Dps-240fb

[rtstorm]

Re: Dps-240fb

i replace 0.22 resistor and UC3845. Connect over dimbulb and bulb turn ON.

[ben7]

Re: Dps-240fb

i check resistor R9 and it is 0.7 Ohm, i think it should be 23 Ohm. Am i looking right?

That is a 0.22 ohm resistor. Since the transistor blew, it is a good idea to replace this resistor. It might be good, since most ohm-meters have a hard time reading low resistances like that.

Also replace that UC3845 chip. Then test the power supply again.

[rtstorm]

Re: Dps-240fb

i check resistor R9 and it is 0.7 Ohm, i think it should be 23 Ohm. Am i looking right?

[ben7]

Re: Dps-240fb

Yeah it's likely that the gate drive circuit is toast.

[rtstorm]

Re: Dps-240fb

Ok, because current in that moment run to the switching ic, but it shoud not pop the fuse

[kaboom]

Re: Dps-240fb

I replace the 20N60C3 (because it is now shorted in all directions), and i replace fuse.

Whatever drives that gate is probably dead now. The shorted MOSFET(s) backfed into the driver(s).

I plug in and my main house fuse pop-down, but fuse on PCB stay OK. I use 6.3A, and original was 8A.

Your power fuse took fault current twice in quick succesion. No surprise that it blew.

Make a dim bulb tester!
http://antiqueradio.org/dimbulb.htm

[rtstorm]

Re: Dps-240fb

i check, it is OK, no break diodes.

[ben7]

Re: Dps-240fb

i check Q1 Q2, they are ok, no short connection. R12 is 0.2 Ohm 2W, it is OK

but i check the big coil and it has 0.5 Ohm, is this ok?

That is ok.

Have you checked the bridge rectifier? Test all 4 of it's internal diodes.

[rtstorm]

Re: Dps-240fb

i check Q1 Q2, they are ok, no short connection. R12 is 0.2 Ohm 2W, it is OK

but i check the big coil and it has 0.5 Ohm, is this ok?

[ben7]

Re: Dps-240fb

Check R12, Q1, and Q2 again (you replaced it, but I want to be sure it isn't blown again.

[rtstorm]

Re: Dps-240fb

Hi, i think i found the problem, i check the big capacitor and it vas 2-3uF on label it is 150uF 450V. I replace him.

But i did some idiot thing, i connect the power when 20N60C3 was not solder (just plug into PCB). and i hear 'bang'

Main fuse was burned out. I replace the 20N60C3 (because it is now shorted in all directions), and i replace fuse. I plug in and my main house fuse pop-down, but fuse on PCB stay OK. I use 6.3A, and original was 8A.
I check varistor = OK.
Disconnect AC/DC converter = OK no 'buum'

[ben7]

Re: Dps-240fb

Be careful working with high voltages in there.

Your meter probably won't see any voltage there because that pin is being switched at a very high frequency - it's not dc.

1. What voltage is on the 5v standby? That is usually the purple wire.

2.If it is OK, then try taking a short length of wire and connecting the green wire to one of the ground (black) wires. The PSU, if it is working right, should start up.

3. Also, do the basic resistance test on each voltage output. Do this with the power supply unplugged. Measure the resistance from the 5v (red) wire to ground (black) wire. Do the same for 12v (yellow), and 3.3v (orange) too. Report the readings to us.

4. Could you please take some good pictures of the insides? Not blurry, or dark, so we can see component part numbers, etc...

Cheers,
-Ben

[rtstorm]

Re: Dps-240fb

i check by line, and i think the fail is FET 20N60C3. I have voltages on pin1 and pin 3, but nothing on pin 2 witch is connected to transformer. So i dont have anything on secundary.

I have (+) on primary transformer but not (-)

[rtstorm]

Hi, i got a PS from HP computer that wont start, cannot find scheme for it.
I look caps and they look OK, the fuse is OK, no burned components. What should i check?