HP 34401A Repair/ restoration - Page 1
Forum section: electronics / repair

author: scenz
author_rank: Contributor
timestamp: May 26, 2022, 08:37:34 pm
content: 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 extendSo 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 openscopeprojecthttps://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 HighVoltageOptional:34401-45021 Handle EUR 12.67Bought from HighVoltage34401-86020 Kit Bumper EUR 22.79Bought from HighVoltage34401-62101 Input Jack AssyAnd 3D Prints:34401-88304 Rear Panelhttps://www.thingiverse.com/thing:2186112Am 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

author: HighVoltage
author_rank: Super Contributor
timestamp: May 26, 2022, 09:31:08 pm
content: Quote from: scenz on May 26, 2022, 08:37:34 pmHopefully you got this instrument for free.I can offer you a used Agilent window in great shape, if you are interested.

author: free_electron
author_rank: Super Contributor
timestamp: May 26, 2022, 10:35:57 pm
content: 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.

author: strawberry
author_rank: Super Contributor
timestamp: May 27, 2022, 04:38:24 am
content: remove shattered VFD and connect meter via serial port to test all functionality

author: scenz
author_rank: Contributor
timestamp: May 27, 2022, 10:43:03 am
content: Quote from: HighVoltage on May 26, 2022, 09:31:08 pmI 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 amThis is what i plan to do next. Maybe this sunday or monday next week. The display pcb is already cleaned and the vfd removed.

author: strawberry
author_rank: Super Contributor
timestamp: May 27, 2022, 11:27:17 am
content: this looks cool and could last longer than OLEDhttps://www.eevblog.com/forum/buysellwanted/ebay-led-display-module-fluke-8840a42a-(diy-kit)-replace-broken-damaged-vfd/

author: coromonadalix
author_rank: Super Contributor
timestamp: May 27, 2022, 05:07:59 pm
content: 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

author: HighVoltage
author_rank: Super Contributor
timestamp: May 28, 2022, 09:49:30 am
content: I also have an older VFD, if you are interested.Probably perfect to play around with.

author: factory
author_rank: Super Contributor
timestamp: May 28, 2022, 09:01:28 pm
content: 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

author: Paceguy
author_rank: Regular Contributor
timestamp: May 29, 2022, 03:52:36 am
content: 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.

author: scenz
author_rank: Contributor
timestamp: June 01, 2022, 01:07:50 pm
content: Quote from: factory on May 28, 2022, 09:01:28 pmHi 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.Quote from: Paceguy on May 29, 2022, 03:52:36 amNah, 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.Quote from: HighVoltage on May 28, 2022, 09:49:30 amHi 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

author: HighVoltage
author_rank: Super Contributor
timestamp: June 01, 2022, 03:52:38 pm
content: Quote from: scenz on June 01, 2022, 01:07:50 pmWow, that was a fast delivery and you installed the VFD already.Looks nice !It seems you have fun.Enjoy the ride.

author: scenz
author_rank: Contributor
timestamp: June 18, 2022, 11:26:55 am
content: 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.

author: strawberry
author_rank: Super Contributor
timestamp: June 18, 2022, 03:01:53 pm
content: K103, K104, current source, : selftest measure 10M

author: scenz
author_rank: Contributor
timestamp: June 27, 2022, 04:48:01 pm
content: 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.

author: ava297
author_rank: Newbie
timestamp: March 21, 2025, 10:48:53 am
content: Hello everyone.I would like to report on the successful repair of the Agilent 34401A with firmware version 11-05-02 and factory calibration from 2003. When starting up, the multimeter did not measure on any of the ranges. Running the self-test recorded 13 errors: 608, 609, 610, 611, 612, 613, 615, 617, 618, 619, 621, 622, 623.Upon inspection, a burnt tantalum capacitor C322 (22 uF x 20 V) was found. When checking the CR305 (3.3 V) zener diode on the board with a tester in both directions, it showed a voltage drop of 3.6 V. When desoldering, the zener diode showed an open circuit in both directions. When replacing both faulty parts, the multimeter resumed its work, all errors during the self-test disappeared.I was once again convinced that the repair of any device should begin with checking the supply voltages.For prevention, I plan to replace all tantalum capacitors with new ones, with a higher operating voltage. In the future, I will calibrate the device and use it in my home laboratory.

author: d-el
author_rank: Regular Contributor
timestamp: March 23, 2025, 04:16:54 pm
content: This is a classic malfunction of old equipment with tantalum capacitors. And this is not related to the operating voltage. It may also be that one capacitor is burnt out, and the neighboring one will work for many years.I suspect that the problem lies in the quality of the capacitors of that time.I also noticed that some HP models were updated from tantalum capacitors to ceramic ones. For example, 33220A.

author: ava297
author_rank: Newbie
timestamp: April 03, 2025, 04:16:44 am
content: Hello.Please tell me what type of switch is used in place of S1 (front/rear terminal switch)? With silver or gold-plated contacts? According to the parameters specified in the service manual for 34401 (0.4 A, 25 VAC/VDC), I chose C&K F8UEE 0.5 A 125 VAC (https://www.digikey.pl/en/products/detail/c-k/F8UEE/3862501), with silver-plated contacts. If the original uses gold-plated contacts, how much will this affect the metrological characteristics?

author: Kleinstein
author_rank: Super Contributor
timestamp: April 03, 2025, 07:22:22 am
content: The main point with gold contacts is getting reliable contracts after long time and with low current. The switch contacts are anyway potential failure points.Somewhat touchy switches are not so uncommon.The type of switch seems to use sliging contacts that are somewhat self cleaning. So chances are good one gets reasonable reliable operation even with non gold plated contacts, but also possible wear on the gold coating. Sometimes just using the switch a few times may clean the contacts.A point that is a bit odd is the voltage rating - the front rear switch may see rather high voltage, though usually in combination with low current. Also with the original switch one should not operate the switch when a higher voltage is present.

author: ava297
author_rank: Newbie
timestamp: April 03, 2025, 07:49:32 am
content: The manufacturer I mentioned has a similar switch, but with gold-plated contacts: F8UEETBAU ( 0.04A 50V ). As I understand it, the indicated voltage and current are values ​​that can be switched without causing the contacts themselves to burn. In switches with gold-plated contacts, I noticed that the values ​​of currents and voltages are always lower than with silver ones. I read that the gold coating is less resistant to the action of an electric arc. And these parameters are not the maximum permissible operating voltage and current through the contacts. The multimeter's operating instructions clearly state that the terminals should only be switched when the device is disconnected from the measuring circuit.