Products > Test Equipment
Metrix MX56C / BK Precision 5390 Multimeter Teardown
Vince:
OK ! Sussed it out finally !
Big thanks to RetiredCaps ! The unlikely indeed happened ! It was the PSU's galvanometer reading garbage !
I first tried with my old kit/toy multimeter fromwhen I was a boy... goes only to 1A max, but that was enough to convince me, as the MEtrix also showed 1.0 A !
So then in order to get more data points, I search my stock of ancient salvaged power resistors... sure enough I eventually found something suitable : 0.47 ohms wire-wound resistors, 6 of them no less. No power rating on them, but compared to similar ones which had markings on them, it's safe to say they must be rater at 5W minimum, most probably 6 or 7.
3+ amps, around 0.5 ohms, 5+ W.... teh resistor is just withing spec power dissipation-wise ! So should do it ! But to be really on the safe side, and keep them from overheating which could increase their value and false the measurement, I decided to parallel 5 of them so as to get around 0.1 ohm, which has also the benefit of allowing for easy reading/direct translation of Volts to Amps.
So I soldered them together, so as to get a good connection between them, and measured the drop with the Fluke, whilst having the Metrix in serie displaying the current. Before running the test, of course I measured this 5 pack of resistors, using the Metrix since it has better resolution and accuracy than my little Fluke : it reads spot on 0.10 ohms ! Happy enough with that ! :-)
Unfortunately the crappy connections of the crocodile clips made the apparent resistance be little higher, so the drop was a bit higher than a one to one ratio. Still, once I processed the 7 data points I took, it was obvious that the reading was reliable : the effective shunt resistance (ie, with the leads and clips), was a very consistent : 0.117 / 0.118 ohms. So there is no doubt that the shunt worked just fine and therefore the experiment was valid.
So... the 10A range of the Metrix reads in fact just fine ! .... sorry for chasing a read hearing ! But boy, this galvanometer was all I had to test the Metrix's 10A range.... thought that would work just fine ! Should have done this shunt thing right from the start then !
But stil, what the hell happened to my PSU ! Been working just fine for the past 25 years, and now all of a sudden the current display reads garbage ?! Suspicious... might have an idea to what may have caused this : when testing the Vdc range, I put the two (floating of course) outputs in series, so I could go/test up to 60V rather than just 30V. Did the same thing then, to test the 10A range : I paralleled the two outputs, hoping to get 5 to 6A instead of just 2.5 / 3A.
I remember that it didn't go too well : the current needles didn't quite do what I expected them to do, and the current display on the second output, would stay to the floor no matter what.
So.... I fear I must have mad some damage in the PSU by paralleling them... but boy, I thought all these such dual output floating lab supplies are all meant and designed to be put in series or in parallel, no ?! :-/
Luckily, if I don't parallele them, taken independently, tehy still work fine : the vltage can be set just fine as it should, between 0 to 30, and the Volt needle reads just fine. So the problem seems to be limited to the current display... but this is just a galvanometer with a wire shunt bolted right across it's terminal, along with the wires. So... I don't know. I guss I will have to take the two galvanometers out, and wire up some test circtuit on the bread-board, to test them. Galvanometers are very sensitive of course, in the order of 100uA for full deviation, but it should be alright now that I know the Metrix works just fine. It can mesure down to 0.01 uA, so plenty sensitive enough to test the galva, see if either the galva itself has suffered and/or if it's the shunt at its back... or some damage on the PCB of the power supply, at worse...
Attached a few piccies of the PSU, vintage chauvin-Arnoux stuff, that's my "2 minute tear down" ! LOL
OK ! So now I guess my meter is just fine and just need some TLC ! ... which I will dispense in the coming days, and put it back together., and hopefully everything will be alright then.
Someone, a silent read of this topic apparently, private message me saying he owns a MX 55C and was interested in the schematics. I hope I won't go to jail but... I find it so frustrating and unfair not to share the stuff with the few who know and like these meters ! So here it is attached... just don't "broadcast" them all over the web please... let's just keep it contained here. I think if Metrix really cared about the privacy of this stuff, they would not have taken the risk to give it to me (never mind in easy to spread PDF form !) in the first place ! Or at least they would have explicitly told me what their policy was... but they didn't do any of that. So... hope you enjoy as much as I do ! :)
Thanks again for the advice and tips you gave me to help me figure it out, I much appreciate it ! :-+
Vince:
Oops, forgot to put some photos, I like piccies ;D
Here, pics of my little toy/kit meter I built when I was little. Still, I like the looks and feel of it. Scale looks nice, and nice rubber housing that does a great job at bouncing the thing all over the bench. Then a pic of it side by side with the Metrix : 1.00 A on the Metrix, and my analog meter reads just a tad under full scale, so really not bad for what is really nothing but a toy ! 8)
Vince:
Hell, can't believe I had THAT many power resistor... that's the opportuniy to organize them a bit ! The carbon film ones, the glazed/vitrified ones (not sure how you call them in English), the wire wound types, plain, and then the really vintage one, wire-wound but hollow, for better cooling I guess. Then the ceramic ones.
Vince:
...
Vince:
Now I have to fix the PSU... tear down time !
The two output are really completely distinct. They have their own PCB, and both boards are strictly the same. Transformer has two taps : 30V of course, and a middle tap at 15V with a relay to switch between the two, so that the PSU can lower the power transistor's dissipation when the outputs is set to a low voltage but draws lots of current.
Each of the outputs have the following connected to their terminals/jacks : a diode (BY550, will look that up. drop tested at only .45V, lower than your usual diode eh ), then a 47uF electrolytic cap, and what looks like a "MOV" ?! Then, there are 4 very high value resistors, (they wouldn't even measure with the fluke), connecting each of the 4 terminals/jacks, to the chassis/earth connection.
You can also see the current shunt bolted right accord the lower galvanometer's terminals.
The boards use the classical at the time uA 723 ... I guess these days there more modern/better replacements. Each board/output actually uses TWO uA 723 ?! Don't know what the second is for ? Constant current regulation maybe ? Could be, because on the PCB, we can see what looks like long wire shunt, that runs alllll along the edge of the PCB, see it ? So could be used as a current sense to perform the current regulation ?!
That was my impromptu 2 minute tear-down, a la Dave, sorry if I lack the talent ! :-DD
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