eagle1109
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Hi,
I agree with Barry.
Your DMM buzzer sends out a DC signal (plus and minus) it checks current flow.
If there is enough current flow, then the bzzer goes ON.
But the threshold is not defined.
With a PC power supply - when flyback type - the 12V output consists at least:
* the secondary winding of the transformer
* the (catch) diode
* the capacitor.
Your testing:
* The secondary winding is very low ohmic. It surely will cause your buzzer to sound
* The diode is low ohmic in one direction, high ohmic in the other direction. It is very likely that with an unconnected diode your buzzer sounds when you connect your DMM in one direction, but it will be quiet in the other direction.
* The capacitor when connected in the correct direction is low ohmic first ... then it becomes charged and thus the current decreases. It is very likely that you hear a short buzz immediately when you connect the DMM, but it will stop buzzing automatically. Connecting the capacitor in the wrong direction will cause the buzzer to sound, but maybe it sounds continously or it will stop.
Now all three components (and maybe others) are connected...this makes it less predictable if the buzzer sounds or not.
Klaus
What's that big blob of solder in the upper left corner? Actually, there are two. That doesn't look right. Neither does that big blog in the center of the board. Maybe those shorts don't matter, but it sure indicates terrible manufacturing.
I could be wrong but I'm very suspicious that so many things seems shorted, especially when checking with a buzzer.
The big rectifier diodes could well be dual diodes with both anodes/cathodes going to windings on the transformer so they would appear to be shorted in-circuit but are probably OK if removed.
Low voltage side faults are quite rare on PC PSUs with the exception of dried up electrolytic capacitors but they usually fail open circuit not shorted. It is also fairly rare to take feedback from the 12V line to regulate the input stage, usually it is done from the 5V or 3.3V outputs which are more critical. That only leaves the rectifiers and maybe two capacitors (one each side of the choke) as possible culprits.
Brian.
That's not what I meant. The 12V rectifier is often in a TO-220 package but has TWO diodes inside it. The outside pins go to a center tapped transformer winding and the rectified output comes from the middle pin. If you measure across it you will probably see the resistance of the transformer, not the diodes.The rectifier bridge is OK and doesn't have any shorts, I have a big assumption that the HIGH voltage side is OK.
Hi,
The big yellow taped device should be the power transformer.
I assume what you named "o23N04" are the diodes.
But I'm not sure. Difficult to recognize..
Klaus
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