterminate:
Mine has the  all red banana jacks. but is a rev 2 display running FW 10-05-02  with Temp function etc
so defenetly look at the firmware version

coromonadalix:
you need to search at the serial number level, there's HP Agilent  papers on witch versions and fw you'll know that it is the newest vfd drivers and associated FW version(s)



thread about this
https://www.eevblog.com/forum/testgear/q-agilenthp-34401a-versions-is-any-particular-one-preferable/
https://www.eevblog.com/forum/testgear/agilent-34401a-revisions/

i did wrote in theses

Brief History  on the   34401A-06     service note  attached here

Up to the fw 06-04-01  you will have the old unobtanium display chip,       pcb: 34401-66502

Past the  fw 06-04-02  you'll get the newest more repairable display pcb   pcb: 34401-66512

You can still order the newest pcb version 34401-66512, but the quantity is low ...  and the price is high,  around 200$ in my country, dont know how much you've paid for it ?

There is a version 10 leaked in the attachement, never tried it, it is supposed to work if you reprogram your (U502) fw chip and put the newest display pcb ... never heard of anyone doing it ???


For the VFD (vacuuum display) pn: 2090-0296 it is not orderable from Keysight anymore, you have to rely on chinesium parts around the web.

std:
It appeared resistance measurement in my HP 34401a is incorrect. I discovered this a few days ago but only now had time to write.
When I tested it in seller place, I shorted probes. Readings were low, so I didn't pay attention, I thought these were crappy probes, dust, old, no-contact, etc... It is impossible to figure out what is going on while standing in the passage between the door and the counter and when a guy is waiting for you. To do this, you need a normal place, sit down and at least 30 minutes to figure it out. Maybe in fact everything was fine, the defect was floating. I called them, they offered to come and look at another HP34401a, but I know that there are devices with even greater defects there. So I decided to forget about their two-week warranty and repair the HP34401a myself.

To test I made two banana jacks shorted with 1.62mm copper wire one inch long and began to compare with (oher) Agilent 34401a taken as a sample.

Reference Agilent 34401a ohm readings when input shorted: 
~   000.000,3  OHM ....  000.000,1  OHM

My HP 34401a ohm readings when input shorted:     
000.564,3  OHM    000.701,1 OHM    000.912,8 OHM    even   1.7 ohm,  1.3 ohm,  2.5 ohm, 4ohm, 6 ohm  ! 
The readings change every time I switching the [DC V] and [Ω 2W] modes, when relays click.

K101 relay problem?

Kean:

--- Quote from: std on December 09, 2024, 10:52:11 pm ---It appeared resistance measurement in my HP 34401a is incorrect. I discovered this a few days ago but only now had time to write.

--- End quote ---

What do you get if you use the rear terminals?

A common problem is dirty switch contacts for selecting between front/rear terminals, and operating the switch a few times can help.

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