No.084 - ***Another*** HP 34401A 6.5 digit Multimeter Repair - Part 1
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- Опубліковано 19 лют 2022
- After acquiring another faulty HP 34401A Multimeter, in this Part 1 I begin repairs. One main symptom but quite a few issues found!
Part 2:
• No.085 - ***Another***...
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So here's my theory......we'll see if Part 2 confirms or not.
I reckon the uP is not starting up due to a fault of its own, the ram or prom. The Asic is not being fed its own code from the uP at boot as a result and therefore its random configuration is pulsing the reset line.
There is a 4u7 10V tant C411on U405 12Mhz Clock 5V, may be well worth replacing the cap and checking the clock operation sooner than later. my 2cents and thanks for the video. Good luck.
I worked for ST Ericsson as an ASIC verification engineer in the mobile platform group from 2009 to early 2011. I got out before the mobile platform group imploded.
This is a great repair =D I love rabbit hole repairs like this!
I've repaired many hp 34401A and I've already encountered this type of failure of loop condition. In my case I suspected first the EEPROM of the firmware (maybe corrupted for any reason) but after reading it and comparing the content with a known-good one I excluded it. The RAM was the second suspect but after placing a new one the problem didn't solve. After some more troubleshooting and check of CPU and caps involved in the RST level, I found out to be the ASIC/CPLD custom hp part. It was the worst condition unfortunately (so the last one checked). Luckily a friend of mine had a spare board (with analog section destroyed) with a good ASIC/CPLD and after I've changed it my multimeter get back to work! I hope this will not be your condition because it's quite hard to find a scrap 34401 just for salvage the custom ic.
I love fault finding vids especially if there about a piece of kit I own, good work so far...cheers.
Another interesting and detailed video, cant wait till part 2, keep up the great work
I can appreciate your troubleshooting skills!
Thanks for talking us through your thoughts doing these repairs.
Didn’t use your Flir camera to look for “hot spots”! That cap may have shown up on the camera.
Great troubleshooting. Thanks for sharing
@23:30 that same supervisor regulator is a part I had to replace in a E3642A, seems to be a common failure point.
Awesome, Ian - thanks.
Been looking for one of these to repair but they are expensive. I wrote some software to stream 10k readings/second over USB, very good for current monitoring in low power devices.
Looking forward for part 2
It's available......link in description above.
As I recall there was a massive move to get every item of test eqmt calibrated to meet ISO 9000 or something processes. Then the bean counters realised how much that was costing and suddenly the category of "Indicator Only" was created and in ST Ericsson that must have been denoted by "Calibration Not Required".
nice repair..well hopefully it will be once you are done..i didn't expect so many components gone..it would be ok if you had all those comp with you..btw from my experience i changed those tantalum caps as well..after all mine was over 20 years old ..I also had to clean that switch( as i didn't see any new one i could use) so i understand pain of desoldering it...
I've ordered a bunch of 20uF 25v tants with same 7mm package to replace those 20v ones.
The AC Circuit on my unit Manufactured Sep 2008 has U301- LF356, U305- OP37 (not a JFET), U312 - AD825 (as far as I know factory installed) and my Flir IR camera shows 50C+ on the JFET Opamps, far from ideal. Also, the AC measurements on my unit are noisier than I would expect, I've just ordered some ADA 4610-1 to replace U301,305 & 312 with higher spec and half the power, so would expect not to get as hot and be more thermally stable.
Replace all the tantalum capacitors, the whole unit is thermally stressed operating at 40C+, which reduces their life, so they all can expect to be in the high failure part of the curve as you have found.
The regulators also operate significantly hotter than ideal, I did a mains voltage plot with energy and where the regulator drop out and found with the unit set to 240Vac (local mains) that the regulators dropped out at below 180Vac, the display 5Vac dimmed and was usable until 210Vac at 4.2Vac. I notice a Keithly 2001 7.5 dig model had a mains input voltage regulator to reduce the power dissipation of analogue regulators and self-heating.
Good luck with the micro fault-find.
The more I delve into this DMM the more it looks like it wasnt designed with a helluva lot of headroom.
I got a nice package from RS yesterday with a load of tantalum caps (35v), some SOT-23 3.3V zeners and a couple of op-amps.
Yes, the Keithley 2001 has an AC pre-regulator circuit on it.
Cheers for the info, i am looking forward to getting back to the 34401A soon.
Any idea where to purchase the rectangular fuse tray that is next to the AC input on the back of these units?
@30:00 replace the 10nF cap, it could be shorting out
29:19: I think you could see this on the oscilloscope... Sharp falling edge on the reset line, but slower to get back to 5V.... Can only happen if the capacitor is charged from the supply regulator (slower charging due to the RC time constant) and from time to time shorted (fast falling edge we see)...
Yep, the devil is in the detail as they say. Certainly a wierd one!
The hunt continues....
okay abrupt ending but await part 2 seems to have a few problems with this one which is good as it makes a great video🙂
Squeak !!!
@@andymouse Red Liecester Cheese!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
When I saw the blow tracks leading to the switch and the flux residue, I thought cascade failure awaits.
This is a quite common problem. U503 SRAM chip is dead on your board. Also U301 is ok. This opamp is always very hot if meter does not boot.
Seems like you were right.
Abrupt ending I thought Wi-Fi had gone down again. I watch these in case my HP34401 gives me trouble and may have provided a few pointers.
Same here !
@@andymouse another HP34401 fan boy :-)
Likewise. You can never know too much about your equipment.
It's actually not an ASIC but a gate array, possibly for glue logic.
Yup, looks like it. The Service Manual BOM for U501 says it's a gate array, but the schematic says ASIC......LOL!
I suspect that the ASIC has a watchdog timer in it that can pulse the reset line if the CPU does not "pet" it periodically. The ASIC's reset line is OR'd with the one from the power supply chip.
Kinda along the lines of what i was thinking. The only doubts i have is that the Agilent psu's i have repaired in the past have had dead ram and even corrupt roms, but the Asic's never pulsed the reset line.
Same uP, same Asic etc i believe.
Anyways, i can't wait to get stuck into it again.
The box around the soldered ribbon connector seems to indicate that there was a connector at some point.
It does!......and going by the last 34401A i repaired those connectors get weak. I had to tighten the pins on that one in order to get a reliable connection.
Seems odd to have a bunch of seemingly unrelated faults like this. Does the reset trace run anywhere near any of these faulty parts or any strange voltages enter either of the main ICs?
Not that i can see. My theory is that the ASIC isnt getting programmed from the uP and its operating eratically without code, hence the iffy reset line.
In regard to the zener diodes, its possible they are being stressed by opamps running outside their spec, consuming a little too much current and at 350mW a pop the zeners are easily blown by just a few additional mA...........maybe!
isnt the difference between rev A and rev B the unobtainium VFD driver chip that has the defect and is guaranteed to fail? because thats the one i have here. and its an open question whether to just replace the vfd on it. or the whole display board for the OLED conversion. there a discussion thread over on eevblog
The conversion looks quite good, I saw that thread a while back. Think I'd go for a wide format TFT though, there's a few from midas displays
@@sdgelectronics yeah specifically for the OLED mod project, I was asking the creator of it what was the known influence or affect on the precision of the instrument. However unfortunately both him (and myself too)... neither of us actually possess any other sufficiently accurate test equipment to verify and be certain that there is no difference. Although we suspect it's allright. The best we happen to have around are some pretty standard jellybean voltage references from digikey (like $1-3 parts) but nothing any better than that. I suppose paying for a calibration before and after is a sensible option. But then are paying for calibration twice over. Which is harder to justify for hobby use here, where the extra precision isn't matter as much.
As for using a midas TFT instead... that would be a nice idea. And thanks for the recommendation. So you think we can use this same PCB board from the OLED mod (by user 'qu1ck')? and just exchange the OLED for a TFT? If i remember correctly, i believe it just uses an stm32 and not much else. Assuming it uses i2c or spi well then... sounds OK come to think of it. And indeed an LCD would end up drawing current from the multimeter's power rails - this is desirable. And maybe has a couple of other advantages. It makes a good sense
That's a lot of failed diodes on the -18V supply, do you think there's a common failure mode, i.e. too negative supply due to the failed caps? Looking forward to part 2!
You'd think so......i guess as i dig deeper more info might be revealed.
I could listen to you for hours, well, I have done!
Have you ever come across a 34401a that behaves OK but after some amount of time starts behaving erratically?
I have one here that I have been using to measure a 5mV signal, I’ve got 2 34401a connected and they’re very close in measurement.
However, at some point one of the units will suddenly start measuring incorrect values, and jumps around to values nowhere near what the static DC input is.
switching modes doesn’t resolve the issue, but power cycling does, at least for a while.
This is potentially something that is happening at the unit gets hotter.
The first port of call seems be “replace the electrolytics” but they measuring OK (in circuit) and there is no signs of bulging or dielectric leaking.
Obviously I’m going to have to go do some fault finding, but just wondering if you’ve ever seen this before.
Thanks for your comments. I haven't seen this on a 34401A from memory.
I'd replace the electrolytics but only after I had explored other reasons for this failure mode. I'm not a fan of "open the box and just change the caps".
Try tapping the Pcb and checking/wiggling components & wiring etc inside the unit. Even the front/rear switch etc.
Try measuring the DC supplies/ref voltages and see what they do when the unit goes wonky.
Try a heat gun and freezer spray and see if you can narrow down the area on the Pcb causing the issue.
Ian.
@@IanScottJohnston absolutely, I’m loathed to change the caps for that very reason, they look ok and as far as I can tell in circuit, they are ok.
I’m going to have to probe about and see what is causing the problem, the lack of silkscreen is a bit of a pain and having to refer back to the service manual.
Since the error doesn’t clear when switching function, I need to actually see if anything else (such a current or resistance) is also affected since that would help locating the source of the problem a bit easier.
Thanks for taking the time to reply!
@@IanScottJohnston the plot thickens.
It does seem to measure AC and frequency correctly by changing mode when the DC reading goes off, however…
I have noticed that simply pulling the ground probe out and plugging it back in resolves it.
It’s not a mechanical issue since I’ve tried wobbling and turning the plug which does nothing and this fault occurs regardless of whether front or rear is selected.
I don’t think it’s the switch, however, that was a very good suggestion and definitely seemed like a good place to start once you mentioned it.
So I think the ADC is good in this, whatever has gone faulty is in the DC measurement circuit.
It now seems likely it’s temperature related, I can’t see any obvious faulty parts or bad solder joints so I’m going to have to get the schematic up and start probing around and fine some reference values when it’s not in the fault mode and then let it run until it faults and then check those points again.