Author Topic: Loud "chirping" sound coming from HP3456A voltmeter (repaired)  (Read 1839 times)

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Offline intabitsTopic starter

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Loud "chirping" sound coming from HP3456A voltmeter (repaired)
« on: November 11, 2022, 01:07:29 pm »
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:-



« Last Edit: December 03, 2022, 01:18:22 pm by intabits »
 
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Offline srb1954

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Re: Loud "chirping" sound coming from HP3456A voltmeter (repaired)
« Reply #1 on: November 12, 2022, 11:17:13 am »
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.
« Last Edit: November 13, 2022, 03:16:21 am by srb1954 »
 

Offline pqass

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Re: Loud "chirping" sound coming from HP3456A voltmeter (repaired)
« Reply #2 on: November 12, 2022, 12:34:41 pm »
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/
« Last Edit: November 12, 2022, 12:47:23 pm by pqass »
 

Offline intabitsTopic starter

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Re: Loud "chirping" sound coming from HP3456A voltmeter (repaired)
« Reply #3 on: November 12, 2022, 02:11:52 pm »
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.
 

Offline intabitsTopic starter

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Re: Loud "chirping" sound coming from HP3456A voltmeter (repaired)
« Reply #4 on: November 12, 2022, 02:19:04 pm »
Also, the MOSTEK ROMs have been known to die of old age.

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
 

Offline pqass

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Re: Loud "chirping" sound coming from HP3456A voltmeter (repaired)
« Reply #5 on: November 12, 2022, 03:30:01 pm »
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.

Barbouri has shared it here https://oshpark.com/shared_projects/EVFWnnBD. I think you get three copies for the price.

Quote
Also, subject to available space, a long PCB with multi-caps might do for the large axial cap A3C6.

Well, at least this cap is still available https://www.digikey.ca/en/products/detail/vishay-sprague/53D183G025JT6/5611315 if you can stand the price.
 
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Offline pqass

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Re: Loud "chirping" sound coming from HP3456A voltmeter (repaired)
« Reply #6 on: November 12, 2022, 04:26:40 pm »
Also, the MOSTEK ROMs have been known to die of old age.

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

I'm in the planning stage of fixing my own 3456. I'm going to start with sockets first too. I certainly don't want a rats nest like this https://xdevs.com/fix/hp3456a/.  I think the simplest solution is just to make an adapter PCB per ROM for a 2764 as described here https://bama.edebris.com/manuals/hp/3456a/.  You could consolidate into one [E]EPROM but that would require you to find A13, A14, and replace/interface with the 74LS138 (U6). Too much thinking (risk?) involved.
 
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Offline srb1954

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Re: Loud "chirping" sound coming from HP3456A voltmeter (repaired)
« Reply #7 on: November 13, 2022, 03:30:46 am »
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.
I used an EPCOS B41231-B7478-M part which is a 4700u 35V part, not a 63V part as I incorrectly stated earlier. It is a 2-pin snap-in cap with 10mm pin spacing that fitted OK into the existing PCB hole pattern after rotating it about 45deg.

In the normal course of events I would have used a modern 4700u 16V radial lead cap, which should have been a adequate replacement for the old 4000u 15V Sprague part. However, I measured the ripple current in circuit and decided to go for a larger cap to provide some ripple current derating and a longer capacitor life. The larger snap-in cap has about double the ripple current rating compared to a standard 4700u 16V radial leaded part.
 
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Offline dietert1

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Re: Loud "chirping" sound coming from HP3456A voltmeter (repaired)
« Reply #8 on: November 13, 2022, 08:46:49 am »
Here we have four of those original Sprague caps 4000uF/15VDC. The LCR meter says

C /uF         ESR /Ohm        DC
4300         0.048               8734
3710         0.057               8647
3120         0.31                 8908
4510         0.076               8713

We used 10000uF/16V caps to replace them. The ones that failed more often are the smaller caps in the voltage doublers. One can add two diodes to prevent reverse voltage during power-up.

Regards, Dieter
« Last Edit: November 13, 2022, 12:13:26 pm by dietert1 »
 
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Offline intabitsTopic starter

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Re: Loud "chirping" sound coming from HP3456A voltmeter (repaired)
« Reply #9 on: November 13, 2022, 12:39:26 pm »
Well, at least this cap is still available https://www.digikey.ca/en/products/detail/vishay-sprague/53D183G025JT6/5611315 if you can stand the price.

I don't know how I missed that, I was looking mostly at local suppliers, but did also look at digikey.

That is the most elegant solution, but comes out to about $45 pacific pesos, and I have 2 to replace, so a bit steep. Especially as the original parts may be just fine. I'll test them, and if dodgy, decide which way to go.

I'll do my own home made PCB(s). A little trickier being single sided, but certainly doable.

On the "ROM Rat's Nest", yes, I would aim for something neater than that...

I used an EPCOS B41231-B7478-M part which is a 4700u 35V part, ...not a 63V part as I incorrectly stated earlier. It is a 2-pin snap-in cap with 10mm pin spacing that fitted OK into the existing PCB hole pattern after rotating it about 45deg.

In the normal course of events I would have used a modern 4700u 16V radial lead cap, which should have been a adequate replacement for the old 4000u 15V Sprague part. However, I measured the ripple current in circuit and decided to go for a larger cap to provide some ripple current derating and a longer capacitor life. The larger snap-in cap has about double the ripple current rating compared to a standard 4700u 16V radial leaded part.

Hmmm, that's a simpler solution, and at $3.43, the price is right!

We used 10000uF/16V caps to replace them. The ones that failed more often are the smaller caps in the voltage doublers. One can add two diodes to prevent reverse voltage during power-up.
Not sure exactly what you mean there, but I'll work it out. Sounds like it's worth doing, thanks for the Tip!
 


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