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Loud "chirping" sound coming from HP3456A voltmeter (repaired)
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intabits:
My HP3456A Digital Voltmeter has developed a fault, manifesting as a loud "chirping" sound (reed relays going nuts)



It did this a few years ago, but only for short time, and then it stopped, so I left it be.
But now the fault has returned, and seems to have gone solid, so I had to open her up to investigate.

The sound was clearly coming from the reed relays on A20, the main inguard analog board.
So I first checked the supply voltages going to that board. All where OK, except the+5V which had massive ripple (like 4V!) .
Fortunately, the fault was the most likely culprit, and the first part I tested: A10C5 - the 4000uF 15V filter cap for the +5V inguard power supply.
It's the large blue cap, and it had gone completely open.

So I'll be now be looking at replacing all the power supply electrolytic capacitors in both my HP3456As, and my two HP3455As.

Video of teardown and repair:-



srb1954:
I had the exact same problem in my 3456A. I found that the 4000uF electro had dried out and dropped to under 100u capacitance.

I replaced it with a 4700u 63 35V snap in capacitor. This snapped into the existing pads on the PCB and the larger case size should run much cooler considering the ripple current flowing through it.

I checked all the other electros and found them to be within specification so I didn't bother changing them.
pqass:
See https://www.barbouri.com/2020/06/24/hewlett-packard-3456a-digital-voltmeter/
for an adapter PCB for that unique blue cap.

Also, the MOSTEK ROMs have been known to die of old age.
See: https://bama.edebris.com/manuals/hp/3456a/
intabits:
Thanks for the ideas.
A snap-in would be simple, but a quick search of the local suppliers shows nothing suitable. And I don't fancy my chances of finding a 5 pin job even with the main online suppliers.
I think the multi-caps on a PCB is a really neat solution, I'll probably go with that.
There's a couple of similar caps in the 3455s, so possibly some economy of scale in the PCB design process.
Also, subject to available space, a long PCB with multi-caps might do for the large axial cap A3C6.
intabits:

--- Quote from: pqass on November 12, 2022, 12:34:41 pm ---Also, the MOSTEK ROMs have been known to die of old age.

--- End quote ---

I knew of that, but thought I'd just deal with it if/when it happened.
But thinking about it some more, I should remove, socket, read and burn replacements.
Just to be sure, and in case my ROMs are different to the online images...
And again, subject to space available, using adapter PCBs
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