power supply buzz troubleshooting

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jcharles00

Well-known member
Joined
Dec 28, 2005
Messages
51
Location
Indiana
I recently bought a linear power supply from ebay and once I connected AC mains to it (for 110V operation) found it to make a fairly loud, audible buzzing noise similar to the one in this post about similar PSU problems: http://www.groupdiy.com/index.php?topic=26730.0

In that thread the solution was a bad cap in a timer delay circuit.. I'm not sure what such a circuit would look like. can anyone point me in the right direction?

This is the schematic:
3102073253_c49a39663a_b.jpg
 
I didn't know you could still GET the '723. (or is this vintage salvage?)

Disconnect the transformer from the rest of the system. Does the transformer alone buzz?

Is anything getting hot? Are the voltages right?
 
First are you exceeding the rated current of the power supply?   You don't have anything about circuit load or supply specs.

Look at C2, that is part of the voltage adder to allow the use of an cheaper NPN power transistor.  A "better" design would use a PNP and not need the base drive to be above the base to emitter drop of an NPN.

I have repaired a lot of linear supplies based on the 723 over the last 25 years and that cap is often the cause of ripple as the current draw goes up you lose drive to the base of the NPN pass devices

Or just change C2s and report back.

EDIT I just reread the first post is the noise without the power supply connect to a circuit?
 
Finally had a minute to play with this again, and found that disconnecting the rest of the supply from the transformer still left it buzzing. So I guess the transformer is bad? (FWIW, I never put a load on this unit)

The seller is going to send me a replacement and said I could keep the current one. Maybe I'll happen into an appropriate transformer at some point down the road.

Thanks for the troubleshooting advice!




as a side curiosity, what might the physical cause of this be? loose wraps on the transformer vibrating from the AC? might pouring some epoxy or similar over it help if thats the case?

 

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