Help identifying blown component in Dell PSU.

[roadrash]

Re: Help identifying blown component in Dell PSU.

your right of course dumb question. I tested all diodes & all were ok but I did find a IC TL431 (shunt regulator) that tests as faulty. oh well fingers crossed. Is it possible faults could have also happened in the rest of the PSU not just the little +5v board.

[stj]

Re: Help identifying blown component in Dell PSU.

i'm thinking if the top exploded it could have blown a fuseable resistor or real fuse.

[momaka]

Re: Help identifying blown component in Dell PSU.

i'm thinking if the top exploded it could have blown a fuseable resistor or real fuse.

Or the 5VSB transformer's primary winding. It's rare, but it happens. Check with a multimeter - you should get a nice short-circuit reading. While at it, also check the primary auxiliary winding - that's the one that connects to D601 on one side and primary ground/negative (-) bus on the other.

The optocoupler's may also be bad, and it would be hard to meter those - it's best to pull them out and make a test circuit with them to see if they work. Or just replace and see if that fixes the problem. I see two optocouplers, so this 5VSB circuit may actually have crowbar / short-circuit protection, which is nice (but also makes it slightly more complicated to troubleshoot).

[roadrash]

Re: Help identifying blown component in Dell PSU.

I just replaced the TL431 shunt regulator & sadly its still dead. Ive now checked all diodes and all IC's except the two in the attached picture (L0050 8170). I have no idea what they are and cant identify them from the part number. anyone know what they are? Ive also tested the transformer and all 3 coils are ok. There is one resister (picture also attached) I dont get a normal reading from but not sure I am reading its colour code correctly. I think its red,yellow,blue,gold, 24 Mega ohm. What does everyone else think? I was also wondering should this PSU start up without this 5V board attached because then I can remove it and see if the PSU starts up on its own and if not I will know there is another fault.

[R_J]

Re: Help identifying blown component in Dell PSU.

817a optocouplers, available everywere. the resistor does seem to be 24meg

[roadrash]

Re: Help identifying blown component in Dell PSU.

will this psu run with the +the suspect +5v board removed?

[momaka]

Re: Help identifying blown component in Dell PSU.

will this psu run with the +the suspect +5v board removed?

No, but if you have a 5V power adapter capable of at least 500 mA (or 0.5 A), you can use it to test if the rest of the PSU works. To do that, connect the secondary-side blue wire (OP3) and black wire to the power adapter's ground. Then connect the purple wire (5VSB) and brown wire (OP4) to the power adapter's 5V supply.

Reason for that is because the PSU's secondary-side monitoring IC needs power to generate the PS_ON and PG signals, as well as start-up the main supply.

Ive now checked all diodes and all IC's except the two in the attached picture (L0050 8170). I have no idea what they are and cant identify them from the part number.

Those are standard "817" optocouplers, as R_J noted.

There is one resister (picture also attached) I dont get a normal reading from but not sure I am reading its colour code correctly. I think its red,yellow,blue,gold, 24 Mega ohm. What does everyone else think?

What reading do you get from your multimeter?

When testing in circuit, many times it is "okay" for a resistor to test lower resistance than what is printed on its color code (as there can be other components in parallel with the resistor that may make the overall resistance read lower on the meter). But, if a resistor reads higher resistance than what is printed on its color code (that is, higher than the % tolerance allowed), then that resistor is likely bad.

If in doubt of course, take out the resistor in question and measure outside of the circuit. Then compare to the color code.

[roadrash]

Re: Help identifying blown component in Dell PSU.

Finally managed to get back on this job after getting taken away by something more urgent. Where I left off I replaced both the TL431 shunt regulator & the two Ive been right through the 5v board now. I repalced both of the 817a optocouplers. I also went through the whole board tidying up some pretty poor joints and replaced one suspect zenner diode. I connected it all up and still dead so there is still something else. Maybe its in the main supply? I just tried what you said and I got a bit of smoke out and a terrible stink from the main PSU & its taken out my bench PSU too. So will have to sort that now too but I will start another thread for that if I cant find anything simple like a blown fuse.

[momaka]

Re: Help identifying blown component in Dell PSU.

I just tried what you said and I got a bit of smoke out and a terrible stink from the main PSU & its taken out my bench PSU too.

Well, that certainly shouldn't have happened. Did test the PSU with the incandescent bulb like I suggested in the previous page? If not, now would be the time to take note and always remind yourself to do it in the future when testing stuff so you don't blow up more components.

As for taking out your power supply... did you also have all your wires wired correctly? Also, does your bench PSU have over-current/short-circuit protection? It's not wrong to use a bench PSU, but if it's a cheap bench PSU without protection, it would have been better to use a 5V switching adapter, as most reputable switching adapters come with short-circuit protection, so there's very little chance of making anything smoke or go bad.

Let us know what you find... i.e. what's the body count now (in terms of PSU parts). If something smoked and it really stunk bad, that suggests blown silicon (diode, transistor, IC, etc.). Resistors usually don't stink that badly (usually! ).