No. 124 - HP 34401A 6.5digit Multimeter Repair - Part 1 

Keeping the original pcb in one piece was the best and strongest solution. Good call, Ian !

Wow, fantastic work on creating that little "corner of the board" replacement =D Great work cleaning out the switch too!

If only they had mounted the pcb the other way up. Then the first 13k that got too hot would have fallen off and the current would have stopped with no fire. Melting solder can make a useful fuse.

Nice to see you did not cut the pcb, so it retained its rigidity.

Bardzo profesjonalna naprawa PCB 👍👍👍👍👍👍👍👍👍👍👍👍👍Ładny miernik

Love the groovy disassembly muzak! I have a similar meter I have to get around to resurrecting.

What an elegant solution to that fire incident. Great job!

I always like your ingenuity on getting things done, I was thinking the same thing small pcb for the burnt area

An elegant solution, Ian. Thank you for showing it to us. If clearance with the shield becomes an issue, you could put a notch in your new board that is aligned with the front panel "clip" and attach your board closer to the old board. Regards, David

Great idea, the sign was perfect! Waiting for the second video!

Nice job!! Looking forward to p2

The one time you're happy getting a positive result from a covid test bud. Thanks for the video Ian.

A perfect job so far! 🤗

Great job!

Awesome, looking forward to part 2!

Nice work

Perfectly Repair Job. Thank you for this very interesting Video

great solution

Great Job.

Great fix with the new PCB section! Proper job. I just received one of these from Japan, for such a low price I was sure it would be like the one you're working on. Not so! All I had to do was use a bit of Goo-Gone on some sticker residue and change the mains voltage selector. There is a sticker from Agilent's Japanese lab on the top that says it was due for next cal in '22, which was a pleasant surprise! It's a good job, too, as I would have struggled (lots) with the repair you just made look easy.

Nice fix; I wish I had that creativity...

What, a pleasure to view...thank you, take care.

So, did the switch have COVID or not?

While you have it disassembled, you might want to add some thin isolation between the spark gap and the COTO relay. Over on EEVblog there is a documented case of the spark gap "scratching through" the COTO's isolation, causing a short. On my 34401A I bodged-in some Kapton tape. Not the most beautiful of solutions but it does the job.

Great video. Very interesting how the input switch could be disassembled for inspection/cleaning. I have a 34401 with dodgy switch and this tip might come in handy. One comment to the new pcb: Is there enough clearance vs the display board?

Nice one....cheers !!

Will be interesting to see if it works. Compared to what else could go wrong with various chips that repair is sort of straightforward. Nice if you can have a pcb fabricated. Us lesser mortals would use proto board or similar.

Glad the COVID test kits were used for something worthwhile...lol

I am guessing that your are going to snake wires around the protective plate then since you went with elevating the replacement circuit piece? Great video as always

For the switch and relay it wasn’t really a large current (17mA) for their low impedance. I guess there could have been some sparks given the voltage

I pinched a few of those Covid test kit swab sticks too. They're perfect for long reach cleaning jobs, wish you could buy them.

Hi Ian, K101 and L102 could as well be damaged. Good thing, that you cleaned the front/rear switch, as this - after 20 years- probably might be covered by a film, originating from the case plastics, i.e. outgassed plasticizer. Me and several others faced this problem, especially in Ohm modes, being evident by erroneous readings (2W) on the front and ok ones on the rear. Curious Marc btw. just a few days ago repaired an older 3478A in the same manner, and had exactly this switch problem. Isn't the elevated PCB with ~1kV above its resistors too narrow to the display PCB? Formerly, there was a bigger distance, provided by the white pin, where the PCB snaps in.

Fit like loon - nice repair work ... ken

Might be an idea to put a fuse on that new PCB too.

Hy Ian! As usual, great job so far. Can you test the Errors before adding the additional PCB in circuit? In theory it should work fine with the front terminals. Some errors are expected given the relay cant perform it's job but would give good indication about the state of U101.

HP 34401 has an error 625. The I/O processor does not respond

Hi Ian, that's an error "C" = Coal 😂 I was reminded of this when I found a burnt hole with a diameter of approx. 5cm on one PCB of power electronics 🤪 You made the replacement PCB really beautifully, I built the replacement circuit on a bastl board because it had to work immediately. Just a question - won't the shielding aluminum cover be a problem, is there enough space? Nice day 🙂 Tom

Wow, toasty... Given the usage case, anyway to beef up the over voltage protection?

Nice beginning of a repair! Ever repaired a sovjet / "eastern bloc" bench multimeter? If you want, i would send one to you ( i would cover all costs, the meter would be for free) :D I tried to repair it myself but well, changing some caps didn't solve anything :D So ye ... if you are interested in the old tech from this era, let me know. Oh, and it's an Meratronik V560 Digital Multimeter.

First seeing a HP 34401A tested for covid.🤣

Once the daughter PCB is fitted, what is the clearance between the R107 pad and the (safety grounded?) nut/standoff? Wouldn't that be a prime candidate for arcing? See detail at 17:18 and 23:42 ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-h938dAywRko.html

IMHO R110 and R111 aren't as toasted as the rest - see at about 9:15. This confirms C100 rather than C101 as the likely short circuit.

From the date on the PCBs has been only 9 days since you laid those out to publishing this video.

I'm sure that we are all glad that your 'elevator music' doesn't involve a certain Scottish 'musical' instrument. :-)

I would've used 5W through hole resistors and stood them up off the board.