Old ATX power supply - no voltages

[Fritz23]

Hello,

I recently converted an old COBA VP-400X PSU into an adjustable lab bench power supply. I made it adjustable by using a WD2002SJ step up converter on the 12V rail.

A few days ago I hooked up a brushless DC motor from an old RC car. At first at 12V and then I cranked the voltage up to 28V for a short time just to see how the motor takes it.

That were the last moments of my power supply.
It did turn off without smoke or anything.
When I try to turn it on I get 5VSB and when trying to start the power supply I get a short pulse on +5V (I suspect also on +3.3V and +12V, but +5V is tied to a LED so I can see it) before it shuts off.

In the Inside there's a A109B V08 board from 2005 whose schematic seems to be pretty much the same as the one I attached. While doing some measurements I noticed that the power transistor Q3 (STD13007 in my case) gets its collector voltage (310V) but on the emitter side I get around 120V. These 120V start decreasing to 0 after my measurements. It does seem the power supply gets kicked on but couldn't keep up the oscillation.
So i measured the base Voltage, which decreases (after my measurement) from 13V to 0V.

My guess is that it just doesn't oscillate and I suspected the two transistors Q1 and Q2 (C1384) to be failed. I tried to measure the resistance between B, E and C and the lowest resistance ist around 700ohms. Am I right, that these transistors are broken? Am I right at all, that this is the oscillation circuit and that might be the cause? Could there be a short circuit I missed? Resistance between 3.3V and GND is around 7ohms but since I didn't use that rail I don't think that's the fault.

If I'm right, the transistors didn't fail as the only ones for sure. What should I check next? What might be the cause of all this trouble? Did the Motor draw too much current? was the current change to fast?

I hope someone can help me here.

Regards.

Edit: Since the image got resized here the original image: http://i.imgur.com/IFXl2tb.gif

[eccerr0r]

Re: Old ATX power supply - no voltages

Odd, usually in these old supplies I see a crowbar circuit that when voltage output is too high, would crowbar the output to GND, usually killing the supply or at least the fuse. I don't see it in the schematic. The main switching transistors end up dying if the crowbar takes effect.

Anyway I'd be very worried about running an ATX supply at 28V, the output usually aren't made for anything above 16V.

So your 5VSB works or not? It should be running at all times while power is applied. You could try disconnecting the control IC and see if 5VSB works...

I'm not familiar with this controller IC, I'm surprised it didn't prevent the supply from staying on if the voltage goes too high.

[stj]

Re: Old ATX power supply - no voltages

so on the 12v output, what diodes and caps is it using?
you probably have a short on the output

[dragos2009]

Re: Old ATX power supply - no voltages

like stj said you most probably have a short on the output first try to remove the step up converter and check again

[stj]

Re: Old ATX power supply - no voltages

looking at the tiny schem - did you remove the -5v regulator? i doubt it is going to survive 28v when it's design maximum is 25v (with a big heatsink)

[Fritz23]

Re: Old ATX power supply - no voltages

Sorry for not answering quite a long time.

like stj said you most probably have a short on the output first try to remove the step up converter and check again

It actually was a shorted diode in on the 12V output, so thank you!
Anyways I did throw away the PSU since it was in pretty bad condition.