Broken 34401a - where to begin - Page 1
Forum section: electronics / repair

author: JJalling
author_rank: Unknown
timestamp: August 04, 2015, 04:37:04 pm
content: Hi,I have a broken 34401a. When the power button is pressed, all the display segments light up, and then turn off.A long beep comes from the speaker and thats about it.The shift button on the front panel can toggle the "SHIFT" segment of the display but none of the other keys seems to do anything.How would you proceed from here?Thanks, Jonas

author: vinicius.jlantunes
author_rank: Regular Contributor
timestamp: August 04, 2015, 05:17:04 pm
content: Have you tried running the self test? I think most HP/Agilent/Keysight equipment allow you to run some sort of diagnostics by holding a front pannel key (not quite sure if any key or a specific one) and then turning it on - release the key when it starts the testing routines.I don't have one of these but it's a pretty popular meter and I think I've seen other repair threads in the repair subsection, in case you haven't checked there yet.

author: radioFlash
author_rank: Regular Contributor
timestamp: August 04, 2015, 05:30:17 pm
content: I think that's the behavior when the front panel display can't communicate with the mainboard. You should download the service manual which as a schematic and check the cable commendations between the display panel and the mainboard and the power supply voltages.

author: JJalling
author_rank: Unknown
timestamp: August 04, 2015, 06:45:17 pm
content: Ok, the reset pin is oscillating...

author: JJalling
author_rank: Unknown
timestamp: August 05, 2015, 06:38:09 am
content: I tried to hook up the display to another meter, and it is working fine. So the fault must be on the main board.As I wrote earlier, the reset line is oscillating- could U553 (lt2925) be broken? I can measure 5v at the output so at least the regulation part is working.Br JonasEdit: Minor corrections

author: HighVoltage
author_rank: Super Contributor
timestamp: August 06, 2015, 08:31:25 am
content: Did you test all the rail volatges?I had several 34401A that showed really weird problems and at the end it was a voltage regulator or a bad tatalum cap.So, I always start there, including ripple and noise on the rails.

author: JJalling
author_rank: Unknown
timestamp: August 06, 2015, 07:41:14 pm
content: Quote from: HighVoltage on August 06, 2015, 08:31:25 amNo, I haven't tested them yet. My plan was to do it tonight, but now my Rigol DS1054z is stuck during boot - just my luck.BR Jonas

author: JJalling
author_rank: Unknown
timestamp: August 06, 2015, 08:18:36 pm
content: Quote from: JJalling on August 06, 2015, 07:41:14 pmFixed my scope - back to the 34401a.All the voltages are ripple free and the voltages are within spec./Jonas

author: JJalling
author_rank: Unknown
timestamp: August 06, 2015, 08:51:13 pm
content: Quote from: JJalling on August 06, 2015, 08:18:36 pmOk, maybe the comm between the earth-referenced circuit and the floating circuit is bad.If I, in my good meter, disconnect power to the floating circuit, I get the same long beep from the speaker, as in the broken meter.The 5V in the floating circuit is good.BR Jonas

author: JJalling
author_rank: Unknown
timestamp: August 08, 2015, 02:08:33 pm
content: I feel like I'm running out of options :-( I'm not sure what to look for.BR Jonas

author: JJalling
author_rank: Unknown
timestamp: August 10, 2015, 08:20:46 pm
content: The LM2925 (actually its a CS8126-1 in this worn - its a LM2925 in the working meter) seems to be working, so what is pulling the reset line low.There seems to be communication on the RAM and ROM data lines, but not on the CAL eeprom./Jonas

author: free_electron
author_rank: Super Contributor
timestamp: August 10, 2015, 08:25:57 pm
content: check the tantalum caps on the power rails. they may cause the 5 volt to collapse.

author: JJalling
author_rank: Unknown
timestamp: August 10, 2015, 09:13:12 pm
content: Quote from: free_electron on August 10, 2015, 08:25:57 pmHi free_electron,Thank you for replying.Well, the tantalums looks ok - at least visually. There is no ripple on the rails (or it is at least comparable to the ripple in the working meter). should I unsolder and replace the tantalums?/Jonas

author: JJalling
author_rank: Unknown
timestamp: August 11, 2015, 03:50:24 pm
content: Quote from: JJalling on August 10, 2015, 09:13:12 pmThe highest ripple I can measure is about 2mV ac. The measurement is done with the working 34401a./Jonas

author: HighVoltage
author_rank: Super Contributor
timestamp: August 11, 2015, 04:22:16 pm
content: If you have a working 34401A, you can compare voltages and signals step by step. In one other 34401A repair thread, someone posted a nice page of the schematics with voltages as they are suppose to be. May be you can start there.

author: JJalling
author_rank: Unknown
timestamp: August 13, 2015, 08:10:18 pm
content: Ok, I measure about 14v between the two LO terminals on the front. The left one is 14V with respect to ground./Jonas

author: tautech
author_rank: Super Contributor
timestamp: August 14, 2015, 12:40:26 am
content: Use the search button next to help button, not the search box and you will find much information including schematics in various threads:https://www.eevblog.com/forum/repair/repairing-an-hp-34401a-dmm-51092/https://www.eevblog.com/forum/repair/help-me-repair-agilent-34401a/https://www.eevblog.com/forum/repair/34401a-test-failures-(many)/https://www.eevblog.com/forum/repair/please-help-to-fix-my-agilent-34401a/https://www.eevblog.com/forum/testgear/34401a-service-notes/

author: JJalling
author_rank: Unknown
timestamp: August 14, 2015, 03:06:59 am
content: Quote from: tautech on August 14, 2015, 12:40:26 amActually, a better way to search the forum is to use Google. Write "site:eevblog.com/forum" plus your search term in the google search box. This way will also give a better overview than the forum search function.Quote from: tautech on August 14, 2015, 12:40:26 amThank you for the links, but none of them has the same problems as me. The reason I keep updating the thread is (mostly) for my own future reference. When the DMM is fixed, the plan is to update the first post of the thread with a step-by-step guide of what I did and what I observed during the repair.BR Jonas

author: tautech
author_rank: Super Contributor
timestamp: August 14, 2015, 03:13:14 am
content: Quote from: JJalling on August 14, 2015, 03:06:59 amDon't forget to add (Fixed) in the thread title when finished.

author: JJalling
author_rank: Unknown
timestamp: August 14, 2015, 03:14:59 am
content: Quote from: tautech on August 14, 2015, 03:13:14 amGood tip!! Will do that./Jonas

author: JJalling
author_rank: Unknown
timestamp: August 15, 2015, 06:52:43 am
content: Does anybody know how the main program of the processor functions? Is it worth mucking with the analog circuits, before I have sorted out the reset problem of the main processor?BR Jonas

author: JJalling
author_rank: Unknown
timestamp: August 16, 2015, 08:17:28 am
content: This is a scope plot of the 5V (CH1) and the reset line (CH2).Working meter:Broken meter:

author: JJalling
author_rank: Unknown
timestamp: September 05, 2015, 08:25:39 am
content: I don't think I'm able to repair this thing unfortunately, so the plan is to use it for spare parts./Jonas

author: jitter
author_rank: Frequent Contributor
timestamp: September 05, 2015, 09:50:37 am
content: Not so fast...You've only looked at voltages and the reset circuit, as far as I can tell.According tothis service guidethe symptoms seem to indicate a failure in serial communication between main board CPU and front panel CPU, see page 127 (as indicated by the browser/pdf-reader, not the number on the page) item 601. Item 601 is also part of the power on self-test (page 126).Next step would be to check if there's a two way communication between both processors.Edit: also look at page 113. The front panel processor can force the main processor to reset. Also interesting: for the floating front panel, 13 V is a logic 0 and 18 V a logic 1.

author: JJalling
author_rank: Unknown
timestamp: September 08, 2015, 05:23:34 pm
content: Quote from: jitter on September 05, 2015, 09:50:37 amI have another (working) meter to compare with. If I disconnect the front panel from the working meter, it still resets the way it is supposed to, so I don't think that the front panel (or the connection to it) is the problem./Jonas

author: jitter
author_rank: Frequent Contributor
timestamp: September 08, 2015, 06:38:53 pm
content: Indeed, I was aware of that from one of your earlier posts. Perhaps my mentioning the logic voltage levels on the front panel pcb suggested I thought that that pcb was faulty.My point was that the front panel pcb may force a reset on the main board if there's no communication with the main board within a certain amount of time. It may keep trying to do this, hence why you see the reset pin oscillate. The main pcb might not monitor this communication and reset normally with the front panel pcb disconnected.QuoteDoes the reset pin on the faulty main pcb also keep oscillating with the front panel pcb disconnected (it's not clear to me if your measurements were taken with or without the front panel pcb connected)?

author: JJalling
author_rank: Unknown
timestamp: September 15, 2015, 04:18:39 pm
content: Quote from: jitter on September 08, 2015, 06:38:53 pmOk, I did some measurements on the connector to the front panel (the front panel was disconnected).FPINT and FPRST are both oscillating at ~4kHz as well.And yes, the reset pin oscillates no matter if the frontpanel is connected or not./Jonas

author: jitter
author_rank: Frequent Contributor
timestamp: September 15, 2015, 05:42:40 pm
content: Okay, it's beginning to look like your hunch that the origin of the reset pin oscillating is to be found at (or rather around) U553 may be correct after all.At first I couldn't find it as you wrote it was the LT2925, but now I tried LM2925 instead and found thedatasheet.I didn't know a voltage regulator with a reset-out function existed, but this is one. Since voltage regulators are normally very robust, I'd first start looking at the health of the components around U553, especially C556, C557 and C558.Dried up caps may wreak havoc on the output of U553 and if that's not what it should be, that also generates a reset. It might also explain why it keeps resetting. Perhaps put a probe on the in- and output of U553 and see if anything strange is happening there...Edit: I read through from the beginning again, and see that you already had a closer look at this reset pin.What I might do to find out if U553 is the cause or if something else is the cause is desolder / cut the reset pin so that it's electrically isolated from the rest of the circuit and see if the behaviour has changed (on the pin of U553).I might also desolder U553 and feed in 5 V from an external PSU and connect the reset trace to 5 V through a resistor and see if that changes anything.What happens if you put another cap of 100 nF across C558?

author: JJalling
author_rank: Unknown
timestamp: September 15, 2015, 07:16:35 pm
content: Quote from: jitter on September 15, 2015, 05:42:40 pmHi jitter,I have tried to desolder the reset pin from U553 (actually it is lifted right now, and connected by a wire to the board), but it doesn't affect the reset pin, so I don't think that U553 is the culprit.I have tried to tie the reset pin high through a resistor, but it didn't work - it still oscillated.Thank you for your comments and ideas./Jonas

author: jitter
author_rank: Frequent Contributor
timestamp: September 15, 2015, 08:08:51 pm
content: Quote from: JJalling on September 15, 2015, 07:16:35 pmWhat does the reset pin on U553 do when you cut that wire so that the pin is floating from the board? Does it still oscillate?Leave that pin disconnected from the board and measure what the signal looks like on the pad/trace where the pin used to be (XPONRST in the schematics).They should not be the same when they're separated.QuoteTrying to pull the reset line high through a resistor may not work with the reset pin of U553 still in place. Try it again with that pin disconnected.The way I see it, the reset of U553 is only an output (going to reset-inputs on U500 and U501), and certain situations described in the datasheet (input transients, low input voltage, etc.) that result in the voltage output going out of regulation can trigger a new reset.If pin 5 of U553 still oscillates when completely lifted off the board, then it must be U553 that's causing the reset to oscillate.But the root cause may not be U553 itself. But that's the next step.QuoteYou're welcome.

author: JJalling
author_rank: Unknown
timestamp: September 15, 2015, 08:52:33 pm
content: Quote from: jitter on September 15, 2015, 08:08:51 pmWhen the reset pin on U553 is disconnected, it stays low - hmmm... The XPONRST pin is still oscillating.Quote from: jitter on September 15, 2015, 08:08:51 pmI have connected XPONRST to +5V(pin 2 of U553) through 1K, but it still oscillates.Quote from: jitter on September 15, 2015, 08:08:51 pmYes, thats how I understand it. But now it seems like it never goes high, so maybe U553 is broken after all - I will try order a replacement.But the reset line is still oscillating - should I try pulling it up with a lower resistor value?Quote from: jitter on September 15, 2015, 08:08:51 pm

author: jitter
author_rank: Frequent Contributor
timestamp: September 16, 2015, 04:59:59 am
content: Now that I know that the reset pin of U553 stays low when disconnected, I'm not convinced that the oscillation is caused by that pin. Your first scope image already seemed to indicate that the reset circuit of U553 was working correctly because the time it takes to change state from low to high after power on is what it should be with the value of C558 that was used (~ 250 ms).Another thing that seems to indicate that something else may be wrong is the fact that the reset pin of U553 stays low when disconnected. If you look at the block diagram of the LM2925, you'll see that there's a line drawn over the text RESET, meaning it's inverted. In other words: low = active. For some reason, U553 seems to detect a fault. The attachment shows situations in which the reset pin will go active.Before ordering a new LM2925, I would try the experiment with an external 5 V PSU and XPONRST tied high. Also measure the current drawn, perhaps a bad tantalum is on the verge and already drawing excess current.Should that not solve the problem, it sure looks like something else is the root cause and not U553 itself.I don't like XPONRST oscillating on its own, it looks like it might be shorted to another trace.

author: JJalling
author_rank: Unknown
timestamp: September 16, 2015, 04:41:41 pm
content: Quote from: jitter on September 16, 2015, 04:59:59 amI now have removed the jumper JM553, and connected an external 5V supply. The meter draws around 130mA from the 5V rail, but the XPONRST still oscillates.

author: jitter
author_rank: Frequent Contributor
timestamp: September 16, 2015, 05:53:38 pm
content: I'm afraid that does not look good, it rules out U553 as the cause and it may mean that U500 or U501 is defective.At this stage I strongly suspect something might be very wrong with U500 or U501, one of the reset inputs seems to act as an output... perhaps internally shorted.Might be worth checking with an ohmmeter on XLPORST and adjacent traces if there's a suspiciously low resistance between them. But I'm running out of suggestions what else to check.Ah well, you can still use it for spare parts...

author: JJalling
author_rank: Unknown
timestamp: September 16, 2015, 06:24:09 pm
content: Quote from: jitter on September 16, 2015, 05:53:38 pmIf I measure the resistance between +5V and gnd, I get 6K5 ohm on the broken meter, and 9K5 ohm on the working meter - capacitors?Quote from: jitter on September 16, 2015, 05:53:38 pmYep/Jonas

author: jitter
author_rank: Frequent Contributor
timestamp: September 17, 2015, 05:15:45 pm
content: Don't know. Caps with leakage would probably result in much lower resistance...

author: teksturi
author_rank: Regular Contributor
timestamp: September 17, 2015, 09:51:59 pm
content: I would still try to cut xlporst trace between U500 and U501. Then you may at least know where the fault is coming from. Try to cut as close U500 (I think it is the reason). If oscillation is coming from trace (very very lucky shot) you can then use jump wire.I found this thread where symptoms are similar to yours.https://www.eevblog.com/forum/testgear/hpagilent-34401a-dmm-startup-problem/But theres no help

author: JJalling
author_rank: Unknown
timestamp: September 21, 2015, 01:33:52 pm
content: Quote from: teksturi on September 17, 2015, 09:51:59 pmI just tried to cut the trace near U500. The pin at U500 oscillates and the other side of the cut, is low./Jonas

author: teksturi
author_rank: Regular Contributor
timestamp: September 21, 2015, 02:41:20 pm
content: I still think that it might be something to do with mcu program. It is very unlikely (I think needs another opinion) that there is something broke inside mcu because oscillation happens also some other pins. Reset pin can also act as open-drain output (page 6) and there is pull-up resistor in reset pin (page 9). There can be some situation where watchdog gig in. Example LSENSE sees only 50Hz or 60Hz because in diode bridge one diode might be faulty. Then there is programming error and there will be forever while.You should think why MCU is doing reset and what states should be in input when system is going up.mcu datasheethttp://media.digikey.com/pdf/Data%20Sheets/Intel%20PDFs/8xC196KB_KB16.pdf

author: JJalling
author_rank: Unknown
timestamp: September 21, 2015, 02:43:14 pm
content: Quote from: teksturi on September 21, 2015, 02:41:20 pmExcellent info. I will try to do some digging in the datasheet. I will get back here, when I get further.Thanks!/Jonas

author: JJalling
author_rank: Unknown
timestamp: December 11, 2015, 08:13:33 am
content: I'm giving up on this - is there anything in the meter worth selling, or should I just dump it in the garbage?BR Jonas

author: HighVoltage
author_rank: Super Contributor
timestamp: December 11, 2015, 12:57:44 pm
content: Quote from: JJalling on December 11, 2015, 08:13:33 amSorry to hear.The Display is sometimes selling well, if it is nice and bright.The rest is probably not much worth in this stage.

author: JJalling
author_rank: Unknown
timestamp: June 05, 2016, 06:57:32 pm
content: Quote from: JJalling on September 21, 2015, 01:33:52 pmI just found this meter in a box in the workshop - does anyone have a last suggestion before I throw it out?Reading through the previous comments and my own notes, it seems like the resetpin on U500 is oscillating when not connected to anything. Is U500 fried?Thanks,BR Jonas

author: teksturi
author_rank: Regular Contributor
timestamp: June 06, 2016, 05:06:25 pm
content: It is probably faulty U500. At least do not trash it. Sell it to someone who need it. You can sell it and tell that U500 is faulty and you do not want to repair it.

author: teksturi
author_rank: Regular Contributor
timestamp: June 06, 2016, 05:19:09 pm
content: Quote from: JJalling on September 15, 2015, 04:18:39 pmDo you mean that U500 reset pin is oscillating same frequency? This is kinda strange to me.Have you confirm that XLPORST work correctly after you cut U500 out. Have you check crystal of U500?

author: JJalling
author_rank: Unknown
timestamp: June 07, 2016, 07:35:44 am
content: Quote from: teksturi on June 06, 2016, 05:19:09 pmI just measured again - the signal on the \RESET is 5.7kHz - so does FPINT and FPRST.The XLPORST just stays low, so something is probably wrong with the regulator (U553) anyway - I thought it was working, so I'll try to order a replacement for that one.For a test, should I try to pull the RESET high through a resistor?ThanksBR Jonas

author: teksturi
author_rank: Regular Contributor
timestamp: June 12, 2016, 08:05:22 pm
content: Yes, you can try. Good find that U553 is broken.