Select-able wall wart power supply - dead

[ajsemtb]

hello, this is probably simple, but I searched and didn't find a thread with this specific repair, so I thought if nothing else, I'll document mine.

This is a variable voltage output wall wart power supply, with reversible polarity. It was mainly used for a power supply for audio pedal boards. It has died. This could go under audio, I suppose, but because it is a universal power supply, I thought this section to be better.

This was my friends, and the cord looks like a candy cane its so twisted... Maybe there's a short there somewhere.

When I plug it in, there is no voltage to any of the optional tips.

I opened it up (after fabricating a split flat screw driver tip with a dremel tool and an old flat screw driver bit) and it looks pretty simple. A transformer, 4 diodes, 1 capacitor 25v 1000uF , and 2 resistors. A switch for dc voltage output and a switch for +- polarity reversal.

I have no idea whats wrong with it. Diodes in circuit read 460v 459v 464 and 464v respectively. reverse reads 598 598 602 and 602v respectively.

Caps not bulged. ESR meter won't be here until next week.

Any thoughts on where to begin on this would be appreciated. I don't know how to test the transformer...

[retiredcaps]

Re: Select-able wall wart power supply - dead

Diodes in circuit read 460v 459v 464 and 464v respectively. reverse reads 598 598 602 and 602v respectively.

Minor nit. Those readings are millivolts or 0.460V.

All those readings suggest all 4 diodes are shorted. A good diode should read between 0.3 and 0.7V one way and 0L (out of range) the other way.

Desolder all 4 and remeasure out of circuit to verify.

[Joun]

Re: Select-able wall wart power supply - dead

But if they were shorted they should read 0 in both direction,right?
Also 4 shorted diodes at once is unsual..

Caps not bulged. ESR meter won't be here until next week.

Any thoughts on where to begin on this would be appreciated. I don't know how to test the transformer...

You don't have to check ESR on that cap.That kind of power supply doesn't stress much caps and it would show you some reading on voltmeter even without any cap at all..
My bet is that transformer primary is open..You can put multimeter on Ohms and check if its resistance is below 500 ohms or ssomething similar.

[vinceroger69]

Re: Select-able wall wart power supply - dead

i would unsolder one leg of each diode and re test them

[Evil Lurker]

Re: Select-able wall wart power supply - dead

You can also put a 60w 120v light bulb in series with the transformer.. if its shorted the bulb should light up.

[Joun]

Re: Select-able wall wart power supply - dead

It is difficult if not impossible for the transformer (or any kind of coil) to be shorted..
How can a piece of wire "jump" and short phase with neutral? It is possible for two or three turns of the winding to short but that wouldn't produce enough current to light any bulb in series..

[Shocker]

Re: Select-able wall wart power supply - dead

I wouldn't trust that capacitor. Even with this being a linear power supply.

[Evil Lurker]

Re: Select-able wall wart power supply - dead

It is difficult if not impossible for the transformer (or any kind of coil) to be shorted..
How can a piece of wire "jump" and short phase with neutral? It is possible for two or three turns of the winding to short but that wouldn't produce enough current to light any bulb in series..

Actually it is quite possible. All it takes is two microscopic damaged areas of the magnet wire's insulation in close enough proximity to each other to create an arc. The heat from the arc melts other winding's insulation creating a chain reaction.

[Evil Lurker]

Re: Select-able wall wart power supply - dead

I wouldn't trust that capacitor. Even with this being a linear power supply.

I wouldn't trust that PSU either. I would at least have a linear voltage regulator on the output, ultrafast rectifier diodes (supposedly they do improve sound quality), and full compliment of x-y caps and common mode choke on the input for no other reason to keep line noise out.

[rogfanther]

Re: Select-able wall wart power supply - dead

Ajsemtb,

Disconnect the diodes. Then measure the AC output of the transformer. The series lamp is a good trick to prevent problems if there is a short.
If you get AC voltages from the transformer, your problems lie in the rectifier circuit, or even in the switch. Voltages probably would be 2V or 3V bigger than the voltages one can select from the switch.

[Joun]

Re: Select-able wall wart power supply - dead

Actually it is quite possible. All it takes is two microscopic damaged areas of the magnet wire's insulation in close enough proximity to each other to create an arc. The heat from the arc melts other winding's insulation creating a chain reaction.

What you say it is quite possible,but:all that ends with a wire melted because too much curent flown from it and so it is open, not shorted..

[Evil Lurker]

Re: Select-able wall wart power supply - dead

What you say it is quite possible,but:all that ends with a wire melted because too much curent flown from it and so it is open, not shorted..

[ben7]

Re: Select-able wall wart power supply - dead

Check the resistance of the primary winding of the transformer, if it is O/C then the transformer is dead (has overheated)

[ajsemtb]

Re: Select-able wall wart power supply - dead

There are about 7 wires that come off this transformer. Which 2 are the primary pair?

[budm]

Re: Select-able wall wart power supply - dead

The two black wires on the left of the picture #2 which should go to the AC prongs are the primary winding.
If it is open, there should be a thermal fuse under that red tape on the transformer.

[ajsemtb]

Re: Select-able wall wart power supply - dead

Ac prongs are open. No tone on continuity test open ohm reading. However all prongs that come off the bottom of it are shorted. They all have continuity which I would Imagine is bad... lol

[ajsemtb]

Re: Select-able wall wart power supply - dead

Oh and diodes are good.

[budm]

Re: Select-able wall wart power supply - dead

If you always leave this wall-wart plugged into the outlet, that power supply would have been running 24/7, so it will exposed to any surges/spike/over voltage/under voltage all day long.

[ajsemtb]

Re: Select-able wall wart power supply - dead

Yeah, so dead transformer? Is it cheaper to just buy another ps or get the transformer?

[budm]

Re: Select-able wall wart power supply - dead

Better off getting new PS, and unplug it when not in used.