Fuse Finder.

moving violently back and forth sounds a bit worrying when it used to work was the audio tone pulsed or continuous.? From what you say the voltage is pulsing now.


In any case, it sounds like the electronics on the board is chopping up a DC supply of about 5V to create a square wave to drive the buzzer.

This is in effect rather like a DC of half the supply voltage on average,  but with an AC superimposed that takes the total down to zero on the negative half cycles, and up to the full supply on the positive ones.

Be aware that some older moving coil meter AC ranges are not AC coupled but are just a DC range with a diode in series, so they will still read on  a DC input, but rather over estimate, as the scale is compensated for the AC case when curretn ony flows in the meter movement for half the time.  The clue to DC present on such a meter is that the 'AC' reading is not the same if you reverse the polarity of the meter probes.


So you may not need the type of sounder that takes a DC input, as if the board is already generating AC, then a simple transducer will do, with no electronics in. 

 example of bare piezo electric crystal


 made up units with electroncis inside


Connecting a pair of walkman style headphones via some limiting resistance ( - which could be your moving coil meter - it is after all a box of switched resistors in series with a meter mechanism) will allow you to be totally sure if the audio frequency AC is created on the PCB. Equally I think it will do no harm to fit a cheap DC sounder and see what it does. Or can you open the broken beeper and see if there is electronics inside or just the bare disc of crystal that changes shape with applied voltages.

moving violently back and forth sounds a bit worrying when it used to work was the audio tone pulsed or continuous.? From what you say the voltage is pulsing now.


In any case, it sounds like the electronics on the board is chopping up a DC supply of about 5V to create a square wave to drive the buzzer.

This is in effect rather like a DC of half the supply voltage on average,  but with an AC superimposed that takes the total down to zero on the negative half cycles, and up to the full supply on the positive ones.

Be aware that some older moving coil meter AC ranges are not AC coupled but are just a DC range with a diode in series, so they will still read on  a DC input, but rather over estimate, as the scale is compensated for the AC case when curretn ony flows in the meter movement for half the time.  The clue to DC present on such a meter is that the 'AC' reading is not the same if you reverse the polarity of the meter probes.


So you may not need the type of sounder that takes a DC input, as if the board is already generating AC, then a simple transducer will do, with no electronics in. 

 example of bare piezo electric crystal


 made up units with electroncis inside


Connecting a pair of walkman style headphones via some limiting resistance ( - which could be your moving coil meter - it is after all a box of switched resistors in series with a meter mechanism) will allow you to be totally sure if the audio frequency AC is created on the PCB. Equally I think it will do no harm to fit a cheap DC sounder and see what it does. Or can you open the broken beeper and see if there is electronics inside or just the bare disc of crystal that changes shape with applied voltages.