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| Metrix MX56C / BK Precision 5390 Multimeter Teardown |
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| Vince:
--- Quote from: Performa01 on August 29, 2016, 04:43:29 pm ---The 500mV range on the MX 5x meters is high impedance, like on a 'real' lab grade DMM. No wonder it shows overload if connected to nothing, as the input current of the frontend will eventually charge the input capacitance to a out of range condition. --- End quote --- Good evening Performa, Well... you are right.. and wrong. The MX5X is not a real high-impedance meter on the mV range. But.. it CAN be.... it asked to ! By default, the mV range uses the standard 10M impedance like all the other ranges. If you WANT the high-impedance (stated in the manual as greater than one Gohms), then you have to explicitly ask for it, by pressing the "range" button on the little keypad at the top of the meter, while at the same time, turning the main knob to power the unit on. The LCD will then briefly inform you what the impedance is, 1G or 10M., tried it, worked just fine. The meter will the remember the setting and store it in the external cal EEPROM. But the over-sensitivity I am experiencing in the mV range is observed when the standard 10M impedance is selected (I measured it to be sure)! Rest assured... I also turned on the High-impedance mode to see it made any difference... it doesn't, not the slightest change. Still, this in itself is a clue, as I mentioned in my previous mail: this comforts me in suspecting that the 10M resistor is indeed faulty (replacement on its way), which then would mean that basically the meter is in high(ish)-impedance all the time, no matter what impedance I set it to ! --- Quote ---And of course, the shields are essential in this "electrometer" mode. --- End quote --- OK thanks, wasn't sure just how critical it was in this situation. Will solder it back , as soon as I have replaced the 10M resistor (don't want to solder it back and off again 2 days later !...) --- Quote ---I would be careful with the current range as well. Don't attempt to touch the calibration unless you have the means to _really_ confirm that the readings are way off. --- End quote --- "really off" ? Hell, a blatant, undisputable 40% off... I do'"nt need a 7 digits 1000 buck meter to tell me it's way off ! Such a meter would be nice to know if the Metrix is off by 39,567876544 % insteand of just "roughly 40%".... but that would not change the diagnostic would it ;-) --- Quote ---(And if they actually are, I'm quite sure something else would be horribly wrong and it's nothing to be cured just by re-calibrating the thing) --- End quote --- Ah, I never said it WAS the calibration, did I ? Don't think so. I only said it was a reasonable/plausible cause, given the age of the meter which is way past the guaranteed retention period of the calibration EEPROM. And old gear losing their car is just common place, the web is flooded with people have to deal with such issues, and not just devices with volatile/battery-backed NVRAM.... So it's just an hypothesis, at this point... nothing more... One argument to go your way, would be to say : "how comes the cal on the 10A range is so off, while all other ranges read just fine ?". Indeed it seems rather unlikely that the cal EEPROM would lose the bytes related to the 10A range, and preserve all other bytes ! ;-) Still, I find it nice and interesting to build the serial cable and get the cal software, if just to be able to "talk" to my meter, even if it's just to say "hi", and not touch any cal data ;-) So... what could explain this huge error on the 10 A range ? Don't know. What we can see so far is taht the DMM chip can read voltage just fine, even very low signals on the mV range. So technically it should be able to measure the voltage drop just fine across the current shunt. So what's next in the current measurement "chain" ? The shunt of course... maybe it is 40% off, which would then translate in a 40% error when the DMM chip reads the voltage across its terminals. BUT... as I noted yesterday, the uA reads just fine ! So clearly the shunt is good. So.. what's left ? The shunt is good, and we know the DMM chip reads voltages just fine. So, one can only assume that there is some external/analog circuitry between the DMM chip and the shunt, which is specific/ only used by the 10A range, which would introduce this 40% error. I am plenty willing to admit this, and this is precisely why I am trying to get hold of the schematic, so I can see exactly how the circuitry is put together, so I can see where the error may be coming from, knowing it's not the chip nor the shunt. I e-mailed Chauvin-Arnoux/Metrix this evening, asking for whatever technical documentation they may have and be willing to send me. They happen to have an office just 40 miles from me, so I offered to pay them a visit if need be, to discuss the issue. |
| ModemHead:
I've had a chance to check out a couple of your issues on the 5390 unit I have here. 1) Regarding the mV alarm: It definitely alarms at anything over 500mV (or below -500mV). In 10M impedance mode, it does not alarm arbitrarily with leads in or out, because even though it wanders a bit as expected (tens of mV), it does not exceed 500mV. No amount of fiddling with the leads would get it over 500mV. In high-impedance mode, it doesn't alarm if you leave it alone with no leads connected. But with leads connected, it wanders quite a bit, especially if you handle or touch the leads, and every time it goes over 500mV it beeps. If you connect stimulus over 500mV and then remove it, it takes quite a while for the charge to dissipate, with the alarm going the whole time. Engaging AC+DC mode does increase the sensitivity somewhat. I'm not sure why this alarm is necessary, but if its purpose is just to go off when the input is over 500mV, I'd say this unit is working as designed. With input impedance over 1G, it's easy to pick up a charge on the leads. 2) Capacitance range: Displays 00.00 with an occasional 00.01. While measuring displays "run" in the top half of the display. Upon disconnecting the cap, it will display "run" again very briefly then return to 00.00. If I short the leads it says "run" for 30 seconds, then displays "0.L". |
| redg:
--- Quote from: Vince on August 26, 2016, 11:35:01 pm ---Thanks Robomeds for this post ! I am French and boy I struggled to tear my MX 55 apart... it took a chap from the other end of the world to tell me how to open "our" meters ! LOL So thank you for detailing the procedure to put it to bits. Armed with this, I will make a new attempt at open it ! :) I agree with everything you said : great meter overall, I just love it and always wanted one when they were new 20/25 years ago and I was just a kid dreaming as I saw them tested in the electronic magazines of that day ! They were worth about a month of salary IIRC. As for the history part, I read some thins wrong above, so I would like to correct. It's easy to get lost because Metrix's life has been such a roller coaster throughout the years ! One day merging with similar companies to form a group, another day getting bought by a group or part of a group, then sold again to XYZ ! Hard to follow. But to put things simply : Chauvin-Arnoux and Metrix are both very old big French names in the instruments business, alwyas have pionnered the market, alwyas designed their stuff in house of course, and still do taody ! Their R&D centers are still in France. Chauvin-Arnoux was formed as far back as 1893, and Metrix in 1936, first famous for they vacuum tube test gear, as it was the technology of that era. The two were then competitors, Chauvin-Arnoux always wanting to buy Metrix... which they eventually did later. As for the ITT thing : In 1964, Metrix got "affiliated" to ITT, nothing more. In 1970, Metrix merged with a few subsidiaries owned by ITT, to form a group. In 1979, the Metric products were rebranded " ITT Metrix " In 1990, Metrix was the leading manufacturer in the whole of Europe, for analogue and digital multimeters. In 1994, and of the ITT affiliation : Metrix got sold to a group of companies (mostly French and German). ... which had overambitious plans for Metrix, 3 years later, the group, and Metrix with it, bankrupted. ... that leads us then to 1997. This were Chauvin-Arnoux offered to buy Metrix, the judge in charge accepted... at long last chauvin(arnoux managed to buy his competitor Metrix ! The two French big names are now united in the same group. Chauvin-Arnoux was/is clever enough to let Metrix design and build their own stuff, sold under their own Metrix brand, so nothing has changed. Metrix is still Metrix... except it's now out of trouble financially because Chauvin-Arnoux owns them. So basically, it wall started a century ago with Chauvin-Arnoud, and a century later, after troubled times, it all ends with the same two brothers ! Except they are now joining forces, instead of competing against one another. Here is a link to a PDF emanating from Metric, that describes the whole story, written in French obviously sorry, but I highlighted the main points above. http://www.geode-annecylevieux.org/____assets/uploads/2012/01/METRIX.pdf As for the chip driving the MX 50 series, I don't know its story. Obviously Metrix were not big enough to have their own manufacturing plant, so maybe they designed the chip in -house and just asked ITT to manufacture it for them... or did ITT already have this chip in their catalog and let Metrix use it to roll their MX50's around it.. or was it a joined effort / co-development, shared between ITT and Metrix... who knows really, not that it matters in the end... the result is a great meter ! :-D As for BK et AEMC of who ever else, it was obiously just a rebadged Metrix. Anyway, sorry for the length... I came here as well to ask Robomed or anybody else... if there is a schematics or service manual available somewhere for the MX 50s series ? .. because my MX 55, which is externally/cosmetically in like new.. must have something wrong deep down, as it acts weird and funny and pretty much every function but the current measurements. I suspect most of the weirdness might be a problem with the input protection circuitry, but there is one misbehavior which I fear might not be as easy to diagnose : in the mV range, it screams at constantly. The "high-voltage"/danger alarm and icons, come on as soon as I set the switch/knob to the mV range, even though the test lead are not connected to anything. shorting the leads makes the alarm go off. Anyway... would be great if I could have a schematic for this baby, to help me trouble-shoot the thing... because I just love this meter and although it is extremely sick at the moment, I will not give up on it without a good fight ! :) No idea what kind of abuse the meter might have experienced to show so many problems... it was already like this when I bought it used, 3 years ago... the seller never mentioned anything about any kind of problem.... that's honesty for you ! :-/ Oh, before I go, for those interested, I attached a couple PDF files below. Commercial leaflets of the day, presenting the MX 53/4/5/6 series (in English) and another leaflet (in French) presenting the later/revised ' C ' version of this range of meters. --- End quote --- Merci beaucoup! Thanks for straightening out the history. Really helpful. |
| Performa01:
--- Quote from: Vince on August 29, 2016, 06:22:05 pm ---So... what could explain this huge error on the 10 A range ? Don't know. What we can see so far is taht the DMM chip can read voltage just fine, even very low signals on the mV range. So technically it should be able to measure the voltage drop just fine across the current shunt. So what's next in the current measurement "chain" ? The shunt of course... maybe it is 40% off, which would then translate in a 40% error when the DMM chip reads the voltage across its terminals. BUT... as I noted yesterday, the uA reads just fine ! So clearly the shunt is good. So.. what's left ? --- End quote --- Be aware that each current range has its own shunt resistor. So it looks like the 10A shunt is bad, even though I cannot imagine how this could happen. A severe overload maybe, but then the fuse F2 should have blown and protected the shunt. You could check the resistor, whether it looks discolored or otherwise damaged, also check the solder joints - even though bad solder joints could not explain too high a reading as the shunt has to have a 4-wire connection anyway. The 10A shunt has to be <50 milliohms. I would check its value. If it looks too high, then I'd calculate a resistance value that can be connected in parallel in order to bring the total resistance down by 40% and finally use a piece of thick Manganin resistor wire to build a shunt for the shunt ;) After that, the 10A range should be close enough so it can be calibrated. Because I doubt that a 40% error could be corrected by calibration. Even if the permitted adjustment range is wide enough (after all it's just a firmware decision), the input voltage might get too high and the meter would then be unable to actually measure 10 amps. |
| Robomeds:
I can't imagine that shunt getting thrown out of value. I suspect something else is the issue. I would start by visiting Modemhead's blog. He talks about repairing several damaged meters. In the discussions he talks about the various innocuous seeming things that can throw your readings to the birds. I would also watch Dave's videos on protection circuits. When those go bad they don't always disable the meter. Sometimes they just result in odd readings. PS: My 5390 had the out of range warning all the time in the mA range. It was actually kind of annoying when you just put the leads down for a moment and the meter starts beeping at you. Still, good meter though I admit, mine has been pasted to more deserving hands. |
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