EEVblog #884 - EEVBlog BM235 Multimeter REPAIR

hi I thought we'd take a look at the first return to eevblog bm2 3:5 multimeter um Frank actually bought one of these he's in that Jacksonville in Florida and unfortunately he got it and it didn't work and he's um you know he described all sorts of things in that um it would you know measuring say a 1.5 volt battery and it measure 0.6 volts and then as a drift down to zero and and all sorts of stuff weird things didn't work at all and I've sold I think over a thousand of these meters so far and this is the first one that's reported any issues like this at all so yeah you expect this sort of thing to happen even though each meter is individually are tested at the factory it has to be because it has to be calibrated at the factory so each one would have been tested but you expect some sort of infant mortality rate on virtually any product out there it's not just this meter but any electronic art product there is going to be some percentage of failure rate um after they're tested and they hit the field it's just you know the way it is for whatever reason and that's no reflection on the quality of the thing or well it could be okay but in this case sold over a thousand no problems and this is the only one so you know who knows Murphy's Law uh where we've got one we got one and sorry my voice is still terrible I haven't made any videos this week because um yeah I just yeah you don't probably don't want to see me on camera either so I look and feel pretty terrible at the moment and give me follow me on Twitter I am breaking in all sorts of ways I've busted my ACL joint in my knee is completely busted anyway um as the probes so yeah I might have to get surgery on that so yeah I'm doing wonderful at the moment they turn to wrap this puppy and have a look and see if I it's repeatable um but the I'm pretty short you know from the stuff he was saying it was pretty obvious that this thing was dead so let's turn it on and see what we've got here switch on he said it worked on AC curiously now that you know function wise it's it's displaying the right stuff okay so there we go okay no drama at all I've got a I've got my voltage art reference here I've got it set to a volt this is my lab referent one of my lab reference standards so let's plug it in and um see if we get a volt why why why why he's right look it's going down and down it's not like it's on AC mode it's on DC mode and sure enough I mean if I bring in good one over here and of course it will la measure a volt no worries whatsoever so um yeah that is one sick puppy what is wrong with it well let's maybe whack it on ohms here and disconnect that and okay ohms works oh no that was continuity that was cut Oh cut nuit e mode look nope okay ohms is just showing direct short that's interesting that's interesting I will it's got it maybe there's some sort of solder short on the board or something that's causing that I mean obviously our chipsets working fine and there's not much in this apart from a the multimeter chipset that's pretty much the only active circuitry and the LCD driver chip the capacitance oh that seems a bit high 30 nano farad's I'm not hang on I've got a reference cap here here we go let's check it out I've got myself a em these are very nice if you can pick them up on ebay cheap they're old but these are Co standard capacitors they're very very nice and and often get them quite cheap ah that's low that's low put it over here but at least it's functional there we go no dramas whatsoever that's well within spec so yeah this one is a bit there and I've checked other meters I think that is yeah so this one's a bit low on cap let's measure some current shall we here we go I'm feeding in one milliamp current range works current range works ten milliamps hundred milliamps Hey okay that's interesting that shows that well because the the current uses a different input arrangement so yeah there's something with that main common terminal which you know is maleic so it's not the chipset it's not like I don't know ESD or something else has gone wrong with the chipset and it's damaged because the current just works fine let's try the micro ampere range 100 micro amps there you go yep it works just fine and dandy and hold on to your hats the millivolt range works fine and dandy as well so um yeah that's interesting so millivolt range works current ranges work so chipset is just fine but all the other functionality like if we go back here to our volts you know like that is that is dead and let's just jiggle the range switch around here jiggle is a technical term and it's not that so looks like something on the front end of this thing is gone on particular ranges the millivolt range is basically straight in it doesn't go through the input divider or anything else unfortunately I do not have the schematic for this Broman will not release it to me they say its proprietary bla bla bla yeah okay fair enough so we don't have that but hey we can at least have a look around let's go the serial number for those playing along at home and we're in like Flynn unfortunately we're gonna have to take this top bored out here that's one of the downsides of having the cat for raiding in such a small meter so only way they can physically do it and that's where a bit of the input circuitry is around there but we can see other stuff on here and I might actually get out the Takano microscope just get the two side-by-side and see what we can see oh and the other annoying thing about having the secondary board here is because I do have to actually de solder this and unplug it has essentially unplugged the jacks and everything else to get in there we can't sort of like feed in stuff and then measure it on the main board underneath at the same time so that's what you can if you go to a lot of trouble to wire it back in and stuff but yeah it just makes it a bit tricky to troubleshoot but the first thing you do with troubleshooting something like this is visual especially if you've got a unit to compare it against alright let's check this out we've got our unhappy camper on the left here and our happy camper on the right and probably just a just because you can do a quick visual inspection to see if there's any missing parts now this you know shouldn't be the case because as I said it would have passed factory test factory calibration everything else to do that it would have had to have everything in place but I don't know something might have made the poor contact and then in shipping it might have vanished or something like that but I obviously can't see under the input board yet but it doesn't look to be any issue here at all I wouldn't expect that but see the nice little ah star grounding point there very nice just going off someone knew what they were doing in terms of PCB layout but yeah there's nothing obvious going on there at all as I said the main chipset down here which is a BTC which is bribe and Technology Corp branded I don't know exactly what what it is they won't tell me but that is obviously not failed because half the functions work and the other half don't so there's got to be something screwy with some with something to do with the input so that all looks hunky-dory the other thing next thing I would look for is any shorts like a little solder balls or anything like that as part of the production process which could have got in there and they may not have been an issue during the during the process I love my Takano microscope here it's beautiful isn't it um and all the way in and go all the way with LBJ look at that you can see the silk screens actually a dot matrix printer it's not a photo image of all silkscreen you can see the dot pattern on there and but yeah I'm solder balls they could um certainly account for it as part because they're like sort of like a little unknown theme got slightly too much paste you know it's not exactly the same every time even though you know there might be a smidgen over or just how the you know the dynamics of the paste and everything else and how it melts and things like that you can often get a stray solder ball so I'm just looking for anything obvious like that and I'm not so I wouldn't expect to see it up the top because this is like as I said this is the LCD chipset all right here we go here's the top board we've got our here's our voltage input of course there's two input protection resistors I can measure those of course there 1k could our blonde brown black red there and I don't think it's going to be our ptcs here because otherwise we wouldn't get anything but anyway oh that got a little and somebody's had a little crimp no no it's all right no worries there but we could measure those they should be about a K each or something like that but as I was like if it wasn't broken like that doesn't explain like the zero ohms and things like that and the mobs here well you know in theory it could be but doesn't explain why the 100 millivolt range was working for example and you know couldn't measure 1 volt so you can measure those they they should be open of course they should only start clamping at you know a thousand volts or whatever they're rated for we've got some 5 Meg input resistors here you can measure those might be doing silly buggers but we've got the input jack detection stuff like that so it's not that so there's nothing on the top board really that you would suspect I'd be going straight for down here and let's so let's have a squiz around here and once again I'm looking for bad joints oh hello hang on you know nine is that my imagination hang on sorry I may not fight oh no no that's all right no thought there was no feeling on that but it's it's all good so I'm looking for bad solder joints I'm looking for solder balls shorts other things but what's going on down there oh hello hello l-3 tada hello get that l-3 has cracked the solder joints on l3 have cracked we got one yep Wow how has that happened that's been down wire out of the way there sorry about the light here I can I can here we go it's going to be better Wow I thought it looked a bit askew and you go there and sure enough that puppy wow that's that's an inductor and that has sheared right off I mean it's not like it's a heavy part so that is very very interesting I'm going to well it's gone look at that there we go okay so what I'm going to do I'm going to take that out now they put some fresh solder down on there those pads I'm going to so well I put fresh pillows solder down a one or week the other off and replace that and I think it might come good again the one next to it sorry about the focus on this the one next door it looks well next to it looks good that is weird because and okay solder joints can crack like that but they're usually on high mass components so Wow I find that totally fascinating how it's able to do that Oh check that out something has gone horribly wrong with that that's the bottom of it it's like got black goo or some crud on there that is really really interesting Wow I wouldn't have expected like this was one of the things I was looking for was a dodgy solder joint I wouldn't have expected a surface mount part like that shear off but what is sup something's gotta what what on earth has happened to that look the beside the cap everything else is that blowin Wow like there's no way that inductor could blow it's not there's no way that could blow and not blow the input circuitry so that is Wow that is toast so I can't just well I could flip it over I could flip it over and resold it back on but jeez you know flip Reese order it on upside down like that but that is horrible has that just been a badly produced inductor you can get that it's um something's gone horribly wrong with that yep I think we just have a bad inductor there's nothing on the solder in look at that there's the there's the other half of the of the cap from the inductor and it's just yeah we have a faulty part of a faulty inductor something's gone horribly wrong with that inductor and obviously it still made contact when they assembled this thing it was still fine and whatever passed it yes but after that it's just gone horribly wrong but as I said there's no way that you could blow that inductor and not blow your inputs so you know not have your input protection trip and you know blow out your tracers and everything else so it hasn't been overloaded I think we have a genuine inductor manufacturing fault so there you go I desalted that from the PCB and that and of course what we're dealing with here this is actually the common terminal this wire here even though it's got red insulation actually it comes from the ground terminal on the front panel and they're just splitting those off different traces to different parts of the circuitry up here so anything that relied upon this ground connection here obviously did not work so there we go we're splitting some more there's another star grounding point very nice they know what they're doing there so what they're doing these you see little slots in there too they got a little isolation slots down in there what they're doing with these these aren't well they're not wound inductors these are our RFI beads actually just for some high frequency rejection to like you know pass emission standards and compliance and all that sort of jazz so yeah I mean you could bridge that over with a short if you wanted to you can put in a zero ohm resistor if you didn't care about the you know the EMI requirements and things like that I'm not sure I have a suitable RFI beam so I might just potentially bridge that with a zero ohm resistor just for now and that should get us up and running alright let's retest this puppy one volt and we'll we have a winner I think we will we don't winner chicken dinner there we go tonight that's all it was we had ourselves a faulty component I have no doubt everything else will work now if we just do the ohms for example the ohms will work tada there we go works a treat no problems that was the only thing wrong it was that was that RFI bead in there which I'm pretty sure that's a faulty component that hasn't happened during the solar in process or you know there's nothing wrong with the solder and process there it was that component was crap there was something seriously wrong with that and well yeah hmm happens so I hope you found that interesting I'm glad we actually found that that was pretty darn easy in the end only took you know seconds it was like one of the first things I saw when I opened that when I took that board off the back of it so pretty easy and we didn't need a schematic to find that so component fault these things happen and to me it doesn't really reflect upon the quality of the product if there was more than one and this was a you know a systemic problem then we might have an issue but hey you know ultimately you've got to trust the components that you're buying you know the genuine complaint you're buying from the genuine manufacturer they're all in the real you expect them to be manufactured properly it's not like the the manufacturer of the meter or whatever product it is can actually inspect those parts for themselves you rely on the fact that you're buying quality parts and well you know occasionally um you can have the odd component weird component failure like that for whatever reason as I said it would have been if it was like the entire reel the entire batch of these little lower ferrite beads that was dodgy then yeah there would have been you know yeah I'm sure we would have had a lot more reports so I suspect this might just be a one-off anyway I hope so um so yeah I'll definitely show Broman this video and they'll no doubt investigate it and see if anything else I might open a couple of other meters I've got in stock here although this one actually you know I've only got the new batch so oh yeah no I don't think I can even open well my current stock anyway just to have a squiz and I'll give a little bit of a wiggle on the ferrite bead out in there and see what happens but yep I I'd be surprised if this was a systemic problem across all meters I think we've just encountered on those weird one-off Murphy's issues anyway I hope you enjoyed that little hunt down of that problem if you want to comment as always links down below leave YouTube comments etc catch you next time hey check this out look amazing symmetrical multimeter stacking just like the Philadelphia Mass turbulence in 1984 aren't believable no human could stack multimeters like this that's a few multimeters 40 to be precise I can't explain it but there's something very therapeutic about doing this oh yeah so I've got my handy banana plug lead here I go oh goodness the things I do

Channel: EEVblog

Views: 159,306

Keywords: eevblog, video, brymen, bm235, multimeter, bm257, repair, how to, pcb, inductor, surface mount, tagarno, microscope, tutorial

Id: YM--cxT6X3k

Length: 21min 13sec (1273 seconds)

Published: Sat May 28 2016