30kg of a resistance meter!

so today let's explore an interesting piece of a laboratory equipment this mli meter donated by a viewer so big thanks for the donation and especially for the ordeal he had to go through to ship this because this bloody machine weighs about 30 kg and it's about 46x 21x 29 CM so it's an absolute unveil and this all to basically measure the resistance of something in mli ohms and it comes with some measuring cables here some high current super thick cables a pair of cables coming from it bloody thick and this mainus cable with a mainus plug and this plug going into the back of it it seems to have a display made of nixes some polarity indicator and for digits and some indicators out of range doesn't comply with the norm unfinished measurement fuse a fuse B and minus I guess this indicator so blown of use this is the minus power indicator and to control it you have the mainus switch here some orang switch with six positions here from 25 amps to 8 amps maybe this knob used to have some cover on it I guess and here's a big knob and this actually chooses the measuring current and I guess this one indicates how to adjust it there are arrows I guess when this lights up you should go to the right and when this one lights up you should go to the left until you get it to the center it's basically a bar graph indicator with seven LEDs and of course to measure very low resistances I guess it has to use a Kelvin connection basically two terminals to connect the high current for the measurement and another two terminals to sense the voltage drop this one has a green cover this one used to have a red cover but it got crashed under it in the shipping I guess and this seems to be a little bit of a botch not sure this is original from a manufacturer not even sure did they make more of these or is it something custom but they can't imagine this produced in bigger quantities anyway now let's take a look at the back panel not that much to see here this plug goes in here there are two fuses with screwable covers they seem fine this one says fast 6.3 amps and it actually is fast 6.3 amps something you don't see very often and this one is 0.1 amp slow and it actually really is 0.1 amp slow I almost can't believe it and it says 1,00 250 bloody Vols basically to measure the resistance of something this is bloody amazing current mohm meter made in a machine and tractor station opava City in Czechoslovakia probably in the80s and some series number looks like 249 and they probably made just a couple hundred of these and of course at this point everybody's screaming power it so let's power it and let's see let's turn the mainus switch on and the display comes on and the brightness went up sort of gradually it says out of range not compliant here and these interesting nice flashing lights in pairs here it says it has minus voltage and not indicating any blown fuse what happens if I turn this by turning it all the way down we activated this led I have to connect the D does it have any effect of course it's a a bit tricky idea to touch it but anyway these have to be connected to it it should actually read zero resistance or a very low resistance well a short circuit is still not a zero resistance for this because it's always some resistance of the conductor is does not do anything it just shows a different random number every time May power it I guess there should be some bloody current going through these cables shouldn't it is it AC or DC nothing I think there's time to open it and to see what the hell is going on in it and how resistance meter can weigh about 30 or kilog each side of it has a handle on it to give you some chance of transporting it and two screws here so let's unscrew it of course slotted the screws I took four screws out of it and cover should come off and here are the internals you can see a panel meter module here with its own Supply Transformer this is basically a module of a panel meter which is meant to be built into machines this module says 220 volt 50 HZ 10 volt amps some type number 89 is it made that light but it's of course possible And1 T 200 40 type panel meter and typically it has a plastic cover all over it but this one seems to be somehow crudely cut out maybe to prevent it from overheating in a double enclosure here you can see the internals of the meter a little bit there is some other Transformer small transformer for the internals well I call it a small transformer because I saw this one there is some I guess current sensing shant on very thick wires this is the terminal block for the primary I guess this is I guess the secondary producing the super high current this is sensing it and again this current sensing shant has a Kelvin connection with two wires basically separately sensing the voltage drop and other two wires bringing the current into it and this basically avoids measuring the resistance of the screw Terminals and here's some board associated with this LED indicator here I guess seven LEDs in seven maybe op pumps or comparators flipping the LEDs here plus some transistors some chip I guess it's some very primitive logic 7437 from 7400 dtl logic this panel meter connected using this connector lot of cables here on this side this is for the range switch not sure what's here but I can see a bloody number of multi potential meters I have never seen that many multi potential meters in one device that's interesting and this is also a connector here the transform has the super thick secondary going from it here the cables go out here's a double switch basically two stck switches to switch the light and a neutral of M using this of course I'm board with LEDs here for indication most of them are red one green for the mainus voltage indication and on this side well this knob actually seems to go to a vak this is an interesting mechanism here's the Rea with a sliding contact sliding on wiing to basically set an ac voltage from Z to full minus voltage or even higher it says 220 volts here that's the input of M25 volt input not used zero not sure what this the slider I guess and 250 volt Top This is the top of the wi ring and as you turn it you also turn this when you basically turn it all the way down this lever presses this switch this is actually what initiates the measurement this is some sort of an interlock that you have to turn it all the way down to initiate the measurement so you always start at the minimum there's some speaker which looks like from a telephone and the fuses the main is in here and here is the bottom of it the screws of the big Transformer which is absolutely huge and is probably most of the way to it but also the VC and a couple smaller Transformers and this entire thing is a big heavy metal box at the bottom here you can see the core of the huge Transformer which basically goes all the way into here and some other components another small transformer here this is mostly passive things capacitors resistors another board here with some transistors this is bloody complex here's the slider of the Via sliding on the wiing and the slider has to have some heat sink I guess but why does it not produce any current here I guess this Transformer has a primary which has to go into the VC and the VC is connected to the mainus probably not straight through this switch but through some contactor where the contactor could be there's a big metal box in it here measuring the voltage of the vak inlet nothing and the life neutral of the terminal block of the big Transformer again no voltage and also the two measuring cables showing no voltage it seems like the control circuitry of it and the panel meter are working but the power section not if I connect this pair to this pair still nothing this indicator stuck at the fairest LED but in either case it seems the measuring current is actually AC not DC it basically just passes a high AC current through a low resistance that you're measuring and since is the voltage drop on it using the other two Terminals and based on the ratio of the current and the voltage drop it measures the resistance well I started taking a closer look at it and it seems there is a lot of connectors for the internal connections another connector here and in the back of it it looks like when a connector actually came off and this plate was par probably originally meant to keep them in place this actually looks like the problem he put the connector back into its place and now let's test it and this is Dody I should probably screw it on some piece of metal turning on yes it's working it's probably too good measurement 0.6 mils another measurement turning it up centering it on the red for long enough and it measures this I guess what if it's open a circuit then I of course can't get it let's try to use this so as a low resistance resistor for the test do enough and and turning it up it still shows a 0.6 mil what if I use a different range here I'll was is this reading but now it no longer indicates out of fren or some other error of course this is getting a bit hot during the measurement or quite hot if I turn it too much up and I don't hear any contactor clicking I guess there is a triac or a th to switch it maybe and when you manage to get the current to the center here and keep it there for long enough it basically takes a measurement and then it deactivates the Transformer but not sure it's working properly because it never reads anything else than this I tried to measure the resistance of multiple meters of a cable and still showing just this and of course this video is taking bloody long to finish because I had to take a break I caught the creature again and I didn't even have a voice I accidentally found this cover for this one in the box at the bottom and now of course let's try to fix it with my brain fog For Your Entertainment and not that I have any use for this machine but at least for Education it's still not measuring I tried to reverse the leads of course polarity doesn't doesn't matter in AC but the phase might matter but in either case it's still reading very low 0.6 as always and measuring the varc it can go up to 260 volt the open circuit voltage of the electrodes is 13.7 Vol at this and it's 230 volts the voltage at the output on the secondary 12.3 volts so it's basically a 12 12 Vol Transformer which could be useful separately 12 volt and maybe something like a kilov volt dump of a transformer of course the vak which I guess is about five hups is also very tempting to just take it out of it and use it separately on the back it says 1250 volt amps and for 250 volts it would be 5s and besides the Transformer and the vak even the panel meter could be used separately I guess I'm trying to disconnect and reconnect ruse connectors in it and it still won't work might actually be damaged from the shipping I guess maybe it actually worked before it was shipped take you look at this screw how it's bent this one actually the not came off and stayed under this plate before it was dropped so it's not bent here but this one is quite bent is the screw of this Transformer nice let's explore it further I guess the meter is just on one screw here let's unscrew it and then this entire module comes up it with the nixes and the screws for screws of something else under it let's unscrew this no screwing the four screws and they're using bigger nuts as washers or spacers you have to take this off this thing should fall out and that's it some power transistor and the 7805 regulator of 5 volts smaller transistors some capacitors it looks like a power supply there's actually more more boards three boards stacked in it here's the switch for the renes and a lot of other circuitry here let's try to take the boards out of the housing there scuse again and bigger nuts as washers spacers I have to say this low volume or custom communist production really doesn't look very far from a DIY construction does it here are the boards some spacers made of absolutely whatever was available this insulation things on the edges of the boards this looks interesting I guess this is the power supply of this section the next the board is this thing you nothing is straight here here's the amazing row of 12 multi- potential meters and a lot of other resistors and the range switch here not sure what's this some capacitors soldered onto this and then this soldered into the board let's pull this one out and after this analog board we can see the last one which is digital a lot of chips and not much else and that's it and this entire board is all 7400 Logic 7493 for bitor base 16 couns you put the board is back maybe there is just a poor contact it seems like a center to Transformer and two diod is rectification here in the power supply is it there is some voltage 17 volts and the Transformer with the bent screws seems to be still working some capacitors this is actually the same voltage from the diode and this should be one of these should be from the 7805 wa it's negative well I'm measuring the heat syn of the transistor not 7805 of course I was probing my negative probe into 5 volt so it was basically subtracting 5 volts from whatever I was measuring now it's right 22 volts is the rectified and smooth voltage from the Transformer there should be some 5 volts here from the 7805 and some other voltage from the power supply there is MAA 723 regulator controlling the transistor the power transistor and I guess 12 volts is generated using this thing I tried to take one more measurement and it's still showing this 0.6 as always so it wasn't a p contact in the connectors it has to be something else and here's the board on the back with all the connectors and of course in the cables one of this is from the current sensing shant and one is from the voltage sensing Terminals and of course I'm not sure how far I want to go fixing this bloody 30 kg crazy complex resistance meter that I have absolutely no use for we haven't seen yet this side of it and the metal box in it it took the bars on the right side of it out and this was just screwed put the bottom using two screws and there is basically some tiny auxiliary power supply Transformer I guess some Bridge rectification some capacitors that's another power supply here and then you can see a big bridge rectifier out of four metal diodes and a thy kt7 8 I looked it up it's 700 volts 15 arms it's a th but I guess in combination with the four diodes it works as a triak switching ac voltage to the Transformer primary and the other side has some beeper maybe some alarm for overload or something some transistors whatever this is it's sort of a rubbery is it some op to isolation there is a huge isolation Gap and I don't know no and lot of transistors here and capacitor and of course the question is what's in this metal box it's out one was missing in it and removing the cover and it seems like some interference filter with inductors and capacitors and a schematic in it nice seems like a typical interference filter double inductor in it terminals connections a capacitor and some other inductor it says 100 Nano plus twice 2.5 Nano capacitors 250 Vol thens an interference filter with a schematic included and in a solid metal box imagine that you can't imagine something like this in modern appliances I put the right side of it back together I don't think there is anything wrong here this double board probably does the primary wiing switching this is basically monitoring the current and giving you the directions how to control the vak nothing wrong with this one as well this is an analog to digital converter converting some voltage like up to 200 300 M volts to digital reading I guess the problem has to be somewhere here and besides the positive 12vt rail the power supply also has a negative 12vt rail as you can see it's working and the plus and minus 12 volt is used for do pumps and the 5 volt for the logic guess nothing wrong with this board of the power supply the problem has to be in this board I guess the one contains three opss here MAA 725 and one more on this additional board and I'm thinking how this was even working because typically a resistance meter contains a constant current source and measures the resistance based on the voltage drop of the resistance at this current but here the current isn't really constant you can never really manually set the current accurately enough using the vac and this crude indicator to take a precise digital reading so I guess instead of just measuring the voltage drop of the resistance you're measuring it has to be measuring the ratio of the current through it using this current sensing shant and the voltage on the resistance you're measuring the resistance basically is the voltage divided by the current and if it's the case it actually requires an analog division which is not an easy thing to do especially not with a high accuracy does this do some analog Division and then of course there is this 10 chip logic circuitry not really sure what this does this is quite a bloody complex thing and I don't think there is any schematic for it I guess the problem has to be on one of these boards and because the chips on this one are soed let's just replace all the chips in it and let's see what happens why not just go about it the lazy way as especially when you're demotivated from fixing it now all the chips in it are changed I put it back together and it still keeps showing 0.6 of course when you have a schematic and you're familiar with the machine you can shrink The Surge down into a small area based on the symptoms and then look for the better component in just a small area but not being familiar with this one and having no schematic I have to search more blindly in a larger area and use a more General approach like measuring all the resistors capacitors diodes not knowing how exactly this one operates testing also can't shrink The Surge to a smaller area so I'm measuring various capacitors resistors and so on let's measure some capacitors and of course I measured all of them already and they're charging no short I basically test all capacitors for short and all elect otic capacitors for high ESR capacitors typically can be tested for a shorten ESR in a board but the capacitance can't be measured in a board he measured some diodes they're good he measured all the resistors basically in it and they're good resistors typically can be measured in circuit and if there is something connected to them parallel it can screw the resistance measurement down not up if you're measuring more resistance than the resistor says on it it's probably bad I've tested all the capacitors ceramic film electrolytic all the resistors the potential meters the resistance measured from the slider to each end of it has to always be the same or lower than the marking of the potentiometer otherwise it indicates that the slider is not making contact or the resistive Trace is broken there is also a circuit with multiple resistors built into it and it also seems good basically absolutely no faulty component here I was almost blindly replacing the op pums because they can't easily be tested of course have having the boards stacked in it you can't really probe into it when it's running it was almost blindly replacing the op pumps but then I had an idea to measure the switch the switch has six positions and in each position it connects two pairs of panes in the first position for example it connects these two and in the sixth position it connects these two pairs I was checking it using a continuity test and it turned out that these are not working the outer ones are making a connection but not the inner ones I guess this is from the ping it got dropped and because the borders are in a heavy metal box the Box moved and it basically bent or pulled or pushed this shaft going through the front panel and damaged the switch trying to take a closer look at it I guess some of the pins are broken here it's also at an angle this is making a contact this is not I just jumpered it permanently for the 80 amp range and let's try to test it I've made a Kelvin connection to my saw blade again and 7.7 milliohms of course as it's getting hotter the resistance goes up a bit and measuring certain length of a cable about 100 m and testing about 13 of the length showing 33 which definitely makes sent I was measuring three wires in this CA in series and then just one so does this machine actually working and of course you can take a very similar measurement using just a bench power supply and a voltmeter setting the bench power supply to a 1 amp current limit and the voltmeter read is 0.03 volts or 30 MS which at 1 amp means 30 milliamps a combination of a bench power supply and a voltmeter can also measure very low resistances except that it measures at a lower current so it doesn't heat up the cable or whatever you're measuring so it can't simulate the temperature drift at High current trying a modern multimeter again in a kelvin connection and it gives a very similar reading a 0.1 mom resolution basically the same as this machine and the bench power supply has two ARS about double the reading he P the jumper here to test it at 25 arm UPS now putting all five wires in a series and it measures 1619 M and the meter seems to have 2,000 Cs and reading actually changes based on how I turn this vak so I guess there is no division it just tells you how to set the vak and measures the resistance based on the voltage drop analog division really isn't an easy thing to do I don't see how this one would do it itally just tells you how to set the V Rea using this scale which is sort of logarithmic much more sensitive in the center so you can set it with some sort of precision you basically set the right current and this just measures the voltage drop on to measure the resistance and the Machine is now working other than this switch is permanently set to 25 arms it's usable but I don't have much use for it and I guess this V is made by some German company which keeps making the same way reacts to this very day just a different color of the plastic no change since the80s but when you think about it how could you possibly improve a v Rea anyway and of course it never took me so long to finish a video but if you like my videos please consider supporting my channel on patreon using the thanks button or subscribing and big thanks to all of you who already support me this channel wouldn't be possible without you

Channel: DiodeGoneWild

Views: 16,317

Keywords: milliohmmeter, 1980's, repair, how does it work, resistance, meter

Id: tWpunHEROvY

Length: 28min 19sec (1699 seconds)

Published: Mon Dec 04 2023