Loud "chirping" sound coming from HP3456A voltmeter (repaired) - Page 1
Forum section: electronics / testgear

author: intabits
author_rank: Frequent Contributor
timestamp: November 11, 2022, 01:07:29 pm
content: 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:-

author: srb1954
author_rank: Super Contributor
timestamp: November 12, 2022, 11:17:13 am
content: 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 4700u6335V 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.

author: pqass
author_rank: Super Contributor
timestamp: November 12, 2022, 12:34:41 pm
content: Seehttps://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/

author: intabits
author_rank: Frequent Contributor
timestamp: November 12, 2022, 02:11:52 pm
content: 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.

author: intabits
author_rank: Frequent Contributor
timestamp: November 12, 2022, 02:19:04 pm
content: Quote from: pqass on November 12, 2022, 12:34:41 pmI 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

author: pqass
author_rank: Super Contributor
timestamp: November 12, 2022, 03:30:01 pm
content: Quote from: intabits on November 12, 2022, 02:11:52 pmBarbouri has shared it herehttps://oshpark.com/shared_projects/EVFWnnBD. I think you get three copies for the price.QuoteWell, at least this cap is still availablehttps://www.digikey.ca/en/products/detail/vishay-sprague/53D183G025JT6/5611315if you can stand the price.

author: pqass
author_rank: Super Contributor
timestamp: November 12, 2022, 04:26:40 pm
content: Quote from: intabits on November 12, 2022, 02:19:04 pmI'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 thishttps://xdevs.com/fix/hp3456a/. I think the simplest solution is just to make an adapter PCB per ROM for a 2764 as described herehttps://bama.edebris.com/manuals/hp/3456a/. You could consolidate into one EPROM but that would require you to find A13, A14, and replace/interface with the 74LS138 (U6). Too much thinking (risk?) involved.

author: srb1954
author_rank: Super Contributor
timestamp: November 13, 2022, 03:30:46 am
content: Quote from: intabits on November 12, 2022, 02:11:52 pmI 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.

author: dietert1
author_rank: Super Contributor
timestamp: November 13, 2022, 08:46:49 am
content: Here we have four of those original Sprague caps 4000uF/15VDC. The LCR meter saysC /uF ESR /Ohm DC4300 0.048 87343710 0.057 86473120 0.31 89084510 0.076 8713We 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

author: intabits
author_rank: Frequent Contributor
timestamp: November 13, 2022, 12:39:26 pm
content: Quote from: pqass on November 12, 2022, 03:30:01 pmI 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...Quote from: srb1954 on November 13, 2022, 03:30:46 amHmmm, that's a simpler solution, and at $3.43, the price is right!Quote from: dietert1 on November 13, 2022, 08:46:49 amNot sure exactly what you mean there, but I'll work it out. Sounds like it's worth doing, thanks for the Tip!