HP 34401a Repair

[TizianoHV]

Hi, my first article on eevblog. Bad news! I was looking for a good quality multimeter to use as a standard. So I bought an HP 34401a on ebay, sold as working used for calibrations, last calibration 2 years ago. After a long wait and a customs fine for a mistake by the seller it arrived. I turn it on, it works, I connect 10V and it marks the correct voltage. I try the ohm range, it works. To increase the resolution, I go to the "MEAS" menu, press the key to enter the submenu and the dispay is displayed with all the characters lit (self-test type). I tried to turn off, turn on, disconnect power but nothing. Before freezing, it remained on for a few minutes (time to connect 10V and a resistor). Do you have any clues? thankyou!

Seller: "The instrument is in an excellent condition - passed self test. Cosmetically, it also looks very good. It just has one tiny scratch on a display's right side.
During its service, the meter was calibrated annually and was only used as a reference instrument to check other DMMs accuracy. When not in use it was stored in a box."

Some photo from the ebay seller.

Tiziano

P.S. While turning on it emits two beeps, dispay remains always on.

[TizianoHV]

Photo of the inside. Looks good.

[chekhov]

If instrument really was described as "working, excellent condition", then it makes sense to shoot a video about how it hangs, and at least try to return a fraction of money from seller - open ebay claim (because shipping back takes money, and it seems you already payed some customs fee which you not able to return).

Having a video is also helpful to show how it looks like Maybe you just have cable going to front panel went off from socket, but you need to open the unit. In this case of course, you cannot claim seller for bad meter since it's just a shipping issue, hard to say what's the best decision.
Btw if unit does not have "calibration void if broken" seals on the back screws under bumper that's a bad sign, and smb may be in the meter before.

Then you may start an attempt to fix the issue.

Unit opened - then only try to fix
All segments might be shown when there is no communication between brains and front panel, or failure on the FP itself.
It was usually mentioned for E3646A-like power supplies that sometimes FP board goes loose and you can make a short even by pressing the button (I'm a bit sceptical about it, but why not), and FP CPU dies ... I hope that's not the case.
Also, there should be beeps when you turn instrument - they also may indicate failure on main board.

[coromonadalix]

normally you push the auto man  button  and with the up down arrows you force the range ??

and to do a display test : hold the shift a few secs before powering it ... you have a youtube  how to test ...

[meandeev]

At first, a complete recapping of the electrolytics is not a bad idea. As chekhov mentioned, 2 beeps means no communication of the main processor an the FP.
And sometimes it is not the FP, which doesn´t work. I had a 34401a with a defective U553 (voltage regulator): voltage was ok, but it held the main cpu in an endless reset state...
The main question is: how much experience in electronics do you have and what measurement gear is available?

[RayRay]

This caught my eye:

What is that bronze/grayish material on em? If you flick your fingers over em, does it take care of it? If it's not dust/dirt, then I'd say you'd be best replacing em, could be an indication that they're bad!

[coromonadalix]

I recapped my 34401a  with nichicon caps, and changed the tantalums (square yellowish caps)    you change them before putting back the big capacitors

One of mine had the gray flex cable having bad mating, but it was hard as hell to move the cable in the white connector, looked like they had fused    had to lift the connector clip  and putting force to push down further the cable in the connector and sligtly wiggling it  until i saw all the pins getting loose

Normally you push down the connector to make a release ...

[TizianoHV]

Hi, thanks everyone for the help!

I am now considering whether to return it or try to repair it (the display connector looks fine). Speaking of "electronic skills" I am an electrical engineer (look at my iron welders;)). So as long as it comes to capacitors ok but if I have to replace IC no!  . Is there a risk that an IC is broken?

Problem summary: When you switch it on, all segments of the display light up, the keys do not respond.
I don't have a gpib connector to try to remotely control it

Thanks, Tiziano.

[coromonadalix]

please check this ;   

[TizianoHV]

Hi, I tried to run the autotest but the key don't works (is like to use ctr+alt+canc but with the keyboard disconnected or with the computer broken)

[chekhov]

This board contains many threads about 34401A repairs, with different failures and outcomes. Looking through them is maybe the best thing you can do at the beginning.

Maybe you need to take off front panel and look whether it has some damage (blown capacitor for example) (well, honestly I found it very hard process, I felt like I'm going to destroy red clip, maybe I was doing it improperly).

At least it good idea to understand which part is faulty. It's a pity you don't have GPIB adapter, maybe somebody nearby has it and can help with diagnostics ?

[coromonadalix]

have you pulled the gray flex cable from its white connector ??  can you check if the wires are okay ?  they are rigid wires, sometimes if you connect it or push it too fast they may bend ?

[robert.rozee]

wondering if the original poster had any joy... one possible cause could be the 'shift' key is stuck in or shorted. stranger things have happened before!


cheers,
rob   :-)

[giacomo]

Tiziano have you done any progress with the repair ?

If you need a USB->GPIB adapter i can sent one.. For the next month i will be busy hence i will not use it 

Have you tried to take a look at the front panel pcb ? It has componets and a microcontroller under the vfd. Maybe one supply rail to the micro is missing (it controls the keyboard)

If it is the micro, you can replace it with the AT89S51, i have done once (on an Agilent power supply, but they share the front panels as far as i remember)

[Dr. Frank]

Hello everybody,
Obviously this DMM came from U.S., as the line voltage is still set to 120V, if the rear picture is done by the T.O.

So check this and set line voltage to 240V; the DMM will probably be damaged, in this case.

Otherwise, it's more probable that due to vibration or shock during the transport either the ribbon cable / connector to the display PCB is not ok any more. Check correct seating and for continuity on all single lines.
With your kind of equipment, I strongly recommend that you stay away from any electronics repair attempt.

Frank

[giacomo]

Distracted as i am, on mine i wired incorrectly the transformer after replacing an optoinsulator (mine had error 625, missing comunication between the main processor and the other that handle the gpib/rs232) and i blew up the fuse.

Maybe, but i think the transformer will be resiliant enought and it will not be damaged instantly, if it is so, i would expect some distinct smell from the burned insulation of the primary coil.

After correcting for the mistake will be easy to see if any voltage rail is missing.