AGILENT 34401A

[lionelkjh]

Good evening everyone! I am having a problem with my multimeter AGILENT 34401A. Here are the symptoms of the failure: it starts to light up, all the digits are displayed during the initialization; then suddenly everything goes off. Nothing remains in the display, however we can not select anything on the keyboard; except the SHIFT function that lights up, but accesses the menu. This failure happened suddenly. Does somebody have an idea?? What are the tensions to check? Can the failure come from the front panel? I attach photos and a short video of the failure. Thank you for you precious help!

[lionelkjh]

a video of 13s is not supported on the site: I can not put it !!!

[lionelkjh]

I found the same case similar to the failure: I copied the link: https: //www.youtube.com/watch? V = 8ybhONvIP0Q

[Mr. Scram]

Reposted with corrected link.

[chekhov]

Reminds me this topic https://www.eevblog.com/forum/repair/agilenthp-e3646a-repair/.

There might be a connection problem between display module and the rest of the unit, and shift lights up only because this action is directly controlled by the IC that is on display module.

Have you tried to communicate with unit through RS-232 (if enabled of course) ?

[vidi]

Hi

0.) Schematics/service manual and lot of useful info can be found here https://xdevs.com/fix/hp34401a/
1) Does it beep when you turn it on ? (hint: The buzzer is driven by separate MCU (U700) that is responsible for GPIB/RS232 communication, opto-coupled from main board. It should work / beep as soon as it has a stable power supply and valid data over optocoupled UART with the main processor).
2) Check with an oscilloscope a voltage input and voltage output of U553. The U553 is a voltage regulator that also drives main MCU reset signal. If the output voltage drops below threshold, it forces a reset. It also means that one of  electrolytic caps (C556?) is probably half dead.
3) In general, check with the scope a power supply on each main digital component, then move to trace digital communication between the components.

[lionelkjh]

Hello, thank you for your help! The multimeter beep twice at startup, and display nothing, suaf when you press SHIFT (exactly as in the video). The exact version is in the photos that I posted yesterday, the video could not be put because it is in mp4 format. I'll be getting my RT809H soon, maybe I can see more clearly? I'm already going to look at these elements to check ..... Cdt

[lionelkjh]

I just downloaded the service manual, and the fault corresponds to error code 601 ......

[Samogon]

Try to reset front panel cable in the molex connector

[SrS]

Press Shift then directly Menu (On/Off).
Your display is probably set to off state.

try System menu -> Display -> On

[chekhov]

Your display is probably set to off state.

Manual says that this state is stored in volatile memory. If I understand correctly, it does not survive power-cycle.

Also, it was not mentioned whether relays are clicking or not.

[Mr. Scram]

I just tested to make sure. A power cycle through resets the display to on, so that doesn't appear to be the problem. It's a fair suggestion though.

[lionelkjh]

Hello everyone! I tried while pressing SHIFT at startup, but the digits all light up and go off, the multimeter emits 2 beeps, then the display remains off. I have raised tensions, and they are good; I strongly suspect U500 to be dead. Here are the voltages: U552 = -18V, U551 = 18V, U553 (pin2) = 5V, CR306 / CR305 (cathode) = 18V, CR304 / CR307 = -18V, CR402 (anode) = -15V; CR401 (anode) = 15V. The 80C196 must be dead, but how to test it to be certain? Is it programmed? or the content of the program is it contained in flash memory Thank you for you precious help

[Zucca]

Are GPIB and/or serial working? You could get some diagnostic out of there.

[lionelkjh]

I do not have the programmer, I am waiting for his arrival, I will be able to know more after .....

[lionelkjh]

Hello; could someone have solved this problem on this multimeter

[corn11]

Hi,

your problem description sounds like the SRAM issue I've had with my 34401A back then:
https://www.eevblog.com/forum/repair/hp-34401a-repair/msg612109/#msg612109

There is a special error code for that issue mentioned in the service manual which can only be read via GPIB if I recap that correctly.
The 80C196 can also drive the reset line if it hits a reset vector which is the case when the SRAM check fails. It could also be a faulty EPROM or the 80C196 but I doubt it.
If you haven't got a GPIB adapter and checked the usual errors like the regulator I would swap the SRAM to be sure and if this isn't the case I would socket the EPROM and get a new EPROM chip.
The firmware is available on the net in various revisions.

[lionelkjh]

Good evening; the model I have is more recent (2008) and does not have the GPIB but the RS232 .....

[HighVoltage]

Good evening; the model I have is more recent (2008) and does not have the GPIB but the RS232 .....

All 34401A have the GPIB and the RS232 interface, this was standard from the beginning to the last model.
I have never seen a 34401A without GPIB, since this is part of the main PCB

Like Zucca said, I would hook up a remote control and see is the instrument reacts to remote commands

 

[corn11]

Good evening; the model I have is more recent (2008) and does not have the GPIB but the RS232 .....

All 34401A have the GPIB and the RS232 interface, this was standard from the beginning to the last model.
I have never seen a 34401A without GPIB, since this is part of the main PCB

Like Zucca said, I would hook up a remote control and see is the instrument reacts to remote commands

The change between the rectangular and the round button units was just a facelift. There is a service note which contains the numbers of the housing parts to convert an unit from the old design to the new design. All interior parts stayed the same.
So as HighVoltage pointed out your unit should have a GPIB connector (maybe covered by a dust cap)

[macboy]

GPIB or RS232 both use the same command set. It's just a different connection.

[lionelkjh]

Bonjour; si le microprocesseur en face avant est mort (N80C51BH) est t'il possible d'y mettre un equivalent comme le AT89S51 (plus facile a programmer et reprogrammer que celui d'origine, et non doté d'un mask rom)?

[lionelkjh]

Hello; if the microprocessor on the front panel is dead (N80C51BH) is it possible to put an equivalent like the AT89S51 (easier to program and reprogram than the original one, and not with a ROM mask) ?

[coromonadalix]

Original chip: Intel N80C51BH, package PLCC44

Other text on chip: 34401-88813 REV 2.0

AT89C51 is suitable replacement.

Here's an cpu dump attached i have found,  Taken from here : https://www.eevblog.com/forum/repair/hp-34401a-repair/


In French : le cpu est remplacable par un AT89C51
Voici un dump du cpu que j'ai trouver venant de ce thread:  https://www.eevblog.com/forum/repair/hp-34401a-repair/

[lionelkjh]

Hello everyone! I did not advance much on the failure of my multimeter. I was able to desolder the lcd display on the front panel, to perform some tests using the manual. All voltages are correct. I did not find anything abnormal. The different signals should be used with the oscilloscope around the microprocessor. But how to go back to the breakdown Thank you for your advice (here is the front panel removed)