BM235 defective? - Page 1
Forum section: electronics / testgear

author: Dr.Joe
author_rank: Contributor
timestamp: May 20, 2016, 05:18:11 pm
content: Hello all,my name is Johannes, im 27 and im from Germany. I am an electronics hobbyist, and i mainly do "audio stuff".I recently bought a BM235 from Dave, and after the german customs had it for about 2 weeks, i finally got hold of it.The first thing i did was test it on some AAA batteries, but the voltage readout was all over the place (sub 1V, and constantly changing), so i measured some SMPS at 12 and 5V:12V:cheapie Voltcraft VC160: 12.3VBM235: 10.06VIf i reverse the probes, it only displays 9.06V.5V:cheapie Voltcraft VC160: 4.88VBM235: 2.84V, and 2.48V with the probes reversed.After that, i emailed Dave and he suggested i measure some batteries:9V battery:cheapie DMM: 9.44V no matter which polarityBM235 in VDC Mode:: 7.17V correct polarity, -6.30V reverse polarityBM235 in AutoV LoZ Mode:: 7.6V no matter which polarityand then i gets weird: When i try to measure anAAA battery, its starts of at around 1,07V and continuously goes down to 0V. This takes about 3-4 seconds. If i then short the probes, it displays around -0,7V and again goes down to zero. the cheapie DMM tells me the battery is around 1,17V.However, if i measure my line voltage, its pretty much spot on: 239.9V at 49.98Hz.The Ohms range is completely broken. No matter what i put across the probes, or if i short them, the meter tells me its off-range, and displays 0.L. Same with the continuity tester.The temperature range doesnt work either. it just says "---F" or "---C", both with the supplied k-type probe and another one.I have not tested the A or mA/µA Ranges yet.So, am I missing something here, or is my meter broken?Thanks in advance,JohannesEdit: i did change the batteries, but that didn't solve it.

author: thisguy
author_rank: Contributor
timestamp: May 20, 2016, 05:32:25 pm
content: Maybe obvious, but you didn't mention whether you put fresh batteries in it. Did you? Edit: (Oh, I see now that you did)

author: retiredcaps
author_rank: Super Contributor
timestamp: May 20, 2016, 05:39:28 pm
content: Hmm, if you are willing to take apart the BM235, I suggest you measure some of the input protection components (MOVs, PTC, power resistor,etc). All can be done in-circuit. A MOV should measure infinite resistance. A PTC should measure around 1k ohm. The power resistor, it varies in different meters, is probably around 1k ohm.I would also do a close visual inspection, and/or post pictures here, to see if anything is obviously bad like bad solder joints, blown components, etc.I can't see Dave denying you warranty/exchange if you take it apart.

author: Dr.Joe
author_rank: Contributor
timestamp: May 20, 2016, 06:09:46 pm
content: thank you for the replies,@thisguy: Yes, i did change the batteries. twice@retiredcaps: i did open it already, and did a visual inspection, but im no pro, so i attached a picture.I just measured the input protection components, and they measure as follows (referencing Daves pic:https://www.flickr.com/photos/eevblog/24781245266/in/album-72157664100975342/)R7 and the one directly above it: ~1kOhmPTC1: 1.6kOhmPTC2: .94kOhmAlls the 3 blue MOVS measure infinite resistance.The black long resistor on the Mainboard measures ~10MOhm.

author: retiredcaps
author_rank: Super Contributor
timestamp: May 20, 2016, 10:47:57 pm
content: Quote from: Dr.Joe on May 20, 2016, 06:09:46 pmAll those measurements above look okay.Can you also verify those pink resistors (5M ohm) in Dave's picture are both reading 5M ohm?

author: Dr.Joe
author_rank: Contributor
timestamp: May 21, 2016, 11:22:22 am
content: Quote from: retiredcaps on May 20, 2016, 10:47:57 pm5.04/5.03 MOhm

author: EEVblog
author_rank: Unknown
timestamp: May 21, 2016, 11:35:13 am
content: I've got another meter on the way back with similar symptoms. Really hard to diagnose these things without having the meter to hand.I hope it's the same issue/cause as yours then we'd have something to go back to the manufacturer with.I don't have a schematic which makes it harder.

author: timgiles
author_rank: Regular Contributor
timestamp: May 21, 2016, 11:58:29 am
content: Keep the faith DrJoe - Dave is on the case and it should not be too long before corrective action can be taken.Its a great unit, I have been using mine for a couple of months now, its never missed a beat.

author: retiredcaps
author_rank: Super Contributor
timestamp: May 28, 2016, 03:14:01 pm
content: @Dr.JoeSee eevblog 884Check for bad soldering around the same area (underneath the board).

author: jitter
author_rank: Frequent Contributor
timestamp: May 28, 2016, 03:29:30 pm
content: Bad soldering could be a cause but it could also be a defective inductor L3 as the investigation in that video revealed.The thread on EEVblog #884 ishere.

author: John_ITIC
author_rank: Frequent Contributor
timestamp: May 28, 2016, 07:14:43 pm
content: I suppose this explains why one would pay more for a "quality" meter such as Fluke or Agilent. The designs may be similar but they have to cut corners somewhere and often this results in "cheap" parts sourcing, which translates to problems like this one.

author: jitter
author_rank: Frequent Contributor
timestamp: May 29, 2016, 05:16:23 am
content: That could be part of why you pay more, but I suspect that the design of Fluke or Keysight stuff might get a little more attention to detail. And let' s not forget that they will most likely also have a huge markup on the actual parts and labour value.

author: Lightages
author_rank: Unknown
timestamp: May 29, 2016, 12:20:08 pm
content: Every company has glitches, big prices or not, good reputation or not. You can't judge anything based on one or two samples. For example:https://www.eevblog.com/forum/testgear/agilent-u1272a-crappy-soldering/?nowap

author: Dr.Joe
author_rank: Contributor
timestamp: May 29, 2016, 02:40:20 pm
content: Quote from: retiredcaps on May 28, 2016, 03:14:01 pmI did see it 5mins after uploading.Guess what? My L3 Inductor was faulty too.Sorry, i currently don't have a macro camera, but on my Multimeter, L3 wasn't as far gone as the one from Daves Video. The solder connection to the lower pad (the one closer to the bottom of the meter) was broken, and therefore it was open circuit. i resoldered the joint, and voila, it works perfectly.thank you all for your suggestions.

author: TheAmmoniacal
author_rank: Unknown
timestamp: May 29, 2016, 02:43:37 pm
content: Quote from: Lightages on May 29, 2016, 12:20:08 pmThanks for reminding me of this thread, I never got around to open my U1272A!

author: retiredcaps
author_rank: Super Contributor
timestamp: May 29, 2016, 03:24:46 pm
content: Quote from: Dr.Joe on May 29, 2016, 02:40:20 pmSo my suggestion to check solder joints in an earlier post was applicable, but understandably difficult to do with that input board covering up some of the pcb.I'm glad your meter is working again!

author: Dr.Joe
author_rank: Contributor
timestamp: May 30, 2016, 08:52:22 pm
content: I guess i spoke too soon.DC Volt Range is now working, and as far as i can tell spot on. The continuity tester is working as well.however...The Ohms Range is not working, and displays O.L all the time. The continuity tester is working fine, but displays O.L when there is continuity. Also the Diode Check is not working. As far as i can tell, all the other functions are working fine.Edit: DC mV Range not working as well. Set up a quick voltage divider, (1M and 10k Resistor with 5V across both of them, probes across the 10k resistor). DC V Range shows 0.050V, DC mV Range shows O.LGiven that Daves Multimeter worked fine on the mV range before and after he fixed it, i can assume i have another failure. Going to pull that input board.Edit2: Forgive me if this question is stupid, but when measuring volts (and amps), the meter just has to measure the voltage that it sees across the input circuitry, and compare it to a reference. But for functions like measuring resistance, the diode check and the capacitance range, the meter does actually have to put a voltage across the component?

author: rstofer
author_rank: Super Contributor
timestamp: May 30, 2016, 11:51:30 pm
content: Quote from: Dr.Joe on May 30, 2016, 08:52:22 pmYes, the meter puts a small voltage across the component. It would need to be at least 0.7V to forward bias a diode and many meters can also forward bias an LED. I use the LED function to find the anode of SMD diodes before I place them.I measured the voltage on the BM235 Diode function as 3.25V and on the Ohms function as 1.7V.

author: retiredcaps
author_rank: Super Contributor
timestamp: May 30, 2016, 11:59:32 pm
content: Quote from: Dr.Joe on May 30, 2016, 08:52:22 pmIf you are going to desolder the input board, post clear focused pictures. In addition, please post pictures of the other side of the pcb. You might as well carefully remove the range switch and check there as well.

author: thisguy
author_rank: Contributor
timestamp: May 31, 2016, 12:10:00 am
content: Quote from: Dr.Joe on May 30, 2016, 08:52:22 pmIt might make sense to look really closely at L2 as well and check that it's not open. It seems very unlikely that a single meter would have two unrelated failures. As others have said and after examining my own bm235, I see how removing and reinserting the big fuse could put a lot of flex and stress on that part of the board.

author: Dr.Joe
author_rank: Contributor
timestamp: May 31, 2016, 12:35:04 am
content: @retiredcaps: Will do when my girlfriend returns from holiday with my camera. I'm afraid smartphone pictures aren't good enough@thisguy: both L2 and L3 are at 0 Ohms.R25 looks a bit weird and i can't get a solid measurement for it. Jumps around a lot, but that could be my cheapie DMM. I will try to take a picture.Edit: could the unstable measurement be because im measuring in circuit?I did take some smartphone pictures, they're better than no pictures. R32 (next to the big 10megohm resistor) looks a lot like R25, but measures &stable at 49.8kOhms. L10 looks weird, but is at 0 ohms.

author: BurningTantalum
author_rank: Frequent Contributor
timestamp: May 31, 2016, 12:33:05 pm
content: Why are you doing all this dismantling and repair work on a brand new product?BT

author: Kean
author_rank: Unknown
timestamp: May 31, 2016, 12:40:01 pm
content: Quote from: BurningTantalum on May 31, 2016, 12:33:05 pmHuh? Do you know where you are right now??

author: Dr.Joe
author_rank: Contributor
timestamp: May 31, 2016, 01:23:54 pm
content: Quote from: BurningTantalum on May 31, 2016, 12:33:05 pm1. Shipping costs (i'm in germany)2. If i send the Unit back for repair, first i have to go through a lenghty customs burocracy war so i dont have to pay a customs fee again when the repaired unit comes back.

author: rch
author_rank: Regular Contributor
timestamp: May 31, 2016, 03:23:56 pm
content: I bought mine at least partly to support Dave (although it is nice for once to own a meter I can safely check things in the mains distribution board with). In the exceedingly unlikely event it fails I won't rush to send it back, unless perhaps it's BER because of an intrinsic fault.

author: retiredcaps
author_rank: Super Contributor
timestamp: May 31, 2016, 03:46:44 pm
content: Quote from: rstofer on May 30, 2016, 11:51:30 pm@Dr.Joe, Are you getting these voltage readings from the BM235 using another DMM?In addition, does capacitance work at all say measuring a 1000uF capacitor?

author: retiredcaps
author_rank: Super Contributor
timestamp: May 31, 2016, 05:31:35 pm
content: @Dr.Joe. One more thing. Does temperature work?

author: retiredcaps
author_rank: Super Contributor
timestamp: May 31, 2016, 05:49:18 pm
content: Quote from: Dr.Joe on May 31, 2016, 12:35:04 amIn Dave's video, R25 is marked as 103 which is a 10k ohm resistor. If your measurement is not stable, it possible that other components are affecting the reading (like C25)

author: retiredcaps
author_rank: Super Contributor
timestamp: May 31, 2016, 05:56:08 pm
content: Quote from: Dr.Joe on May 31, 2016, 12:35:04 amWhat is the marking on R32?edit: Found some high resolution photos from Dave athttps://www.flickr.com/photos/eevblog/24180623623/sizes/o/R32 is 223, a 22k ohm resistor.

author: Dr.Joe
author_rank: Contributor
timestamp: May 31, 2016, 08:34:32 pm
content: @retiredcaps:Temperature does work, i can' test the other functions right now because i want to keep the soldering on this thing to a minimum.As far as i can tell, R32 and R25 have no markings on them. All the other resistors do.R32 is definitely not 22k on my meter.

author: BurningTantalum
author_rank: Frequent Contributor
timestamp: June 01, 2016, 08:18:32 am
content: OK Kean- Silly me... You got me there !Dr Joe- Now I see...BT

author: Dr.Joe
author_rank: Contributor
timestamp: June 01, 2016, 05:12:48 pm
content: Backside looks okay, so does the range switch.I think i will replace R25 and R32 with the values from Daves picture and Video.

author: retiredcaps
author_rank: Super Contributor
timestamp: June 01, 2016, 09:23:04 pm
content: Quote from: Dr.Joe on June 01, 2016, 05:12:48 pmBefore you change those resistors, what about those voltage measurements in post #25 and capacitance? The more we know what works and doesn't work will narrow down the problem.

author: Dr.Joe
author_rank: Contributor
timestamp: June 06, 2016, 07:08:35 am
content: Sorry, i was out of counry on a business trip.@retiredcaps: sure.Continuity works, and i can measure a stable 1.628V between the input jack. It just doesn't display a ohms valueTemperature works as well at a stable 0.908V.Capacitance doesnt work at all, when i put any capacitor across the probes, the meter switches to "----nF" and has 1.4mV across the probes.Diode Tester does not work, but has stable 3.104V between the probes.Edit: So far, i have only resoldered L3, all the other beads check out fine. R25 and R32 look a bit weird. i have not resolderd or reflowed those.Now Ohms: Doesn't work, and the weird part is: Voltage across the probes starts at around 60mV, then goes up to about 350mV, then my cheap DMM (4000 Count i presume?) autoranges, and i get the reading of about 1,47V for a fraction of a second, before it starts at 60mV again. this whole cycle takes about a second.Edit: A/mA DC range also not working. Shows O.L on both ranges with about 100mA going through the meter.

author: PedroDaGr8
author_rank: Super Contributor
timestamp: June 06, 2016, 12:37:46 pm
content: Forgive me if I am stepping on toes but is Dave not taking care of this? Are you just trying to fix it for fun? Not implying that Dave is intending to avoid his duties, just curious on why all the work when an RMA might be more appropriate.

author: Dr.Joe
author_rank: Contributor
timestamp: June 06, 2016, 02:05:09 pm
content: Dave has been well aware of this, in fact I contacted him before I opened this thread.if this was a 5 minute fix like the other one in his video, it would have been stupid to send it back to Australia, let Dave repair it and let him pay for return shipping. Also, i don't want to pay custom twice for the same product. avoiding that is a bit of a headache with german customs.

author: jitter
author_rank: Frequent Contributor
timestamp: June 06, 2016, 03:11:57 pm
content: Quote from: PedroDaGr8 on June 06, 2016, 12:37:46 pmPerhaps the "Waranty void when seal broken"-seal is broken?Edit:Quote from: Dr.Joe on June 06, 2016, 02:05:09 pmGood reasons and good to know it's with permission...

author: retiredcaps
author_rank: Super Contributor
timestamp: June 06, 2016, 03:28:34 pm
content: Quote from: Dr.Joe on June 06, 2016, 07:08:35 amHmm, the results above, in addition, with all the other data so far does not bode well. I'm thinking something is wrong with the main IC, but I still may be missing something? All your input protection checks out okay.Let me think this for a bit longer.PS. Wrt to your cheap DMM, put it into manual range mode (4V) and then it shouldn't have to autorange and might work faster to get a more accurate reading. Maybe retest with it in manual range?

author: retiredcaps
author_rank: Super Contributor
timestamp: June 10, 2016, 05:46:41 pm
content: Right now, I don't have any more ideas, but I will re-read and re-analyze all the data again. I might have missed something really obvious?If there are no more suggestions from members, you may need to contact Dave and get a replacement meter. You may want to wait until Sunday night to make a final decision. Maybe during the weekend, some other members can offer help or have more time to look at this?I don't know how Dave handles the return costs and shipping charges. Good luck with customs (again). I know it can be frustrating.It is funny how it can pass calibration, but fail immediately after being shipped. I mean, some of the functions don't even work at all like current.

author: ez24
author_rank: Super Contributor
timestamp: June 10, 2016, 08:31:28 pm
content: Quote from: retiredcaps on June 10, 2016, 05:46:41 pmIt is a physical problem - right? So being shipped will break something lose. Not funny but physics, right? What I wonder is what does future mold changes mean. Does this mean when the current molds wear out?

author: imidis
author_rank: Frequent Contributor
timestamp: June 10, 2016, 08:38:49 pm
content: looking at the pics, how does the solder join on Q9 look? I notice it's a little crooked and looks like the solder is a little sparse.

author: retiredcaps
author_rank: Super Contributor
timestamp: June 10, 2016, 09:29:55 pm
content: Quote from: ez24 on June 10, 2016, 08:31:28 pmI don't know if it is phyiscal or not. OP has already resoldered L3. There could be another dodgy soldered component? Micro crack on the pcb?OP says DCV, continuity and temperature work fine, but ohms, capacitance and current does not. While continuity works, OP says there is no reading on the screen, just 0L.

author: retiredcaps
author_rank: Super Contributor
timestamp: June 10, 2016, 09:41:18 pm
content: Quote from: ez24 on June 10, 2016, 08:31:28 pmOn a lighter and completely true note, I have no idea how I passed that year long course in University.

author: kolbep
author_rank: Frequent Contributor
timestamp: June 10, 2016, 09:54:59 pm
content: Maybe that inductor that you resoldered has some damage inside still.Just bridge across it to see if that does anything.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 11, 2016, 09:32:48 am
content: + 1 to bridging L3 for a test. Form Daves repair thread is seem there is a mechanical weakness in the area of L3 and L2. So it's likely not bad soldering, but just the mechanical load that causes the solder to fail. It's well possible the part itself has a micro-crack and soldering did not really fix it.For passing calibration, the damage might well occur during shipping, from vibration or temperature.It's rather unlikely, but the broken part could have defeated some input protection, so that there is a slight chance to have a follow up error, maybe from ESD. A partial reverse engineering of that area might exclude this.