AGILENT 34401A - Page 1
Forum section: electronics / repair

author: lionelkjh
author_rank: Frequent Contributor
timestamp: November 23, 2018, 06:52:08 pm
content: 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!

author: lionelkjh
author_rank: Frequent Contributor
timestamp: November 23, 2018, 07:12:55 pm
content: a video of 13s is not supported on the site: I can not put it !!!

author: lionelkjh
author_rank: Frequent Contributor
timestamp: November 23, 2018, 07:29:34 pm
content: I found the same case similar to the failure: I copied the link: https: //www.youtube.com/watch? V = 8ybhONvIP0Q

author: Mr. Scram
author_rank: Super Contributor
timestamp: November 23, 2018, 07:33:40 pm
content: Reposted with corrected link.

author: chekhov
author_rank: Regular Contributor
timestamp: November 23, 2018, 08:00:29 pm
content: Reminds me this topichttps://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) ?

author: vidi
author_rank: Contributor
timestamp: November 23, 2018, 10:53:16 pm
content: Hi0.) Schematics/service manual and lot of useful info can be found herehttps://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.

author: lionelkjh
author_rank: Frequent Contributor
timestamp: November 24, 2018, 08:53:51 am
content: 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

author: lionelkjh
author_rank: Frequent Contributor
timestamp: November 24, 2018, 11:41:56 am
content: I just downloaded the service manual, and the fault corresponds to error code 601 ......

author: Samogon
author_rank: Frequent Contributor
timestamp: November 24, 2018, 05:07:37 pm
content: Try to reset front panel cable in the molex connector

author: SrS
author_rank: Contributor
timestamp: November 26, 2018, 11:34:19 am
content: Press Shift then directly Menu (On/Off).Your display is probably set to off state.try System menu -> Display -> On

author: chekhov
author_rank: Regular Contributor
timestamp: November 26, 2018, 12:07:12 pm
content: QuoteManual 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.

author: Mr. Scram
author_rank: Super Contributor
timestamp: November 26, 2018, 12:48:45 pm
content: 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.

author: lionelkjh
author_rank: Frequent Contributor
timestamp: November 26, 2018, 01:42:02 pm
content: 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 memoryThank you for you precious help

author: Zucca
author_rank: Unknown
timestamp: November 26, 2018, 06:07:28 pm
content: Are GPIB and/or serial working? You could get some diagnostic out of there.

author: lionelkjh
author_rank: Frequent Contributor
timestamp: November 26, 2018, 06:30:31 pm
content: I do not have the programmer, I am waiting for his arrival, I will be able to know more after .....

author: lionelkjh
author_rank: Frequent Contributor
timestamp: November 27, 2018, 12:37:37 pm
content: Hello; could someone have solved this problem on this multimeter

author: corn11
author_rank: Regular Contributor
timestamp: November 27, 2018, 01:26:15 pm
content: 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/#msg612109There 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.

author: lionelkjh
author_rank: Frequent Contributor
timestamp: November 27, 2018, 04:55:26 pm
content: Good evening; the model I have is more recent (2008) and does not have the GPIB but the RS232 .....

author: HighVoltage
author_rank: Super Contributor
timestamp: November 27, 2018, 06:15:30 pm
content: Quote from: lionelkjh on November 27, 2018, 04:55:26 pmAll 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 PCBLike Zucca said, I would hook up a remote control and see is the instrument reacts to remote commands

author: corn11
author_rank: Regular Contributor
timestamp: November 27, 2018, 06:33:06 pm
content: Quote from: HighVoltage on November 27, 2018, 06:15:30 pmThe 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)

author: macboy
author_rank: Super Contributor
timestamp: November 27, 2018, 06:33:34 pm
content: GPIB or RS232 both use the same command set. It's just a different connection.

author: lionelkjh
author_rank: Frequent Contributor
timestamp: December 16, 2018, 01:14:05 pm
content: 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)?

author: lionelkjh
author_rank: Frequent Contributor
timestamp: December 16, 2018, 01:15:23 pm
content: 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)?

author: coromonadalix
author_rank: Super Contributor
timestamp: December 16, 2018, 02:27:31 pm
content: Original chip: Intel N80C51BH, package PLCC44Other text on chip: 34401-88813 REV 2.0AT89C51 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 AT89C51Voici un dump du cpu que j'ai trouver venant de ce thread:https://www.eevblog.com/forum/repair/hp-34401a-repair/

author: lionelkjh
author_rank: Frequent Contributor
timestamp: March 05, 2019, 04:36:30 pm
content: 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 breakdownThank you for your advice (here is the front panel removed)

author: EE-digger
author_rank: Frequent Contributor
timestamp: March 06, 2019, 04:06:17 am
content: I had a VERY similar problem on a 33120A function generator. I have no idea if the two have any similarity but your symptoms give me deja-vu.On the 33120A, a small reset chip was not coming up to full logic high output. It was stuck midway up, hence resets were not clean for both the main and display processors.I don't know if the 34401A actually has a separate display processor or not.

author: wictor
author_rank: Regular Contributor
timestamp: March 06, 2019, 11:15:29 am
content: There is separate processor in front panel. It's getting all display data from main board cpu. If there is no data arriving to front panel, there is nothing displayed.

author: coromonadalix
author_rank: Super Contributor
timestamp: March 06, 2019, 11:26:19 am
content: the best course would be to have another display to do some tests, or a logic sniffer ...... to be sureI wont argue with all voltages, but do a recheck, theres many of themIts an old meter, in mine i have done the recapping to be sure, they where all working before, but the tantalums where ginving me frills, in one they where slightly brown"ish"

author: SpencerTC
author_rank: Contributor
timestamp: March 06, 2019, 04:34:52 pm
content: Have you tried to get any response through GPIB (or other external communications)?

author: coromonadalix
author_rank: Super Contributor
timestamp: March 06, 2019, 05:53:45 pm
content: With no screen how can we know the gpib adress ? unless you see it briefly

author: HighVoltage
author_rank: Super Contributor
timestamp: March 06, 2019, 06:21:33 pm
content: Quote from: coromonadalix on March 06, 2019, 05:53:45 pmYou just try them all, there are not too many possibilities.

author: picburner
author_rank: Frequent Contributor
timestamp: March 06, 2019, 06:56:13 pm
content: With NI MAX you can do an automatic scan and it tells you which instruments are connected and their address.I think you can do the same with Agilent's software.

author: HighVoltage
author_rank: Super Contributor
timestamp: March 06, 2019, 07:56:21 pm
content: Yes, the Keysight software scans automatically for what is hooked up to any of the GPIB ports.

author: lionelkjh
author_rank: Frequent Contributor
timestamp: March 10, 2019, 05:31:47 pm
content: Good evening; I made no attempt with the GPIB (because I do not have this big connector, nor software, nor interface!): in summary; I'm a little lost! I specify that this version and 2008 it is more recent than the first models released in the 90s. How to

author: SpencerTC
author_rank: Contributor
timestamp: March 11, 2019, 02:33:08 am
content: Well, the first thing I generally check with nonfunctional equipment is the easiest things...external communication (GPIB, serial), ensuring voltages are correct (that 3.3VDC test point is close to 3.3V, same with any other voltages noted on silkscreen/in service manual), unseat and reseat any socketed IC's, see if there are any unpopulated {or populated} footprints that could be looked at (ex. JTAG, serial), sniff around with logic analyzer.I own a 34401A, although I've never had an issue with it and have not had a reason to become familar with the troubleshooting process of it. If you happen to have a second instrument that you can compare voltages, signals, and responses to, then it can make the whole process much easier/quicker.Try what you can and let us know what you find and I'm sure some more people will try to help (either I or some of the much more knowledgeable people on this site)Good luck!

author: coromonadalix
author_rank: Super Contributor
timestamp: March 11, 2019, 02:47:14 am
content: just had a small problem with my second 34401 the white display connector would not "get up" or stuck at "pressed position" not closing on the flex cable center pins when i pushed it to do a verificationWorth to check@Lionelkjh you can surely build the project ar488 usb to gpib adapter for almost nothing, you can communicate thru serial console, or even try with the rs232 if there is somthing going out of the meterhttps://www.eevblog.com/forum/projects/ar488-arduino-based-gpib-adapter/But for the rs232 normally you set the communication type with the meter menu ..... the talkback (you only read / see the meter readings / no interaction is possible, is done at adress 37 if i recall

author: Keith956
author_rank: Regular Contributor
timestamp: April 14, 2021, 06:15:54 pm
content: I'm having the same problem with a 34401A I picked up today. The reset line (pin5 of U553) is oscillating at about 4.5KHz, see pic. Disconnecting pin5 from the XPONRST line shows that signal still oscillating (and pin5 is at 0v) so it looks like U500 is perhaps driving the signal low every 225uS?Someone mentioned this could be the SRAM failing self-test, is this SRAM still available? Any way to confirm this is the fault? Thx.

author: artag
author_rank: Super Contributor
timestamp: April 14, 2021, 07:51:13 pm
content: U500 is an 80c196 and has a bidirectional reset pin. So yes, it could well reset itself as a result of a test failure and assert reset outwards.The SRAM is a common 8kx8 cmos ram and is easy to find. However, it's supposed to be able to test the sram itself: the very short time it takes to reset suggests it's crashing rather than failing a self test. I'd expect it to use internal memory to start with. It may only use the external sram for more complex functions. So I'd be more suspicious that the eprom is faulty (or the data bus getting corrupted).If you have a wide logic analyser you could record address and data bus for the short period it runs for - that may show up a lost bit.The continuous reset can actually be used to help you find the fault. Trigger your scope on reset and then probe the address and data bus with another channel. You're looking for all 0s, all 1s, a deformed waveform, or something similar that prevents eprom or sram data reaching the processor.

author: Keith956
author_rank: Regular Contributor
timestamp: April 14, 2021, 08:14:41 pm
content: Good idea about the logic analyser - I've got a siglent 2104X+ with the logic probes - would that work?

author: artag
author_rank: Super Contributor
timestamp: April 14, 2021, 11:45:05 pm
content: Quote from: Keith956 on April 14, 2021, 08:14:41 pmIt's not enough to do a full bus capture, but it can do something useful. To be honest I'd start with the scope probe - it's far quicker and will show up something like a broken track fairly quickly. It's a multiplexed bus so you'll have to follow through address going out while ALE is active, that address appearing on the eprom, and then data getting back from the eprom to the cpu.If that fails to find the problem, you'll at least have a good understanding of what the address is doing (you should be able to ignore the higher address bits if you're sure the eprom is getting its chip select) and you can dig in a bit further and try to understand why its crashing or resetting after such a short time.You do need to be able to read the data and address off the scope's LA inputs. Most scopes will have a list mode that lest you do this but I don't know the Siglent.

author: Keith956
author_rank: Regular Contributor
timestamp: April 26, 2021, 07:58:05 pm
content: Just to mention - I managed to get a Cypress CY62256L-70SC sram from ebay (well actually 10 of them for $5.76). After replacing the chip the 34401A works fine, and passes the shift+power on self test.

author: HighVoltage
author_rank: Super Contributor
timestamp: April 27, 2021, 07:53:26 am
content: Quote from: Keith956 on April 26, 2021, 07:58:05 pmNice, congratulations !I have had great success with a few 34401A to bring them back alive with such repair.What I have not found out was the reason for this sram to die.Maybe the lifespan of these chips was reached.