Meterman 34XR Multimeter Fault: Beeper always on. - Page 1
Forum section: electronics / testgear

author: MsJaye
author_rank: Contributor
timestamp: February 18, 2014, 06:30:53 am
content: Hello there,I've got a Meterman 34XR Multimeter which is misbehaving, and which I'd much rather fix than replace.As soon as the unit is powered on, the beeper starts sounding, and stays on constantly. By putting the unit in its rubber holster and banging the lead end (rather than the display end) of the unit I'm able to make the beeping change pitch, or occasionally shut off completely. If the beeping does shut off, it comes back on constantly after the next beep it makes.I've opened the unit and had a poke around: nothing obviously blown or broken. All other functions of the meter seem to be in perfect working order (as far as I can tell).Anyone experienced a similar failure mode?Anyone got any debugging tips for such a fault?Thanks,Jaye.

author: rexxar
author_rank: Frequent Contributor
timestamp: February 18, 2014, 06:54:16 am
content: Start by following the signal to the buzzer back to the main IC. Hopefully there's a transistor in the signal path that's died. If the signal just comes straight from the IC, you're probably out of luck.Though, if banging it makes it stop, it could be a bad solder joint. Can you post some pictures of the board?

author: MsJaye
author_rank: Contributor
timestamp: February 18, 2014, 08:14:26 am
content: Sure can: have attached photos.It looks like the buzzer goes into a 4011. I've traced out the circuit comprising the 4011 and bits around, see attached. Most of the action seems to be on the back side of the PCB.

author: MsJaye
author_rank: Contributor
timestamp: February 18, 2014, 08:15:18 am
content: Top side PCB images.

author: Wytnucls
author_rank: Super Contributor
timestamp: February 18, 2014, 08:35:46 am
content: It looks like the 2 kHz buzzer output signal comes from pin 74 of the Cyrustek ES51960, which, I think, is the trace going through C29 on the silkscreen, if that's any help.

author: MsJaye
author_rank: Contributor
timestamp: February 19, 2014, 12:59:28 am
content: Yep, that did help. The brand name and correct model number of the main IC lead me to the datasheet. It seemed pretty clear that all of the circuitry between BUZOUT and the actual buzzer was some sort of driver for the buzzer that takes the 2kHz when active signal from BUZOUT and turns it into a signal that will drive the buzzer at screeching volume. I disconnected the buzzer, and reconnected it directly between BUZOUT and V-, as per the datasheet.Success! The buzzer is much quieter, but now functions correctly. I could trace out the rest of the circuit from BUZOUT through a couple of caps, resistors, a transistor and a diode, which leads down to R127 and try to figure out what's gone wrong in the driver circuit, or I could just take it as it is with a quieter buzzer.

author: MsJaye
author_rank: Contributor
timestamp: February 19, 2014, 01:00:15 am
content: (Kind of unsatisfying to not fix it completely... Hm...)

author: rexxar
author_rank: Frequent Contributor
timestamp: February 20, 2014, 01:28:44 am
content: Quote from: MsJaye on February 19, 2014, 01:00:15 amYou could always just stick a new opamp in there.