EEVblog 1520 - Troubleshooting a Faulty BM786 Multimeter
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- Опубликовано: 19 дек 2022
- Looking at a faulty returned BM786 multimeter.
Warning: Some components were harmed in the making of this video.
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#ElectronicsCreators #Repair #Multimeter 
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LOL. EVERYONE including myself missed a major goof in this video. Can you spot it? It's not the fault, but it's something I was wrongly assuming. I can't believe not a single viewer picked me up on it!
broken ground plane continuity?
Dave, you did that deliberately, in order to do a second video on the same subject. I'd watch it however!
No fuse?
what was it ?
@@seanwilkinson2291 It looks like the fuse was removed during the troubleshooting...
My 22 year old Fluke 177 and 15 year old Fluke 85-V still has the plastic film on the display. I enjoy the fact that it drives people nuts.
Especially if it has little bubbles in it 😂
I would remove it for you without asking for permission and expect a "Thank you" from you.
@@UnrealTransformer Rude. You don't touch other peoples stuff.
"It's a collector's item! It'll be worth a fortune one day!"
@@UnrealTransformer Me too.
Repair videos are always welcome, thank you. Always good to learn about troubleshooting electronics.
yes repair videos is where the knowledge come to the fore because it is not easy at all.
About those protective films. I try to keep them on as much as possible (well unless they are annoying). I remember that my Nokia 3310 had the protective film during all the years that I used it (I nicely cut off the smal lip to peel the film off so that it would stay on nicely).
A real funny story is when I bought a plastic cutlery tray for my kitchen drawer I saw that the surface started to look bad after a few years and was about to throw it away when I saw that it actually had a protective film on it leaving me with a brand new tray after all those years (peeling off that film was ... omg, best day of my life).
I had a meeting at a newly opened remote office a few years back, and one of the locals was complaining how poorly the new white board looked after erasing. Even using the cleaner fluid didn't help much. I walked up to the board and noticed that the protective film was covering the entire board. In slight fairness, there were no obvious bubbles or tabs to remove it. Worked great once the film was removed.
My parents were complaining about how bad the control panel on their refrigerator looked when it was only a couple years old. I saw it still had the film on and went to peel it off, they made me stop, thinking I was going to damage it. I waited until they were gone, peeled it off and...viola! The control panel looked good as new.
@@Okurka. nah, violist’s heads are just smaller. Also, why do violists put a cloth under their chin? No spit valve.
@@briansauk6837 What's the difference between a violin and a viola? The violin takes twice as long to burn!
How many years did you have to take violin lessons before you realized you'd never live up to your parents' expectations?
It failed because they didn't take off the protective film. That'll teach them! 😉
Drinking game: take a shot after Dave says "schematic"
0:40 A while back I got a magnifier for viewing DSLR sensors, and it seemed to have mediocre (but usable) optics. Then I peeled the film off!
Thanks for sharing the journey, because the journey is always worth it! I know I learned a few things, got to flex my grey matter and find out that I was right a couple times during your journey. And that I was wrong a couple times as well, too!
At this point of the troubleshooting without having a replacement chip I would suggest trying to reflow the BTC chip, just in case one of the pin is not soldered properly
We learn more from how you troubleshoot, than from you repairing this one fault on this one device. Much appreciated Dave
Been using one of these for over a year now, well worth the money. No hesitation to probe around the 480v sprinkler systems we use, the 240v wells, etc and it still has all the features for the electronics stuff
For whatever it is worth, I would clear that debris between the multimeter chip pins. It takes pretty much no effort and removes that small possibility. Maybe my ocd, but I'd do that.
Looks like the debris did get cleared at some point.
It's not OCD. I've seen motherboards with QFP pins shorted by fine dust. Blew the dust off and then it booted.
Sometimes things miraculously fix themselves upon being cleaned! So to me thorough clean is a good early step. Even if there is no evidence of moisture ingress or anything like that.
Loved the journey, and goofs, helps us learn a lot! Thanks Dave.
You should make more videos like this Dave! This is great content and you’d always get my little thumb up! Would you try replacing the cpu from a donor working board? That would roule out the cpu right?
Thanks for exposing errors and the whole debugging process.
Thanks for this, nice to see what’s inside my EEVblog multimeter
This is why I love this channel.
Dave dives in the multimeter like a hero ;)
Totally agree. It is very hard to fix modern meters or any equipment nowadays as everything is integrated chips and surface mount components. It's almost impossible to repair or get parts. Not like the old days.
I LOVE this kind of videos! Thank you! Btw the follow up would be nice in case you find the way to get it working
Now I'm learning more electronics fixing.
God bless, Rev. 21:4
Will someone get this man a schematic?!
@EEVblog Merry Christmas , thanks for the years education & entertainment .
just back from holiday's Happy New year Dave to you and your family.
When you suspect a bad mosfet, or any transistor, it's prudent to check the voltages at the terminals first of all. That will reveal quickly if it is faulty or not. Before changing components and potentially lifting traces.
this video is a success as it learn about troubleshooting electronics.
I still have the protective film on my Fluke 75 series III, and that came out in the mid 80s iirc - that's getting on for 40 years now :)
"Warning: Some components were harmed in the making of this video" LOL.
Brymen should release firmware updates to public!
Great work.
This is why I come here. The teardowns and mailbag are fun, but this is prime brain food right there
Great way to learn real electronics repair and the diagnostic process. I've managed to fix 1 out of 5 so don't have a good track record but man it's a great feeling when I did succeed fixing that one.
as a software guy who's hardware-curious, I was surprised by how seldom Dave referred to his known-good unit to check his understanding.
My lovely Fluke 179 still had the film on it and I just can’t bring myself to tear it off. Still in very good nick.
I still have the protective film on mine! Don't really notice it and it's been there several years. I just think of it as a screen protector 😋
Hence the name _'protective_ film'
I figured it out after 2 minues, well theirs yer problems, its no Fluke (lol), it's obviously the flux capacitor lol. Merry Xmas Dave when it comes to you, the lovely wife, sagan and Huxley. thanks for all the amazing videos you upload.
I let the Film on Displays and anything else when I buy something new! My Bike have the Displaysecure on it after 20k km and i love it!! You are a very symphatic RUclipsr. I hope my English is good enough to understand! Greetz from Germoney!
My dishwasher still has the original Film over the entire front
Thanks for another great video. I'm amazed that with your name on it they don't allow you the schematics. Merry Christmass.
I still need a new DMM. Once these are back in stock (Amazon US) I'll be buying one.
If you can't wait for them to show up again on Amazon (if they ever do), you can get them from weletron. Shipping to the USA is a pretty fair 10 EUR or so. I think it ciphered out to less than the Amazon price for me even after shipping and exchange (USD was close to par with EUR back when I bought). Keep in mind that when shipping to the USA welectron will not add in the VAT and they didn't charge my state's sales tax, so the 10 EUR shipping was more than offset by that.
Thanks - that was fun.
I always keep the film on mine, I basically never remove it at all unless it is starting to work itself off due to dust, etc...
Certain products I have no choice but to remove it, but if I can get away with keeping it on, I will.
I think the U1 "microcontroller" you are pointing to is actually the HY3131 DMM front end (48 pin). The NCs pins seem to line up. The pins you pointed to are AGND, REFO and VSSA on the front-end. The location of Xin/Xout seem correct and the HY3131 spec sheet specifies that 4.9152 Mhz Xtal. Perhaps U16 (64 pin) next to the buzzer is the microcontroller? Maybe U16 is a HY16F 32bit microcontroller?
Действительно это он, надо подумать над его модификацией .
"...and bodge is your uncle." Still a very entertaining video! Thank you!
I could watch Dave probe meters all day ; )
sometimes part numbers are on the bottom and the top number is the batch, and you have to physically remove them to figure out what they are (the sot23's)
I peel it off as soon as I confirm it works and decide to keep it.
In my experience Engineers and Fellows are the worst Technicians. If you were one of my Techs, we would have to have a talk. I am glad you guys still need us Techs to help you think!
same as needing nurses to keep doctors from killing their patients.
Also, I can never resist taking the protective films off, once it powers up 🙃
10:09 "sounds like a voltage that you'd have" yep I've seen that voltage before too! lmao
Excellent troubleshooting video
there appears to be a small solder bridge under the leads of U1, under R32,and an inability to detect or program is a common symptom of that. I'll find out what it is in part 2 though! I do rework, and those crystals are VERY difficult to remove. you have to wick up one side, heat it and get a blade under it, lift it the tiniest amount up, with liberal flux. then add more to the other side and lift up the wicked side to kind of tombstone it up, and you can pull it off.
23:18 Doesn't R104 look kinda suspect? No idea what im looking just feels crusty
So did the cap above it. It had a black spot.
Although R104 looked like a solder splash.
Nicely spotted. It measures 0.5ohms which seemed odd, so I checked a good board and it's 10k. So somethign up here. Cap not shorted. It was just flux residue, and it's not part of the micro circuit so doesn't explain the lack of boot, but somethign it certanly up here. Also, the resistor above is populated on the good board.
@@EEVblog probably micro blew up thats why the resistor got charred
Check C46 (nearly next to it). Might be some burned flux ...
@@EEVblog No wonder they don't want to give you one (of many of) the schematics.
Often (including at 28:01) you can see that C107 is stuffed with a 0 ohm resistor (marking 000)! It's not odd to have parts unstuffed, but to stuff a capacitor location with a jumper is very odd. C101 is the same way.
It's an 0805 near the top center of the board on the side with the processor. Near the edge fingers (zebra fingers?).
The BM786 looks like a wonderful multimeter, is there a Bluetooth version? 😎 Thanks for the great repair video.
Oh those were the days: 1971 do you feel lucky punk Dirty Harry... Clint Eastwood his journey from Raw Hide TV.
Worth the trip! I would touch the pins with my iron bit.
i just saw whole your vidoe repair the DMM , but it take a long time
Nice.
12:00 - This was my thought process as well. Especially if it has a buzzer in it.
Texta mark is a nice touch, just like doing your timing belt on the car ;)
I bought an EEV Blog branded multimeter (model 121GW) about a year ago and although it's a bit more pricey than the Radio Shack meters I'm used to, it's a huge upgrade and well worth it in my opinion. Just used it a couple days ago to measure some zener diodes (using the 15V range).
I had two, killed both with a range sweep at 240v. junk meters.
to be fair, Dave did send me a replacement meter and then a refund, but I still had to pay import duty on both. Kept both test leads and fuses, so I guess it worked out ££ wise.
@@ritecomment2098 I've never needed to measure above 120 volts AC but I've had zero problems -- I'll try it on 240 volts tonight with my welder plug. You have the 121GW model? I searched the "EEVblog 121GW Multimeter Issues" thread and found no mention of killed units like you describe so I'm guessing this is a fluke :)
Of course you have to know what you're doing when you use a multimeter; reading current or resistance will at minimum blow fuses for sources that are not current limited!
@@IamJohnanderson I had a 121GW, I have a 101,175, 325 and 17B+ now in daily use. The fault with 121GW is that after a range sweep it seemed to lost all calibration or blew something in the precision measurement circuit as it was reading 10's and 100's of volts with nothing connected. I think it must have been a common flaw as Dave was selling upgrade kits shortly after. I was a kickstarted backer and had such high hopes so I must have got the first batch.
Whenever I get a new meter I always do a range sweep from OFF to all the ranges, AC,DC, resistance, capacitance, mA, A and then back again slowly a few times with the leads in the COM and V/ohm plugs. If it's input circuitry can't take that (as noone is perfect when working), then I won't use it. This proves that the high impedance circuit is working. Obviously doing this in current mode is pointless as its just a short circuit, but ensuring there is no crossover in the range switch section of the PCB during these modes is just as important.
I work on AC servo systems with an shared DC bus that can be between 600-800VDC so its a complete reject for me, I didnt even take it onto site, can't trust it at all.
Its worth saying that the safety specifications for meters only specify that there should be no explosion during a failure mode, not that there should be continuing function. But this is 2023 now (nearly), I just can't throw away a meter because I had a 1 second moment on site.
@@ritecomment2098 Thanks for the further info! You know what you're doing.
Sounds like hopefully this issue has been resolved but I can see why you'd be hesitant when you have other options that are reliable. I use my meter mostly for low voltage circuits. I do like the fact that the firmware is hackable and perhaps some day I'll get a chance to play around with it.
I've got a bootleg fluke that I repaired. I don't remember what was wrong with it but I put the selector switch in backwards.
It works most of the time.
My initial thought was maybe the zebra strip under the display was messed up somehow, but I know it’s not that now. Second guess would be a bad trace in the layer of the board
I removed a blue protective film from my coffee machine earlier this year. For years I just assumed it was blue metal...
I have the same type of blue film on my kitchen scale. Instructions say take it off, but I like the look.
My 2yo BM789 is just starting to have range switch issues while my sons BM786 is still going strong. Its possible my meter had something heavy placed on it during storage/transport
F2 fuse out for most or all voltage checks. It's 0.4A , so not for protecting the high current input.
Instead of replacing the crystal fot testing, could you have paralleled it with a good one to see if oscillations would start (even if the frequency was off)? Alternatively, could you tack on wires from the crystal to another (battery-operated) oscillator circuit to see if oscillations will occur? (The latter assumes that the meter’s IC wouldn’t interfere.) Either way you would have avoided unsoldering the crystal and, of course, the less you have to unsolder the better.
I have a feeling that break before connect on the rotator thingy just momentarily switches on the chip so it could latch the soft power on
Some time you just cant coax the blue smoke to go back in the part. "It's dead Jim"
I would refit the original crystal (if usable) and then do a pin-to-pin check (DVM then scope) between the programming connector and the micro, comparing with a working one. I wish I had £1 for every-time you said a schematic would be helpful, nice etc.
For film replacement you can cut a piece of simple mobile phone film, sticks better than original one
Is it just shadow or is there a crack between C75 and the adjacent via?
Just a couple of silent hints to supply the schematics.
Why are you doing business with a company that won't supply diagrams and other resources to provide repairs.? It doesn't give me much incentive to buy one.
I don't offer a repair service anyway. Neither does any othr Brymen dealer AFAIK.
every journey matters - especially if its one with Dave.
C107 and C101 look like them new fangled zero ohm capcitatorizersistors thingys, but another online image of the PCB's of this meter shows the same so it's probably a BOM item replacement fix.
Good thing sir you have an honest video. it's easy to break such a class meter. I have been saving money to buy a meter like that.
I think the fault is on PP3V3_G3H , wait wrong channel.
I think you came in on the short PPBUS.
Could that make b4 break be for the timeout if you put it away in the on position?
Yes I always keep the film on my devices.
Does the button mezzanine need to be in place? Are signals jumpered across the connectors?
ML images to schematic should be here soon ❤
R104 by the chip is that a fuse on board black mark on it or is that nothing?
To see if the processor runs you van use a scope. After removing the P-channel mosfet you can test it. I love repair videos (but not TVs or laptops) even if they are not succesfull
On not being willing to give you a schematic, is it possible they don't have one?
That doesn't seem likely at all. What EDA software doesn't co-maintain schematic and artwork?
I have put too many amps through my CEM DT-21. It doesn’t seem to have a fuse. So I think a component died because it doesn’t work anymore. I wonder how I might diagnose the failure point.
Plastic film is left on display simply to avoid scratches when meter gets tossed in bag with other tools, its cheaper to change clear plastic film than the hard plastic......
Bodge it on there the same way I would.
It's good to show the failiurs, after all failure is a teecher & one should not deprive the people of his lessons.
makes me wonder if the cpu needs a 3.5v supply jumping in, in case a via or something has failed, or been made to fail from a usually shorted multilayer cap.
Sometimes you eat the bar and sometimes the bar eats you.
34:18 Just curious. Did you have access to an "underside preheater" or something similar? I know it's water under the bridge, but my experience with pre-heaters has significantly helped with those type of components.
Yeah they're usually pretty good, but this is a fairly densely populated double-sided board so I'm not sure if it would have been much use.
I do, but it's not something I would have normally considered for a simply crystal. By the time I realised it wasn't budging the pad had lifted.
@@jhonbus I somewhat agree, but to further add... any highly successful techniques wouldn't be straightforward. I've worked with 6+ layer mil-spec avionics boards with much higher component density and LOTS of disassembly and "pre-desoldering" would need to take place even before employing any sort of desoldering techniques, even with a "simple" crystal. That would be just the beginning. I've successfully removed more root cause fault components using these techniques more times than I care to admit, mind you for over 30+ years and still going, but other components that could absorb significant amounts of heat may need to be sacrificed meaning more possible tradeoffs. Again, just the beginning.
EDIT: In this case, this is a "ruggedly built" DMM, which those robust components definitely would absorb/dissipate massive amounts of heat away quite quickly.
Just my 2 cents.
😎
@@EEVblog Yep. Which could mean more disassembly and even sacrificing other components that could be possible "quasi heatsinks", yet still no guarantee. Considering the cost of the DMM, It would be for entertainment/education purposes.
1. FKUX DLUX FLUX is your friend
2. Low-Temperature solder for removing components
on multi-layer boards ----
ONLY used to aid removal - NOT for soldering
Maybe try to find the reset pin!
I thought perhaps you skipped a bit too fast over that rework that had obviously been done already on that board. That is suspicious isn't it?
Dave blames the mythical "Murphy" a lot when the problem is actually "Dave" and his rusty diagnostic skills. We all get rusty if we don't do the diagnostics work frequently. Dave makes videos now, so I'm not making any attempt at criticism, it's just reality. This is why a lot of places have a dedicated diagnostics person, or team, even for reversing competitor gear.
Too bad we don't know what happened, would like understand it (and take in account for the design i work on).
Good work.
Count how many times Dave says "Well if I had the schematics..."
I ALWAYS remove the plastic film because over time - especially in the hot summers we have here - it gets baked on to the underlying window (so it’s hard to get off cleanly), and then crinkles up. Yuck.
I kept the film on the screen of my PROMAX multimeter for more than 26 years.
This was probably a DOA - it did switch on, but did not function due to missing fuse 200mA 250V (5mm x 20mm)
Is D7 on the “bottom” (ref from programming port) cracked?
What's up with R175? It has solder on the pads but missing the component.
Can't win them all !......cheers.
while you put your "summaries" on youtube (amazing), have you thought to make the entire troubleshooting video stream on twitch? (or youtube itself)
it would be nice to interact with the community while troubleshooting.