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Old 29th Nov 2021, 7:53 pm   #1
cmjones01
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Default Over-reading Sifam meter, and the dismantling thereof

I've recently been tidying up a Solartron AS1164.2 power supply. It dates from about 1962. I got it as more or less scrap in about 1985 and have basically been using it since then. It's been getting more and more erratic, so I decided it was time for its first proper service in 59 years.

Most of its ills were cured with a good dose of switch cleaner, but there's an odd problem with one of the ammeters. Now that I've properly refastened its glass, cracking it in the process because I thought it was perspex (doh!), it over-reads by about 25%. Its range is 1A and it has an internal shunt. Its twin in the other channel of the power supply reads correctly, near enough not to worry about.

I can't quite work out if could have come to over-read like this. To find out why, I'm trying to dismantle it, but the screws that hold the movement in to the case seem to be stuck fast with varnish and in spite of scraping out as much as I can, I can't shift them.

Any ideas of the right way to get it apart? Or, for that matter, what might plausibly cause it to read 25% high? I would have guessed damage to the shunt by overload, but it's spent its entire life in an electronically current-limited power supply so that's not very likely.

Chris
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Old 30th Nov 2021, 7:24 am   #2
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Default Re: Over-reading Sifam meter, and the dismantling thereof

Solatron used some sort of Loctite, in my VF252 every screw has green stuff on the threads making them a pain to undo. The only solution I've found is to use heat which means a blow torch carefully applied in some cases, a %^&*&* hot iron should do though.

If the shunt has changed in value that might explain it reading over. It may need tweaking/re-shunting.

Andy.
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Old 30th Nov 2021, 11:51 am   #3
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Default Re: Over-reading Sifam meter, and the dismantling thereof

Thank you for the commiseration! Whatever they used on these screws is certainly pretty solid. Looking at it again this morning, I buzzed out the connections and figured out a way to improve the meter without dismantling it further. There's a little wirewound resistor in series with the moving coil, but the main shunt was tucked away behind the movement. I tried some experiments with adding resistance in series with the coil. I found that adding a 1.5 ohm resistor brought the reading spot-on at full scale, and it over-reads by about 3% in the middle of the scale, which is not perfect but it's as good as I'd expect for a meter like this and certainly more useful than it was before.

I'll call that a fix.

Chris
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Old 30th Nov 2021, 5:22 pm   #4
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Default Re: Over-reading Sifam meter, and the dismantling thereof

This is a PMMC movement? One other possibility if the meter's been jostled is one of the hairsprings has a coil that has accidentally overlapped with the adjacent one. Have had this happen once and it generated a discrepancy of around 12%. Easy enough to see (if you can get to it!) with light and magnification. I managed to untangle mine, probably with a cocktail stick or thin copper wire; can't remember.

The fact that yours has now gone non-linear at ctr.scale following alteration of shunt value hints that it might be a mechanical issue of this type...

Dave

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Old 30th Nov 2021, 5:58 pm   #5
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Default Re: Over-reading Sifam meter, and the dismantling thereof

Yes, it's a permanent magnet moving coil meter. A tangled hairspring is possible - the power supply has been moved around a lot in its life. Though the meter's read high for as long as I can remember. Actually, perhaps mechanical damage was caused when the front glass was loose. In any case, I can't see a tangle from the front and I can't remove the movement safely to look at the rear hairspring so I think I'll just live with it. At least it's not showing hysteresis, sticking or any of the other unpleasant symptoms of mechanical problems.

Its partner on the power supply's other channel reads about 5% low across the scale. I'll see if I can do a similar internal fix for that one.

Chris
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Old 1st Dec 2021, 7:59 am   #6
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Default Re: Over-reading Sifam meter, and the dismantling thereof

These cheaper sorts of meters are always ish and is never going to be accurate as a DMM, I use my bench PSU with MM meters for rough indication only and in conjunction with a DMM.

Andy.
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Old 1st Dec 2021, 9:32 am   #7
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Default Re: Over-reading Sifam meter, and the dismantling thereof

Me too. I like having the analogue meters as a quick indication and to show clearly when the current is changing and in which direction but for precision measurements I use a digital one too.

Chris
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Old 2nd Dec 2021, 1:28 pm   #8
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Default Re: Over-reading Sifam meter, and the dismantling thereof

More news on this. Having got the meter working to my satisfaction, I refitted it to the power supply. To my dismay, it started over-reading massively again. It soon became apparent that tightening the nuts on its positive terminal made a difference to the reading. Something was clearly amiss.

I now determined to get it apart. Determined scraping of the varnish and carefully grinding a screwdriver to fit the screws' slots perfectly got one screw out, but the other wouldn't budge and its brass head was soon mangled. I resorted to drilling it out which wasn't too hard in the end.

Inside the meter all was revealed. One end of the shunt was dry-jointed as can just be made out in the attached picture. It's presumably been that way since coming out of the factory in 1962. A dab of flux and a blast with the Metcal soon had it soldered properly.

Now the meter read 15% low. Quite possibly it had been adjusted during manufacture with a bad joint on the shunt. I added a resistor in parallel with the coil's series resistor to bring the readings in to within 2% of FSD right across the scale, which is plenty good enough (the power supply's manual says 3% FSD).

I think the job is finally done.

Chris
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Old 2nd Dec 2021, 2:54 pm   #9
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Default Re: Over-reading Sifam meter, and the dismantling thereof

At last, reinstalled and usable again. The whole thing took far longer than I expected but I now have an excellent reliable power supply. After minor adjustments and the replacement of one 22k carbon composition resistor which had drifted high in value, the output voltage is within 5mV of the value set on the decade switches and both ammeters work properly. Not bad for something approaching its 60th birthday.

Chris
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Old 2nd Dec 2021, 4:11 pm   #10
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Default Re: Over-reading Sifam meter, and the dismantling thereof

I can't exactly remember, but to increase the current passing through the meter as opposed to that passing through the shunt, if carefully "nick" the shunt with a fine saw blade, you will increasing the shunt resistance very very slightly until you have FSD. On one occasion many years ago I visited a company making shunts for big DC loads and ladies were doing the shunt "calibration" with a junior hacksaw. But they wasn't looking a milli amp or two, nearest 50 was OK. Needless to say its now a housing Estate. Ted
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Old 2nd Dec 2021, 4:51 pm   #11
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Default Re: Over-reading Sifam meter, and the dismantling thereof

Yes, that practice of notching is apparent in high current avometer shunts where during calibration they have sought to increase the volt drop across them slightly. (The resistance is thus raised in small steps- the opposite of wound high voltage shunts which would have been overwound slightly then reduced in steps)

Dave
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Old 2nd Dec 2021, 4:59 pm   #12
cmjones01
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Default Re: Over-reading Sifam meter, and the dismantling thereof

Quote:
Originally Posted by ex 2 Base View Post
I can't exactly remember, but to increase the current passing through the meter as opposed to that passing through the shunt, if carefully "nick" the shunt with a fine saw blade, you will increasing the shunt resistance very very slightly until you have FSD.
Good thought. In this case the shunt is a coil of resistance wire so 'nicking' it would risk cutting it altogether!

Chris
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Old 2nd Dec 2021, 5:54 pm   #13
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Default Re: Over-reading Sifam meter, and the dismantling thereof

Indeed, it's only the thick shunts that get that treatment.
2 or 3% of scale is good for that panel meter- fixed!
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