HP34401A - Page 1
Forum section: electronics / repair

author: lucian
author_rank: Contributor
timestamp: February 04, 2024, 09:02:53 am
content: Solid Beep on boot, I have a hp34401A and it won't stop beeping on boot the display switches ons but nothing works, the beeping does stop if I change to use the rear jacks but it still doesn't go past the boot screenhttps://youtu.be/GEM9esejeUU

author: lucian
author_rank: Contributor
timestamp: February 04, 2024, 09:04:40 am
content: More photos

author: iMo
author_rank: Super Contributor
timestamp: February 04, 2024, 09:47:49 am
content: fyiPS: do start with the checking of all supply voltages..Download the Service manual, there is the schematics with all the components on the pcbs..

author: lucian
author_rank: Contributor
timestamp: February 04, 2024, 10:47:57 am
content: 5v was fine and 18v looked fine, but I will look to order a replacement for that part any ideas about the buzzer

author: trobbins
author_rank: Frequent Contributor
timestamp: February 04, 2024, 11:23:41 am
content: Suggest start with all the voltage rails - there are more than two rails, and they include regulator circuitry. Afaik there is/are beep(s) as mains voltage increases (eg. when using a variac), and likely related to under-voltage lockout style activation of sections of circuitry.

author: lucian
author_rank: Contributor
timestamp: February 04, 2024, 11:41:03 am
content: Just noticed the 3amp fuse in the rear holder is burst just an fyi

author: lucian
author_rank: Contributor
timestamp: February 04, 2024, 12:30:07 pm
content: Any further steps for diagnosis or fixes

author: trobbins
author_rank: Frequent Contributor
timestamp: February 04, 2024, 01:20:37 pm
content: The fuse has blown, so do you have a Variac, or can power from lower than mains, or have a bulb limiter? Start by disconnecting the 8-pin connector from the transformer. But you need to have the service manual, and need to appreciate the power supply section schematic first??

author: factory
author_rank: Super Contributor
timestamp: February 04, 2024, 01:35:49 pm
content: The 3A fuse is for the current measurement function, I can't imagine it would stop the DMM from working on other functions?Mains fuses are much lower values.David

author: lucian
author_rank: Contributor
timestamp: February 04, 2024, 02:01:33 pm
content: Sadly I do not have either of those, Ive been looking at the schematics for the power supply and I really doubt its an issue with that

author: lucian
author_rank: Contributor
timestamp: February 04, 2024, 02:13:48 pm
content: Just noticed that this part is getting very hot but its still supplying a pretty clean 5v so Im a bit stumped

author: lucian
author_rank: Contributor
timestamp: February 04, 2024, 02:24:48 pm
content: Still Stumped on why when the front selector is pressed to use the rear inputs and the multimeter is rebooted the beeping stops

author: iMo
author_rank: Super Contributor
timestamp: February 04, 2024, 02:54:27 pm
content: Quote from: lucian on February 04, 2024, 02:13:48 pmThat 5V voltage regulator powers the outguard logic only. There are i/o interfaces (HPIB, serial) driven by an MCU where the chips might be damaged. Also the tantalum capacitors need to be checked.The +/-18Volts in the inguard logic (with the ADC, input stuff, main CPU etc) go through various serial zeners (3.2V) thus creating +/-15V for all important analog circuitry there (). Doublecheck them as well, as they get damaged easily.

author: lucian
author_rank: Contributor
timestamp: February 04, 2024, 02:57:55 pm
content: Ok thanks, could that be the root cause?

author: lucian
author_rank: Contributor
timestamp: February 04, 2024, 02:58:50 pm
content: Is this meant to be unpopulated

author: iMo
author_rank: Super Contributor
timestamp: February 04, 2024, 03:05:28 pm
content: Yes, unpopulated, that goes to the RS232 connector, there are 2 free pins without a signal on them.

author: lucian
author_rank: Contributor
timestamp: February 04, 2024, 03:36:53 pm
content: I am assuming by serial zeners you mean this i checked them at it ends at 14.8v

author: tggzzz
author_rank: Super Contributor
timestamp: February 04, 2024, 03:44:07 pm
content: Quote from: lucian on February 04, 2024, 03:36:53 pmTo understand the input protection circuits, there's a good explanation in "The Art of Electronicsx-Chapters".https://x.artofelectronics.net/

author: lucian
author_rank: Contributor
timestamp: February 04, 2024, 03:49:32 pm
content: Thanks will take a look at this tmr morning but Im assuming what I did was incorrect?

author: iMo
author_rank: Super Contributor
timestamp: February 04, 2024, 03:55:04 pm
content: Those are not the power supply zeners I mentioned above (those are around the ADC part, look at the schematics and pcb view in the Service manual).CR304,305,306,307CR400, 401

author: lucian
author_rank: Contributor
timestamp: February 04, 2024, 04:04:06 pm
content: Oh you are referring to the ones here correct?

author: alm
author_rank: Super Contributor
timestamp: February 04, 2024, 04:13:37 pm
content: Did you get it like this or did it suddenly break? If so, did anything particular happen to it before it broke?Did you measure both voltage and ripple on all power rails (+/- 18V floating, +5V floating, +5V earthed, 6VAC for front panel)? Ideally you use a scope to measure.The blown fuse and damaged C109 cap (in parallel with the MOV) both suggest excessively high input signals, though on different inputs. But otherwise after the power rails I'd start looking at the floating or front panel logic circuits. Study what the service manuals says about start up steps (does all segments lighting up indicate correct operation of the front panel?), and based on that identify a suspect. Then look for clock, activity on address bus, reset line, etc.Can you connect to it using RS-232 or GPIB? Maybe it's just the front panel that locks up.

author: J-R
author_rank: Super Contributor
timestamp: February 04, 2024, 10:33:59 pm
content: I say focus on testing all the capacitors right away. You might be able to do some in-circuit, but if you can't tell for sure, remove for proper testing. If you search for 34401A repairs, you'll see a lot of folks just replace all of them, especially on the older units. Maybe you will get lucky and quickly find one that is shorted.

author: lucian
author_rank: Contributor
timestamp: February 05, 2024, 12:09:22 pm
content: Ok all the caps are fine anything I shoud check next

author: lucian
author_rank: Contributor
timestamp: February 05, 2024, 12:14:03 pm
content: all the big ones

author: lucian
author_rank: Contributor
timestamp: February 05, 2024, 02:07:42 pm
content: Ive noticed that these ic and the MCU are sitting around the mid 40's is this to be expected?

author: Paceguy
author_rank: Regular Contributor
timestamp: February 05, 2024, 03:27:51 pm
content: Open power supply filter caps can cause regulators to run very hot. Did you check the ESR of all the power supply caps ?

author: lucian
author_rank: Contributor
timestamp: February 05, 2024, 04:38:44 pm
content: I sadly don't have a esr meter but I have checked the resistance using a multimeter and they are all very high so I don't think thats the issue

author: Kleinstein
author_rank: Super Contributor
timestamp: February 05, 2024, 05:10:33 pm
content: DC resistance is very different from ESR. High ESR would cause increasing power supply ripple. Instead of looking at the ESR one could also check the supply ripple with a scope and as a last resort a DMM (in AC mode).The CPU can get a little warm, as it is a relatively old type and still running on 5 V. So it will use some power. It would be a bad sign if it is not getting warm.

author: coromonadalix
author_rank: Super Contributor
timestamp: February 05, 2024, 06:15:15 pm
content: i would say, yep could be a case of bad capacitors ... and while they are removed, you could replace the small tantalums near them toowith hot airnot an iron ..

author: J-R
author_rank: Super Contributor
timestamp: February 06, 2024, 12:10:40 am
content: https://xdevs.com/fix/hp34401a/

author: lucian
author_rank: Contributor
timestamp: February 08, 2024, 03:57:44 pm
content: Does anyone have a list of the sizes of the different tantulum caps

author: J-R
author_rank: Super Contributor
timestamp: February 09, 2024, 05:11:59 am
content: Quote from: lucian on February 08, 2024, 03:57:44 pmThe link I posted has the service manual which has all that info.

author: lucian
author_rank: Contributor
timestamp: February 09, 2024, 01:18:29 pm
content: It doesn't have the specific sizings

author: JohanH
author_rank: Frequent Contributor
timestamp: February 09, 2024, 02:00:47 pm
content: Quote from: lucian on February 09, 2024, 01:18:29 pmHere's the list of tantalums that I wrote down once, from the manual, but please double check:C552, C554, C753 (main PCB) 1 uF ±20% 35 V tant (+18V rail) B-case (3.5 x 2.8mm)C616, C617, C602, C603, C604 (display) 1 uF ±20% 35 V tant (+18V rail) B-case (3.5 x 2.8mm)C411 4.7 uF ±20% 10 V B-case (3.5 x 2.8mm) (C411 wrongly marked as L411 on component placement diagram)C311, C312, C322, C323, C557, C559 22 uF ±20% 20 V D-case (7.3 x 4.3mm)C706, C512 47 uF ±20% 10 V D-case (7.3 x 4.3mm)

author: J-R
author_rank: Super Contributor
timestamp: February 09, 2024, 11:53:29 pm
content: Quote from: lucian on February 09, 2024, 01:18:29 pmOK, I thought you meant values not dimensions. The service manual does have part numbers listed which can direct you to that info and an appropriate replacement.If you don't have an LCR or ESR meter you could test the capacitors using a function gen and scope (out of circuit).