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Old 09-06-2019, 08:24 AM   #1
tmcw
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Default Multimeter Repair - continuity buzzer (RAPID 328 DMM)

I have one of these cheap multimeters, and when set to continuity/diode setting, and the probes touched together, the 20mm piezo makes a scratchy, muffled, weak noise, barely noticeable. The display does show the response, going from "1" to "0.00".

The piezo is ok, I transplanted it to a similar multimeter, and it makes a nice loud tone. I tried a couple of smaller piezos, the type pulled off old motherboards (12mm type), and it makes the same scratchy, muffled noise.

However, I did find a bigger piezo (about 22mm diamter), that I think was pulled off a UPS board, and when I try with this in place of the original piezo, it makes a nice loud noise. But whereas the other piezos are just a piezo, this one that works has some extra circuitry built in (picture 3).

So it looks like there is something wrong with the electronics going to the original piezo on the DMM board. Will the multimeter have a similar circuit to the 22mm piezo somewhere?

Is it possible to get a smaller piezo with a circuit inside it like the bigger 22mm piezo? I couldn't get the bigger piezo to fit the case, but a smaller 12mm piezo with a circuit built in could fit in one of the corners of the multimeter case, if there is such a thing.

Are there any other viable options?
Attached Images
File Type: jpg dmm_01.jpg (162.4 KB, 14 views)
File Type: jpg dmm_02.jpg (196.2 KB, 23 views)
File Type: jpg dmm_03.jpg (262.2 KB, 21 views)
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Old 09-06-2019, 10:08 AM   #2
stj
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Default Re: Multimeter Repair - continuity buzzer (RAPID 328 DMM)

your problem is unlikely the piezo,
show the other side of the board.
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Old 09-07-2019, 09:46 AM   #3
tmcw
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Default Re: Multimeter Repair - continuity buzzer (RAPID 328 DMM)

Yeah, those piezos would probably survive a bombing, and it does test ok in another multimeter.

Attached are a couple of photos of the other side. The center of the piezo (red wire in photos in first post) is connected to the small pad above component Q4.
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File Type: jpg dmm_04.jpg (562.3 KB, 18 views)
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Old 09-07-2019, 01:34 PM   #4
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Default Re: Multimeter Repair - continuity buzzer (RAPID 328 DMM)

Some piezo drive circuits are done wrong.
A piezo is a capacitive load and needs push-pull (source and sink) current to move. If you drive one with an open-collector transistor Q4 (charge but no discharge path), the piezo will be way quiet.
The fix is putting a resistor like 330R-1k in parallel (with disk) so the piezo's capacitance gets a way to discharge. I would try that.

Changing to a piezo buzzer (built-in oscillator) is a last resort.

Last edited by redwire; 09-07-2019 at 01:35 PM..
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Old 09-07-2019, 04:24 PM   #5
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Default Re: Multimeter Repair - continuity buzzer (RAPID 328 DMM)

well you could trace the drive circuit before the transistor,
but to be honest it's such a low spec mastech meter that you may as well get an ANENG 8009 for 10 or something.
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Old 09-13-2019, 08:30 PM   #6
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Default Re: Multimeter Repair - continuity buzzer (RAPID 328 DMM)

I think scratchy piezo sound is typically caused by either:
a) low/dying battery
b) dirty contacts in the rotary switch

... at least that was the case for all of my multimeters.
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