Need help for restoring an HP34401A multimeter - Page 1
Forum section: electronics / repair

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: October 01, 2021, 02:23:30 pm
content: Dear All,I scored a HP34401A in pretty bad shape. no power button, no case (but front panel).I will attempt to restore it with your help.I found various service manuals on the web, but not always very explicit.I have shunt the power button, and power it on from an external switch.The unit powers on, the display is on, but nothing on the display ( all segments on ) ( see the attached picture).I have measured the voltages at the connector that connects to the display (P500)This is what I find1: GND2: 5.029 V3: 18.95 V4: 5.036V5: -12.22 V6 : 5.036V7: -18.20V8: 09: GND10: 011: -12.22 V12: 2.525VAny hint ?

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: October 01, 2021, 02:35:28 pm
content: In the service manual of the HP34401A, they say to check the main supply voltage, but where do I ned to check it ?The only connector I found was the P500Where do I check the +7REF and the +5REF ?

author: chekhov
author_rank: Regular Contributor
timestamp: October 01, 2021, 06:20:59 pm
content: Manual does not show test points, so according to schematics (page 162) at the input and output of appropriate voltage regulators. I believe there are a few dozens of "34401A repair" topics on the forum.Also, maybe it's just shadow, but this tantalum capacitor near display connector already looks fried.

author: merox
author_rank: Regular Contributor
timestamp: October 01, 2021, 08:27:55 pm
content: Quote from: chekhov on October 01, 2021, 06:20:59 pmThat thing looks more than fried, it even looks kind of bulged

author: floobydust
author_rank: Super Contributor
timestamp: October 01, 2021, 08:33:40 pm
content: The Front Panel connector P500 voltages all look fine. This is the 1st generation uPD7527 MCU?You have to be careful because the (Front Panel board) circuit common (GND) for the IC's is actually at -18VDC and everything is shifted to accommodate the high voltage VFD. It can be confusing.On the Front Panel board, check the +13VDC rail is OK, and I would see if there is life in the Front Panel MCU if you scope the RC clock at pin 2/3. Check VPRE pin 4 is around +8V I think.The tantalum cap C323 in the picture does look a bit discoloured. It is for the ACV circuit -15V rail/CR307-A but I don't think it would affect the display.

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: October 01, 2021, 09:08:35 pm
content: Thans for the feedback.If the voltage on the P500 connector are fine, could it be that the display unit is faulty ?The problem with it is that I am not very eager to desolder the display. Unfortunately, it seems that to inspect the circuit, you have not much choice.I would really like to be sure that this is the thing to do before going further.I attach here a few other pictures of the main board.

author: chekhov
author_rank: Regular Contributor
timestamp: October 01, 2021, 09:24:09 pm
content: What I see on your last picture (maybe I'm not right) - something like little pieces of solder here and there - sign of warning to me.A few things may fail, just a few thoughts:* tantalums on display board or on main board - check them.* RAM chips (btw, does meter beep at all ?)* CPU or it's reset line - check as well.So still there are a few things before VFD desoldering. Maybe you have GPIB adapter or try to use RS-232 to communicate with unit - you really have to figure out whether brain has issues or is that really front panel itself.

author: floobydust
author_rank: Super Contributor
timestamp: October 02, 2021, 03:22:53 am
content: You can still take readings from the backside of the Front Panel board. pic taken fromhttps://xdevs.com34401a repair.The Main CPU issues a RESET pulse to the Front Panel processor if a watchdog trips. I thought the meter beeps when this happens.

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: October 02, 2021, 09:30:11 am
content: Quote from: merox on October 01, 2021, 08:27:55 pmThe capacitor looks OK on the binocular. No bulge, no decoloration. 23 uF on the multimeter ( without desoldering). No heat on the infrared camera.

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: October 02, 2021, 01:15:55 pm
content: A few IR image of the unit under power done with my C200 InfiRay camera + ZnSe lens

author: factory
author_rank: Super Contributor
timestamp: October 02, 2021, 01:45:32 pm
content: There is currently a newer one on ePay in the UK withpossiblythe same problem (all display segments lit), I suspect it's this one that had blank display a few months back, it had an SRAM failure that time;https://www.vintage-radio.net/forum/showthread.php?p=1365376I've no idea if the current fault is the same problem, or something else.David

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: October 02, 2021, 02:40:44 pm
content: Quote from: factory on October 02, 2021, 01:45:32 pmThanks for mentioning this. Indeed, this is similar. Although I have never hear any beep when powering the device on.

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: October 02, 2021, 04:40:57 pm
content: Quote from: floobydust on October 01, 2021, 08:33:40 pmThanks for your feedback. This is very helpful.It is indeed the first generation display : 34401-66502QuoteI have checked the pin 2 and 3 . I found clock signal on both pins : nice square, 5.36 V pk to pk , 563 khz on pin 3, more deformed signal 460 khz, 5.44V pk to p on pin 2.SO for the moment, it seems that the display is not the culprit.

author: floobydust
author_rank: Super Contributor
timestamp: October 03, 2021, 03:06:20 am
content: Upon power up, the Front Panel processor will light all segments, like what you are seeing. It waits for serial data from the main CPU.It might work fine, is not the problem. It's just getting no data from the main board CPU.You could look at the serial data lines IGFPSCK, IGFPDI, IGFPDO and see if there is communication. Try hitting buttons. No comms and error E601 happens and I thought it beeps.

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: October 03, 2021, 05:03:03 pm
content: Excuse my lack of experience, but I must say I am a little bit confused with the use of floating logic and Earth referenced logic.I am contemplating the block diagram from page 9-7, and I am sure that with some concentration the light will come out, but a little help will certainly be welcome.DO we agree that the floating earth is an AC signal of 50 hz and 6.82V amplitude (13.7 Vpp) ?I have nevertheless been able to check that the oscillator is OK. With 12Mhz frequency and two outputs at 3.66 Vpp and 2.24 Vpp.

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: October 03, 2021, 05:08:28 pm
content: Quote from: floobydust on October 03, 2021, 03:06:20 amI dont now if I did it right, but I could not find any activity on these lines, even pressing the various key of the keypad.

author: floobydust
author_rank: Super Contributor
timestamp: October 03, 2021, 08:38:03 pm
content: The entire multimeter is isolated so it's floating from earth-ground, EXCEPT for the RS-232/GPIB interface.So multimeter common GND will have a little weak AC hum and noise present. It's OK to connect your scope ground there (which connects the floating logic to earth-ground) as long as you aren't making measurements to items that are floating to a potential themselves.You might have to see which MCU seems to be hung or not running. The beeper should be making noise, it's driven by U700 on the earth-grounded side by the RS-232/GPIB section. Just weird. Is there data flowing across the opto-isolators at IGTXD, OGTXD? You could try connect to the RS-232 port and see if it's alive.I would back up a few steps and check power the 5V rails like Vcc and PONRST from U553 in the power supply, and +5V_D.Check for a stuck pushbutton, that will also cause the display to stay all segments lit - but it causes two beeps from the main controller MCU which you are not getting.

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: October 04, 2021, 05:38:28 pm
content: Quote from: floobydust on October 03, 2021, 08:38:03 pmAre the infrared pics useful for this ?QuoteI confirm that I never heard the beeper.QuoteI already connected to the RS232 port. I had nothing. But I read in the doc that the GPIB is activated by default, but not the RS232 which need to be activatedQuoteWill do that when I access again my workshop.QuoteI doubt on this one as I had the same behaviour when the keypad was removed ( display connected ).

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: October 10, 2021, 06:40:12 pm
content: Quote from: floobydust on October 03, 2021, 08:38:03 pmIt seems that there is indeed a problem here. Still investigating.By the way, I am surprised to see that the LM340 and LM337 are in To-220 packages and not in special SMD packages. Is that common, to just bendthen pins of a TO-220 to make it SMD, or is it a special version ?

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: October 11, 2021, 10:47:24 pm
content: I was too optimistic. The +5V_D is perfectly at 5V, with no ripple, and still no beep .I am not sure +5V_D was not always good, even before I mande any change ( replaced the LM340) and the associated tantalum cap.

author: Swainster
author_rank: Frequent Contributor
timestamp: October 12, 2021, 06:21:40 am
content: Hi Jacques,I'm working on a 34401a repair at the moment. I'm think it is likely that your display is fine, or at least not the only problem, as when I remove the display on my 34401a it still beeps. With a working display attached it gives one or two short beeps, and with no display it gives 2 long beeps.I would suspect an issue with the mainboard floating logic. Perhaps a good place to start (after the obvious power supply stuff) is the XPONRST signal (power good) from pin 5 of U553. Can't really add much more as my problems are all analogue related - my floating logic section seems to be working fine. Also might be worth scoping out the signals on the optocouplers (U596, U704) to see if the ground referenced logic is talking to the floating logic.Good luck!edit: the LM337 on this board is a variant of the SMD D2PAK TO-263 package. These dont normally have the mounting hole though. You can easily buy the D2PAK variant without mounting hole, but I bent the legs of a TO-220 to save buying more parts.