Electronics > Repair

HP 34401A Repair/ restoration

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scenz:
Hello everyone,

I got a relativly cheap but in bad shape HP 34401A multimeter with the serial number 3146A00141. The unit could be from ~1991-1992 based in the date codes on the components of the mainboard.

Now lets get an overview about the shape of the multimeter:

- It's missing parts (a list of the parts that I could identify will be listed below)

- The display is a bit rough around the edges and needs maybe a replacement :-DD

- But it seems to be working to an extend  :-+

So what did i check for now:

Firstly i checked the Transformer/ Fuses / Power Supply (LM317T, LM337T, LM2925T, 7805 (GPIB Power)) and everything was fine. No component an the mainboard was getting hot therefore for now i assume the meter works to some extend.

On boot i'm getting to two beeps which probably indicates a missing LCD panel that is not attached now (are beep codes mentioned in the service manual?)

The display:
I think that I have two options regarding the display:

- Get a "new" / "alternative" VFD from China and just use it. But I have concerns regarding the used NEC IC which could be probably dead and is unobtainium. Is there a way to check the working status of the NEC IC?
   My Idea would be to check the output pins with my scope and check if I get a signal?
   
- Built the OLED mod by openscopeproject https://github.com/openscopeproject/HP34401a-OLED-HW (maybe if the NEC IC is dead?)

The next step I'm going to do is to find a cable (stupid 2.0mm pitch) to connect the front panel board to the Mainbaord (as the original one is broken) and check if GPIB or RS232 is working and if I can get a reading of the meter.

Then i would probably work on a new VFD or OLED screen.

And then i would get the missing parts:

34401-40231   Panel, Front      EUR 38.26 (Price from Keysight website)
33220-84101   Cover            EUR 32.99 (Price from Keysight website)
34401-81932   Keypad            EUR 7.00  (Price from Keysight website)
34401-45012   Latch-Terminal      (Bought already on ebay)
34401-43731   NFTS Pushrod-Power   (Bought already on ebay for the Agilent version and the original HP version)
34401-49321   NFTS Window/Front   (Maybe I can get it over taobao or so for a resonable price or on ebay for ~40€....) Bought from HighVoltage

Optional:
34401-45021   Handle          EUR 12.67 Bought from HighVoltage
34401-86020   Kit Bumper      EUR 22.79 Bought from HighVoltage
34401-62101 Input Jack Assy

And 3D Prints:
34401-88304 Rear Panel https://www.thingiverse.com/thing:2186112

Am I missing something? It would also be great if you maybe have some of the missing parts mentioned above if you are living in europe or to be more precise in germany.

Thanks

HighVoltage:

--- Quote from: scenz on May 26, 2022, 08:37:34 pm ---
34401-49321   NFTS Window/Front   (Maybe I can get it over taobao or so for a resonable price or on ebay for ~40€....)

--- End quote ---

Hopefully you got this instrument for free.

I can offer you a used Agilent window in great shape, if you are interested.

free_electron:
sorry to say but you didn't buy a 34401. you bought scrap....
This is the first revision of the machine using the upd75xx microprocessor to drive the VFD. that is a mask rommed part and there is no substitute for it.

strawberry:
remove shattered VFD and connect meter via serial port to test all functionality

scenz:

--- Quote from: HighVoltage on May 26, 2022, 09:31:08 pm ---
Hopefully you got this instrument for free.

I can offer you a used Agilent window in great shape, if you are interested.


--- End quote ---

I got the multimeter for a resonable price in my opinion. I payed ~50€. This is maybe to much, but i bought with the intention to make this a project to tinker with and if everything works maybe i get a working multimeter for ~150-250€ if not then im also not mad about it. Then I had fun playing with it. Therefore is the price for me resonable. I will write you a pm later regarding the display window.


--- Quote from: strawberry on May 27, 2022, 04:38:24 am ---remove shattered VFD and connect meter via serial port to test all functionality

--- End quote ---

This is what i plan to do next. Maybe this sunday or monday next week :-+. The display pcb is already cleaned and the vfd removed.

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