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Author Topic: Fried UT60-A: it shouldn't be complicated, but somehow it is  (Read 634 times)

Arnold the Frog

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Yes, the UT60-A is nobody's idea of a brilliant bit of kit, and I did it no good, but all the same - y'what?

I would be very glad if someone who has taken apart a working UT60-A (or sufficiently similar model) would let me know just how the 10A fuse should be mounted.  I realise that this sounds like the world's stupidest question.  Shouldn't it be obvious?  If you are just gasping to know how it might not be obvious, read on.

It all started out as my own damn-stupid fault.  I was testing the current-control accuracy of a fairly high-current bit of kit (up to 30A, and up to 60V too, at the same time), and in applying larger and larger loads I went significantly over the 10A nominal limit (it didn't complain).  After quite a few runs of several seconds at about 15A, it stopped measuring current at all, and after a few baffled seconds I realised what I'd done.  Silly Arnold.

Well, I obviously needed to replace a fuse.  Didn't I?  Um, maybe.  The first thing I noticed when I actually picked the meter up was that something was rattling inside.  This counts as "not good".  One doesn't expect a mere blown fuse to do that.

Like every plastic-boxed bit of kit emanating from the Far East, the UT60-A is held together by a combination of screws and plastic under tension.  The screws are easy; coaxing the plastic into letting go is only a generically similar exercise from one box to the next.  One never really knows how to do it, and it always feels perilously like destructively the Wrong Thing before the parts of the box snap apart.  On a good day, I get to that point without having actually done irretrievable damage.  Sometimes I even manage to remove all the necessary screws first, before beginning to wrench and wring.  It could easily have been worse.  In this particular case, I really did get all the screws out first, and the remaining sense that the case just didn't want to come apart was entirely down to the perverted genius of the designers of the plastic mouldings.  I did eventually persuade top and bottom to go their separate ways without creating plastic confetti or alarming white creases.  I was beginning to feel almost competent.

Once the case was open, I could see what had been rattling around inside.  It was the fuse.  Well, yes, but not quite.  The fuse was apparently quite intact (not really expected).  It still had a plausibly low end-cap-to-end-cap resistance.  Also rattling around with it, still gripping the ends of the fuse, were the clips of the fuse-holder of the main board.  Oops.

Now, when there's too much current passing, one rather hopes that it's the fuse itself that will blow.  A fuse allowing 50% over-current isn't especially surprising, but one doesn't expect its mounting to be the weak link.  This threatened to be quite ugly and nasty.  I am not quite the world's worst at removing bits from a PCB.  I once met someone in Eastbourne... but this looked OK.  The fuse was mounted at a decent distance from any sensitive-looking components.  I had a chance!

Firstly, of course, I had to identify where the fuse ought to go.  I know, I know.  There's a manual!  There are diagrams in it, and descriptions of replacing the fuses!  Anyway, the high-current-terminal island is, for obvious reasons, mostly electrically separate from the rest, and there was only one remotely plausible place for the high-current fuse to go.  The board does have two fuses, but working out which oceans of PCB copper the loose fuse was meant to bridge was honestly not a problem.  So why am I making such heavy weather of it?  Because it couldn't go there.  It was meant to be on the top of the board (I haven't unscrewed the board from the bottom yet).  All the diagrams showed that, all the descriptions implied it.  But where could it go?  The problem wasn't working out where it *should* go.  That was easy.  The problem was that other components of some sort were through-mounted on the other side of the board.  There were little puddles (actually, rather large puddles - no prizes for minimal use of solder) around the legs on the top, and no trace of where the legs of the fuse-holder had gone.

Looking back at the fuse, the clips holding its ends didn't seem to have melted off, or snapped off, or anything.  They were quite possibly complete and intact.  Had they ever actually been through-mounted at all, as they were clearly designed to be?  I don't have much confidence in my interpretation of the evidence, but for what it's worth, it seems to me that the clips had simply been pressed into place into those little puddles of solder.  It's hardly surprising that such a physical junction was waiting to melt at the first opportunity.  It is, perhaps, vaguely surprising that both clips would melt off at the same time thoroughly enough to leave no trace behind.

What now?  Can anyone send me an actual picture of the 10A fuse actually mounted on the board?  Is your mileage different?  Should the fuse (despite the manual) actually be mounted on the other side of the board, and are those legs sticking up merely the remains of the clips which I already know about?

It's driving me slightly crazy.

ProBang2

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Re: Fried UT60-A: it shouldn't be complicated, but somehow it is
« Reply #1 on: July 09, 2018, 06:42:48 PM »
Hello.

Perhaps this picture helps:



(Source: Inside the UNI-T UT60A Multimeter)

BTW: Sometimes is Google your friend. Research time: Less then 1 Minute...

Good luck!
If you think, my english is bad, then you should read my french. :(


 

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