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| Metrix MX56C / BK Precision 5390 Multimeter Teardown |
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| retiredcaps:
In this picture, circled in red, is your range switch. There seems to be a lot of dirt and debris which could cause problems. See modemhead's blog for 2 case studies. http://mrmodemhead.com/blog/fluke-83-iii-rotary-switch/ http://mrmodemhead.com/blog/fluke-83-dmm-repair/ A generous amount of 91% IPA and letting it dry thoroughly will help prevent problems in the future. |
| Vince:
--- Quote from: retiredcaps on August 31, 2016, 08:25:44 pm ---On the BK5360, I get using a Fluke 187 with leads relatived out 100.84 ohm on 5mA range 10.51 ohm on 50mA range 1.54 ohm on the 500mA range 0.03 ohm on the 10A range --- End quote --- Hi there, Thanks for that. I attached below an extract from the schematics Metrix sent to me yesterday. It shows the Amp section. I highlighted in yellow the current shunts. Your measurements are not bad at all, they confirm what teh schematics predict. In the low values you read a bit high, but it gets more and more accurate as the current shunt' value increases. Main 10A one is 0.005 ohm Then it's 1,005 (the mA ranges appear to go through the main shunt, but it's value is so low in comparison, that it doesn't really matter I guess !). Then it's 10.014 then 100.914 ohms. I guess it's obvious that all the shunt values must be close to 10XXX, because it's a 5XXXX count meters, and the ranges are 5XXX full scale of course. So 5/5 =1.... wow, just won Noble prize here ! LOL We also have another/extra resistor up the ladder, around 900ohms, which would male for a 1k shunt hence a 500uA range... but it looks like this does not go through the fuse, and it's also decoupled by a series capacitor, so no relation to our DC current ranges. Don't know what it's for.... maybe meant for the MX54 Performa mentioned, since it has such a 500uA range. But the presence of this cap in the path of the signal, and it not going through the fuse, has me puzzled. The various taps from this shunt "ladder" then goes to what that multiplexer switch, and the two "AUX1/2" digital inputs are actually coming straight from the main DMM5 chip, so it can select the ranges he is interested in. The voltage drop is read via the "IN2" terminal. We can also see how the fuse blown detection circuitry is put together. The circuitry is not the same for the two fuses. Thanks for the link to ModemHead's blog, looks well put together, will spend some time on it to see the contents. The golden tracks which he describes as "real bad", are actually better looking than those on my meter eh ?! So yes, will make sure to clean that up ! I don't have any of that fancy IPA I keep hearing about on the EEVBLOG. Though it was some gimmick, but the other day I tried cleaning an old board with regular alcohol and boy what a disaster, white traces all over the board, oops.... So looks like this IPA is indeed a necessity ! I will make sure to order some from Farnell. I have not yet sent my order for the LCD header and 1% 10M resistor, as I didn't want to spend 15 Euros of shipping for 4 Euros worth of goods ! Was trying to think of something else I could make use of, to increase the order... well let's add a bottle of IPA to the shopping cart ! :-) About the accuracy of the current indicator/galvanometer on the lab power supply, well I doubt it could be so wrong as to display 2.7 A (max I got) instead of 3.9A (what the Metrix showed). I mean, to start with.. the suply is rated at 2.5A, so no way the manufacturer would sell you a transformer rated/capable of 4A and only charging/advertising you for a 2.5A model ! It would had to be profitable, being so generous ! ... However,, slightly over-specifying the transformer to make sure the customer does get his advertised 2.5A, so he doesn't fell "betrayed", would seem a reasonable practice to me... and that's precisely what I am witnessing on the galva : 2.7 instead of 2.5, so 5 to 10% more. This seems OK to me. However... since it's easy to rule it out, then why not. I do have an modest analog meter, a kit I built when I was a boy... actually from Chauvin-Arnoux ! ;-) It's obviously more of a toy, but still, plenty good enough for we have to do here ! From memory it can only read up to 1A, but still, should be enough to see who is right : the Metrix, or the galvanometer in the power supply. My bet is on the supply, but I want to remain open minded, so will perform the test and report ! There is even a simpler test to do ! ! Well... ti's a dual output supply, and both halves are completely independent. so I can just connect the Metrix to second output of the supply, see how it compares ! Having a galvanometer go nuts is unlikely enough, but having both go nut, and by the same amount, would be a tragedy ! LOL Also... I do have a box full of power resistor (ceramic, glazed, wire-wound...) which I salvaged as a kid 20+ years ago. Never did much with them, but looks like they could be put to good use here ! I could try and find suitable enough to act as a current shunt, and then read the voltage drop across it using my Fluke. That would give me a measure of the current way precise enough, to be able to tell where the truth is ! I am currently looking for cheap-ish meters to measure currents as you said, something simple and rugged but reliable and well built and designed, and affordable. I set my heart on the Fluke 70 series. Had a look at the specs. Seems the there is the 73/75/77(23), with accuracy ranging respectively from 0.5%, then 0.4 and 0.3 for the 77. So I am on the look out for these meters. But find one in good condition AND at a decent price AND which comes with it's holster is not as easy as I thought... so the search continues. This one is nice, but at 100 Euros/100+ USD, I am out ! :-/ https://www.leboncoin.fr/outillage_materiaux_2nd_oeuvre/988990358.htm However this week I spotted a MX53C on ebay, which does come with it's holster (hence, a decent tilting stand to actually be able to use it on the bench...), with a very reasonable starting price... but we all know that bidding can skyrocket in the very last few minutes, so I ma not holding my breath, it will probably end up at a silly price and I will have to back off. But who knows, so I keep an eye on it anyway, can't help, just love those meters so much ! LOL Anyway, enough talk. Time to get back to the bench and do the above mentioned tests, and report ! See you later.... |
| retiredcaps:
--- Quote from: Vince on September 01, 2016, 02:06:27 pm ---Your measurements are not bad at all, they confirm what teh schematics predict. In the low values you read a bit high, but it gets more and more accurate as the current shunt' value increases. --- End quote --- I recognize the limitations of the Fluke 187 wrt to its resistance measurements. I just chose it to give more resolution, not necessarily accuracy. I don't have a 4 wire ohm meter. --- Quote ---I will make sure to order some from Farnell. I have not yet sent my order for the LCD header and 1% 10M resistor, as I didn't want to spend 15 Euros of shipping for 4 Euros worth of goods ! Was trying to think of something else I could make use of, to increase the order... well let's add a bottle of IPA to the shopping cart ! :-) --- End quote --- I find fuses and capacitors also make good filler items to meet the minimum for free shipping. It is like I always need 1000uF 16V capacitors when I'm fixing things. --- Quote ---From memory it can only read up to 1A, but still, should be enough to see who is right : the Metrix, or the galvanometer in the power supply. My bet is on the supply, but I want to remain open minded, so will perform the test and report ! --- End quote --- When one clock says 10AM and another says 2PM, you need a 3rd one. Since we do not have the equipment in front of us, remote troubleshooting, time zone differences, etc prompts these questions. --- Quote ---I could try and find suitable enough to act as a current shunt, and then read the voltage drop across it using my Fluke. That would give me a measure of the current way precise enough, to be able to tell where the truth is ! --- End quote --- The above would also work. --- Quote ---I set my heart on the Fluke 70 series. --- End quote --- On ebay.com, these regularly sell for less than $50 USD. Shipping to your country could be expensive though. Alternatively, you can ask on the buy/sell subforum here if anyone wants to sell their Fluke 70 series? |
| ModemHead:
It is not uncommon to have current shunts in a "stack" with the 10A shunt at the bottom. The input is injected into the stack at various places to select the shunt value. Any unused shunt resistors at the top become part of the high-impedance voltage sense line. I can't make complete sense of the schematic. It appears to show 5 switch positions, when there is only two actual current range positions. "Z1 ARC1" appears to be a CMOS bilateral switch of some sort, but not sure of the exact layout of the switches. AUX1 and AUX2 are the select lines. IN2 is the voltage sense. (All guesses.) This is one of those designs where it is difficult if not impossible to probe the device while it is operating. I hate that. Anyway, I ran 1.000A through the unit here and measured the voltage drop on the shunt at 4.811mV. Seems like it should be closer to 5.000mV, but in any case the 5390 display reads 00.999A. It makes a huge difference where you measure this, I choose the bottom of R43, and the large screw-hole pad, which appears to be connected to the lower Kelvin point of the shunt. I presume the strange UFO symbol on the schematic at that point means analog ground. It seems your best bet to provide a known current to the DUT for testing is measuring voltage drop across an external shunt resistor. Be aware that the resistance of that shunt will change as it gets hot. |
| ModemHead:
--- Quote from: retiredcaps on September 01, 2016, 05:07:18 pm --- --- Quote from: Vince on September 01, 2016, 02:06:27 pm ---Your measurements are not bad at all, they confirm what teh schematics predict. In the low values you read a bit high, but it gets more and more accurate as the current shunt' value increases. --- End quote --- I recognize the limitations of the Fluke 187 wrt to its resistance measurements. I just chose it to give more resolution, not necessarily accuracy. I don't have a 4 wire ohm meter. --- End quote --- I do have a 4-wire meter, and from the jack pads, it measures 0.028 ohms. The fuse/fuse-holder has higher resistance than the shunt... |
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