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Try to fix Electric Door bell

pharaon

Oct 28, 2014
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i'm having this electric door bell that make bird sound
it's not working the transistor is C945
i think the big resistor is burned, but i can't determine it's values since the middle color changed due to overheated







 

kellys_eye

Jun 25, 2010
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Resistors don't generally burn out without good reason so perhaps there's another issue elsewhere? Either way I'd make a guess at it being 27 ohms (a standard value) - it looks like it's a simple dropper resistor - maybe 270 ohms. Can you show how the board is otherwise connected i.e. power source (what is it, where is it from etc)
 

Minder

Apr 24, 2015
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There must be a transducer of some kind (not shown) what is the nature of the one you have?
 

pharaon

Oct 28, 2014
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Resistors don't generally burn out without good reason so perhaps there's another issue elsewhere? Either way I'd make a guess at it being 27 ohms (a standard value) - it looks like it's a simple dropper resistor - maybe 270 ohms. Can you show how the board is otherwise connected i.e. power source (what is it, where is it from etc)




first resistor color from left is red then purple
3rd i don't know and the fourth i don't know
 

Minder

Apr 24, 2015
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Red Violet is 27, if the third is brown = 270ohm, gold = x0.1, silver x.01
Measure it if you have a meter.
You don't show the transducer.
 

davenn

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Sep 5, 2009
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what transducer? that's the whole circuit.

it cant be, as Minder said ... there needs to be something that makes the sound, speaker etc

so there are 4 wires coming off the board ... 2 red and 2 blue
where does each pair go to ??
 

pharaon

Oct 28, 2014
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it cant be, as Minder said ... there needs to be something that makes the sound, speaker etc

so there are 4 wires coming off the board ... 2 red and 2 blue
where does each pair go to ??
oh yes the blue are conected to speaker and the two red are to AC 220 V
 

Minder

Apr 24, 2015
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Have you checked the LV output of the transformer, some have a thermal fuse embedded in the windings that sometimes opens.
Is the Spkr OK?
 

pharaon

Oct 28, 2014
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Have you checked the LV output of the transformer, some have a thermal fuse embedded in the windings that sometimes opens.
shouldn't i first know the resistor value to replace it so i can test that?
 

Minder

Apr 24, 2015
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Have you checked the value?
It is a high wattage resistor so it would get warm and maybe discolour.
I would also do the check on the transformer.
 

davenn

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shouldn't i first know the resistor value to replace it so i can test that?

in time, as someone else said earlier, there's a good possibility that the resistor overheated because of some other problem
If this is true and you dont find that other problem, then putting in a new resistor may cause its demise

Do the things suggested first and then sort out the resistor
 

pharaon

Oct 28, 2014
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Have you checked the LV output of the transformer, some have a thermal fuse embedded in the windings that sometimes opens.
Is the Spkr OK?
can i check it with blown resistor ?
 

pharaon

Oct 28, 2014
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Yes, should be possible, like I said, it should be simple to trace the circuit and document it.
what current should be at the speakers leads ? AC or DC
since the speaker is connected to the secondary transformer
 

Minder

Apr 24, 2015
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It would be an AC signal of some kind.
It must generate some kind of higher frequency if it produces a bird chirp etc?
 
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