BM235 defective?

[Dr.Joe]

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: 12.3V
BM235: 10.06V  If i reverse the probes, it only displays 9.06V. 

5V:
cheapie Voltcraft VC160: 4.88V
BM235: 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 polarity
BM235 in VDC Mode:: 7.17V correct polarity, -6.30V reverse polarity
BM235 in AutoV LoZ Mode:: 7.6V  no matter which polarity

and then i gets weird: When i try to measure an AAA 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,
Johannes

Edit: i did change the batteries, but that didn't solve it.

[thisguy]

Maybe obvious, but you didn't mention whether you put fresh batteries in it. Did you? Edit: (Oh, I see now that you did)

[retiredcaps]

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.

[Dr.Joe]

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: ~1kOhm
PTC1: 1.6kOhm
PTC2: .94kOhm

Alls the 3 blue MOVS measure infinite resistance.

The black long resistor on the Mainboard measures ~10MOhm.

[retiredcaps]

R7 and the one directly above it: ~1kOhm
PTC1: 1.6kOhm
PTC2: .94kOhm

Alls the 3 blue MOVS measure infinite resistance.

The black long resistor on the Mainboard measures ~10MOhm.

All those measurements above look okay.

Can you also verify those pink resistors (5M ohm) in Dave's picture are both reading 5M ohm?

[Dr.Joe]

Can you also verify those pink resistors (5M ohm) in Dave's picture are both reading 5M ohm?

5.04/5.03 MOhm

[EEVblog]

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.

[timgiles]

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.

[retiredcaps]

@Dr.Joe

See eevblog 884



Check for bad soldering around the same area (underneath the board).

[jitter]

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 is here.

[John_ITIC]

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.

[jitter]

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.

[Lightages]

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

[Dr.Joe]

@Dr.Joe

See eevblog 884

Check for bad soldering around the same area (underneath the board).

I 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.

[TheAmmoniacal]

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

Thanks for reminding me of this thread, I never got around to open my U1272A!

[retiredcaps]

The solder connection to the lower pad (the one closer to the bottom of the meter) was broken, and therefore it was open circuit.

So 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! 

[Dr.Joe]

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.L
Given 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?

[rstofer]

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?

Yes, 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.

[retiredcaps]

Going to pull that input board.

If 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.

[thisguy]

I guess i spoke too soon.

It 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.

[Dr.Joe]

@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.

[BurningTantalum]

Why are you doing all this dismantling and repair work on a brand new product?
BT

[Kean]

Why are you doing all this dismantling and repair work on a brand new product?
BT

Huh?  Do you know where you are right now??   

[Dr.Joe]

Why are you doing all this dismantling and repair work on a brand new product?
BT

1. 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.

[rch]

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.