Coil replacement - is it worth it?

[Francesc V.]

Hi:

I am trying to repair this ATX SMPS and I realised once I've replaced the 5V IC that also the coil is gone.

Where (and how) do you look for these parts? same piece or similar that I could try to use it? is it worth it? :-)

Regards,
Francesc.

[momaka]

Re: Coil replacement - is it worth it?

What do you mean by "the coil is gone"? Open or short-circuit on a winding of the 5VSB transformer? Or entire transformer was lost?

Either way... yes, you can get the PSU running by taking a 5VSB transformer from another PSU. You just have to match the pins correctly.

On the primary side, you should have two windings: primary high-voltage (the one that switches on-and-off and gives power to everything), and the primary auxiliary (this one provides Vcc power to the 5VSB IC, 2-transistor feedback circuit, and PWM IC on the primary, if PSU has one.) On the secondary side, you will have either one or two output windings - 1 winding if the PSU is single-transistor or double-transistor forward config (generally PSUs with only 2 transformers in the middle of the board) or 2 windings if PSU uses half-bridge (2x BJTs on primary and 3x transformers in the middle of the board) design and PWM chip is on the secondary side (i.e. TL494, KA7500, SG6105, ATX2005, AT2005b, SDC2921, and etc.)

You PSU seems to be a half-bridge, so it would likely need a 5VSB transformer that has the 2 output windings on the secondary (one is the 5VSB itself, and the other is about 10-15V, used for the PWM controller and BJT driver circuit.)

Alternative method of repair:
Get a 5V 2 Amp power adapter and a 12V 0.2 to 1 Amp power adapter. Wire the 5V 2 Amp power adapter to 5VSB output and the 12V adapter after the diode output on the secondary side where that 10-15V auxiliary secondary rail goes. Now you should have a working PSU when PS-ON signal is held to ground.

*EDIT*
Looks like your primary side BJTs likely burned up, along with their driver components.
Gonna be a complete rebuild at this point (new BJTs, new driver components, new 5VSB IC or alternative method of power suggested above... and that's about it, if you're lucky. )

Worth it?
-Maybe.

It depends on how much you value your time for this kind of stuff.
I do enjoy rebuilding PSUs, so I tend to do it more often than not.
Some people don't, and it can be a hassle to them to try to repair a PSU vs. just buying another one.

It's good for learning purposes too, if that's what you're after.

[Francesc V.]

Re: Coil replacement - is it worth it?

Hi Momaka:

Yes........one of the primary circuit is gone (open circuit).

I was thinking to buy a universal flyback circuit and see whether I can inject 5V just on the diode catode and with a little bit of luck, the PWM circuit starts working and magic appears.

So far I've removed coil and IC. I was thinking to conect back to negative capacitor, red to the return of the coil........then orange to the output diode and blue to ground.

Do you think it might work?

Regards,
Francesc.

[momaka]

Re: Coil replacement - is it worth it?

I was thinking to buy a universal flyback circuit and see whether I can inject 5V just on the diode catode and with a little bit of luck, the PWM circuit starts working and magic appears.

Sorry, but no luck would help you here.

Like I mentioned, you need TWO voltage outputs on the secondary side for the PSU to run:
1) 5V (injected into 5VSB pin) for the PS-ON and other signals to be generated
2) 10-20V for the PWM controller and its BJT drive circuit.

The easiest way to do this (and without buying any modules, which I'm a bit weary of, TBH) is to get a 5V 2A power adapter and connect to 5VSB and ground. Then connect your 10-20V (a 12V source from a router or modem or 19-20V from a laptop adapter) to the to the Cathode of that smaller diode next to the 5VSB rectifying diode (I can't read the board designators from your picture, but that smaller diode is right under the small toroid inductor close to the middle transformer.)

[Francesc V.]

Re: Coil replacement - is it worth it?

Oki........I have ordered an extra DC booster to get this 10-13V. I will keep you updated with the outcome of the "experiment".

[Francesc V.]

Re: Coil replacement - is it worth it?

Could I make 2 voltages from 1 by using a resistor divider? I was thinking to use the universal fying circuit to provide me with 10V for the second circuit and then using 2 1K resistors get the 5V for the second diode......

[momaka]

Re: Coil replacement - is it worth it?

Could I make 2 voltages from 1 by using a resistor divider? I was thinking to use the universal fying circuit to provide me with 10V for the second circuit and then using 2 1K resistors get the 5V for the second diode......

Won't work for testing motherboards and likely won't work even just to test the PSU as-is without load. I don't think even two 100-Ohm resistors would cut it for the voltage divider.

The problem is, not only is the supervisor circuit supplied from the 5V rail, but also the motherboard... hence my recommendation of using two power adapters. As a minimum, you really need at least 500 mA of current for the 5VSB... and even then, you'd be lucky to find a motherboard that doesn't go over this current limit (especially with an attached USB keyboard and mouse that's powered from 5VSB.) I'd say, 1 Amp would be (barely) sufficient for a motherboard test. 2 Amps or more --> OK.

Meanwhile, the "10V" supply might only needs to provide probably somewhere around 100-200 mA or so. But I don't know if 10V will actually cut it or not, depending on the BJT driver design. So 12-15V would be recommended here... and up to 20V probably won't hurt anything.