HP 34401A Repair/ restoration

[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

- 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]

34401-49321   NFTS Window/Front   (Maybe I can get it over taobao or so for a resonable price or on ebay for ~40€....)

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]

Hopefully you got this instrument for free.

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

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.

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

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.

[strawberry]

this looks cool and could last longer than OLED https://www.eevblog.com/forum/buysellwanted/ebay-led-display-module-fluke-8840a42a-(diy-kit)-replace-broken-damaged-vfd/

[coromonadalix]

there is an oled project for this meter  from qu1ck   eevblog member, 

and sadly  yes   you have the unobtanium display with that pesky driver IC, it depend if the display leaks badly  or a simple vfd change could resurect it ?

a 50 euro buy price seems ok, but if you get lots of parts to change or buy,  you can be lucky to find them on fleabay .... but this and that in the end may cost you  lots of cash, you have to add the shipping fees in the pricing ...

good luck

[HighVoltage]

I also have an older VFD, if you are interested.
Probably perfect to play around with.

[factory]

Your list is missing the PCB shield plates, or whatever they are called.
Internal picture of mine below, same Rev as your early one, but s/n about 8000 later.

David

[Paceguy]

My advise, for what it's worth: I would keep this unit as a parts donner unit and look out for another defective unit at a reasonable price that has all of it's parts and a good display and see if you can make one good one from the two. It will take time and money to find everything that's missing on this one and you are taking a chance to ever get it up and running. I suggest checking ebay seller Surplus2d to see if they have any scrap ones that they haven't listed. Emailing them is the best.

[scenz]

Your list is missing the PCB shield plates, or whatever they are called.
Internal picture of mine below, same Rev as your early one, but s/n about 8000 later.

David

Hi David, I'm not missing the shield. For the first picture i just took it off. You can see the shielding in the third picture.

My advise, for what it's worth: I would keep this unit as a parts donner unit and look out for another defective unit at a reasonable price that has all of it's parts and a good display and see if you can make one good one from the two. It will take time and money to find everything that's missing on this one and you are taking a chance to ever get it up and running. I suggest checking ebay seller Surplus2d to see if they have any scrap ones that they haven't listed. Emailing them is the best.

Nah, I'm having fun fixing this unit. We will see how far i will take this this. Everyone please remember I'm doing this for fun. It does not interest me much if I have payed 50€ or 100€ to much for such a unit at the end. If I need to measure something I have my Keithley 2700 at home and a alot of Keithley 2000s and Keysight 34461As at work.

I also have an older VFD, if you are interested.
Probably perfect to play around with.

Hi HighVoltage, all the parts have arrived today. Thank you very much. The VFD looks good enough for me. I will probaly stick with it  .

I will now begin to check the unit and check if the self test will past. I assume it will probably not 

[HighVoltage]

Hi HighVoltage, all the parts have arrived today. Thank you very much. The VFD looks good enough for me. I will probaly stick with it  .

I will now begin to check the unit and check if the self test will past. I assume it will probably not 

Wow, that was a fast delivery and you installed the VFD already.
Looks nice !

It seems you have fun.
Enjoy the ride.

[scenz]

Now that i have a better ribbon cable for the LCD and had the time to solder everthing together i was able to test the unit a bit more.

The basic funcionality of the unit is available but the DC voltage measurement does not work at the moment and the culprit is also already found!

The resistor R104 is/was cracked so that the unit was working while i put pressure on it. I have now ordered a replacement 13k resistor in 2512 package. Ordered some more than i need to match it to the other resistors used.

This could also be a reason why the unit was giving me sometimes the error codes 612, 613, 615, 617, 618, 619 and sometimes not when i used pressure on R104.

But to be also on the safe side i have also ordered U153 as these also can break and cause these errors.

Once the resistors arrive I will post if now everthing works.

[strawberry]

K103, K104, current source, : selftest measure 10M

[scenz]

Have now replaced the R104 resistor and the self test passes . Now I need to contact Keysight to get a new cover, frontpanel and button panel. I probably need to print myself a new rear/front switch as the old one will probably not fit with the new panel.