BM235 Quiet Beeper

[fishymamba]

Hello everyone! SO I got the BM235 about a year ago and am really liking it compared to my old cheap meter. But I've been having a problem with the beeper being super quiet.

When I first got the meter, the beeper was super loud, but after a couple months it suddenly got super quiet. So quiet that I could barely hear it without holding it up to my ear. Then a couple days ago I got it out to use it and it was back to original volume, but today its back to being super quiet.

So my question is: Is there some kind of volume adjustment in the settings that I have accidentally changed? Or is it something else? I have checked the manual and it doesn't mention anything and I haven't found anything by searching the forum(sorry if I missed something). The batteries are still at 1.4v+ so that's not the issue. The solder joints for the resistors inline look OK, they could maybe use a reflow. But I thought I would ask here before attempting to reflow and doing some damage.

[bitter_mike]

I got mine a couple months ago and I have a similar problem with the quiet buzzer. Not sure what it is either.

[Compulsive Fixer]

Likewise, I got my BM235 from the EEVblog Store in November 2020 and, while doing some continuity tests just yesterday, the buzzer suddenly lost it's voice. I thought it had stopped altogether until I read some other posts on it going very quiet and I found I could barely hear it if I put my ear up close.
After carrying out some checks I found it to be faulty on all functions where it should be working.
I have yet to test the battery but the fuses and leads test OK
My dilemma now is whether I should sent it off under warranty for such a minor fault or just have a go at replacing the piezo.
I would appreciate hearing what others have done with this problem.
Trev

[james_s]

Put a piece of tape over the hole in the beeper. I've done that many times in various devices and it makes them much quieter.

[pjwum]

Have you checked if there is still a driver signal? Usually a buzzer needs a PWM-signal at around 3 Volts.

[james_s]

Oh I misread that, I thought the beeper was too loud, not too quiet.

You could try temporarily connecting a different piezo across the existing one.

[Compulsive Fixer]

I just replaced the batteries with fresh ones and still no improvement.
Interesting to note that the ones it came fitted with were standard carbon/zincs and not alkaline as alluded to in the manual.

Have you checked if there is still a driver signal? Usually a buzzer needs a PWM-signal at around 3 Volts.

I guess it wouldn't hurt to connect the scope across it to see what gives although I am resisting delving into the innards for now since it is still within the 12mth warranty.
Somewhere in another post I read that a replacement piezo successfully restored the sound so I would be inclined to suspect a connection fault to the piezo disk inside the buzzer.

@james_s
I was just about to comment on that when your reply came through as I was typing my response.

[Compulsive Fixer]

I found the topic I mentioned above -" EEVblog BM235 Not Beeping Anymore " (sorry, don't know how to link to this!)

He posted a photo of the dis-assembled Buzzer in post #16

It appears that electrical contact is made to the piezo disk with a couple of flat spring contacts which seems a pretty unreliable way of doing it. Loss of spring tension or contamination at the contact point could easily cause the low/no volume fault. I am tempted to give mine a 'technical tap' with the handle of a small screwdriver to see if this fixes it!

BTW, the replacement used looks a bit sus. Possibly an active piezo buzzer or a small speaker, hence the reason it sounded strange.

[Compulsive Fixer]

Sorry, that should have been post #7

[pjwum]

I guess it wouldn't hurt to connect the scope across it to see what gives although I am resisting delving into the innards for now since it is still within the 12mth warranty.

I would suspect a defective buzzer. You could easily replace it yourself and be the proud owner of your self-restored meter. Probing if there is a driver signal will make sure that the buzzer is the problem. If there is no signal (or questionable PWM waveform) I would send it in.

Also worth watching:
https://youtu.be/cvOpBCs3aJg

Cheers
Patrick

[pjwum]

Out of curiosity I just opened my BM829s and probed the buzzer...

[robert.rozee]

I found the topic I mentioned above -" EEVblog BM235 Not Beeping Anymore " (sorry, don't know how to link to this!)

He posted a photo of the dis-assembled Buzzer in post #16

It appears that electrical contact is made to the piezo disk with a couple of flat spring contacts which seems a pretty unreliable way of doing it.

https://www.eevblog.com/forum/testgear/eevblog-bm235-not-beeping-anymore/msg1000299/#msg1000299

over time, as the disk vibrates in and out, the end of the (centre) sprung contact will rub back and forth against it and over time wear through the silvered coating directly underneath. when the coating is worn through, the remaining contact area onto the piezo crystal material will be miniscule, resulting in severely reduced volume.

the 'quick fix' is to open up the device and move the centre contact slightly sideways so it touches a new area of the silvering. but it will wear through again in future. a better solution would be to bend the end of the contact back (ie, reverse the direction of the existing bend) so that a larger flat area of the metal makes contact with the silvering, rather than just the point at the end. the best solution would be to also add a small piece of thin conductive rubber between the contact and the silvering, so that the interface between the two is even larger and cushioned - this is effectively what is used in the fluke 77 meter.

you can carefully solder a wire onto the silvering, but need to use low-temperature solder to prevent destroying the silvering. the wire also needs to be very fine multi-stranded, otherwise it will just fracture at the piezo end and break at some later time.

btw: many years ago, i spent several months at work analysing just such a piezo sounder that was failing in a product. as i recall, no sensible solution could be found, and we ended up replacing it with a small low-impedance sounder that used a coil + magnet and steel diaphram!


cheer,
rob   :-)

[Compulsive Fixer]

Thank you both for the above replies with some very useful information.

I feel confident that Robert's explanation is most likely to be the cause of the fault and, if this is the case, there could be a lot more of these meters showing this problem in the future unless they change the design of the sounder.

I had a chuckle to myself over the irony of this whole saga as I bought the Brymen as a replacement for my HP973A which died due to battery leakage after many years of reliable service. In due course I managed to get the HP working and it is still performing well but it's replacement is giving problems. Not only that, the Brymen started playing up while I was circuit tracing a piezo driver board!!!
So, for now I am a little over repairing multi-meters and hence the indecision over whether to send it back as a warranty claim or fix it myself.
At least it is good to have some idea of what the fault is.

[Compulsive Fixer]

Following Rob's advice, I opened up the meter and, using a tiny screw driver, managed to pop the cap off the piezo without having to unsolder it from the board. I then pushed the piezo disk out of the plastic cap through the hole in the top and gave it a thorough clean with a tissue. For good measure I also bent the two spring contacts so they put a little more pressure on the disk. Once I refitted the disk to the cap it was a simple matter to snap it back onto the plastic base and re assemble the meter.
Happily, the piezo buzzer is now working as it should although the volume is a little low now but back to what it probably was originally. At least it is usable.
Thanks for your sound advice Rob!

[Zero999]

Out of curiosity I just opened my BM829s and probed the buzzer...

Interesting. The piezo is driven with 4.34V, yet the meter is run off two AA cells, giving 3V. I wonder if there's a boost converter to give a 5V supply and some of the voltage is being dropped by the driver transistors?

As this seems like a common fault, I think people should be returning it under warranty, rather than reparing it, if possible. I know it's a pain, for such a simple fault, but if the manufacturer doesn't know about it, they'll never fix it!

[BeBuLamar]

Why don't people return it for warranty? If people don't do that then Brymen doesn't think there is anything wrong with their design. I am in the USA and I think I won't have warranty if I buy a Brymen.

[Zero999]

Why don't people return it for warranty? If people don't do that then Brymen doesn't think there is anything wrong with their design. I am in the USA and I think I won't have warranty if I buy a Brymen.

Because it's much easier and quicker to fix it?

I agree though, as I said, even though it may be easier to fix it youself, you should always return it, otherwise they'll never know there's a problem!

[james_s]

Generally speaking I can't be bothered to return stuff for warranty service, usually shipping is on me, and there is the risk of it getting lost or damaged, and I'll be without it for potentially several weeks or more. If it's something I can fix myself I'd rather fix it myself, and contact the company to inform them of the problem.

[Zero999]

Generally speaking I can't be bothered to return stuff for warranty service, usually shipping is on me

Are you sure that's the case? Normally, if it's faulty, the seller has to pay for the return shipping. They should send you a label and because they're paying for it, they take the risk of it getting lost. If this isn't the case, the consumer protection laws in your state are crap.

[james_s]

Generally speaking I can't be bothered to return stuff for warranty service, usually shipping is on me

Are you sure that's the case? Normally, if it's faulty, the seller has to pay for the return shipping. They should send you a label and because they're paying for it, they take the risk of it getting lost. If this isn't the case, the consumer protection laws in your state are crap.

If it's faulty out of the box then yes, many sellers will cover return shipping although nothing legally compels them to, unless you bought it from China direct in which case you're usually SOL unless it's a reputable seller that will just refund you. The consumer protection laws here are crap though, as in they don't really exist at all, it's just never been a thing here, in the whole nation. The warranty is determined by the manufacture and/or seller and if you don't like the terms you can refuse to buy.

[ebastler]

Thanks for your sound advice Rob!

Whether that pun was intended or not, it made me smile. Thanks!

[Compulsive Fixer]

 

[lukas2511]

My BM235 suddenly got very quiet today, mid-use.

I first thought about disassembling it to replace the buzzer but that seemed way to complicated...
Since the buzzer was broken anyway and would have had to be replaced I thought about opening it first to have a look inside.

There is a small gap at the side that allows to easily remove the buzzers top cover with a small flat-head screwdriver or a knife.
Inside is the actual piezo.. there was an easily visible layer of some kind of dirt on it.
The dirt was covering parts of the crystal and was thicker in some regions (unfortunately I didn't take any photos).
Using some paper towels and a bit of rubbing alcohol it was very easy to remove.
Afterwards I placed the piezo back into it's housing (smooth side up) and closed the meter.
Before closing the buzzer I also bend the springs a bit up for better pressure (probably not necessary).

After closing everything back up the piezo now sounds as loud as it probably should.

I guess this is some kind of manufacturing defect and the buzzer just moved slightly and didn't make good contact anymore. I'd guess that other meters have a similar issue. So before taking out the soldering iron maybe check if my solution also works for you