Old Compaq Armada internal PSU repair

[Th3_uN1Qu3]

Re: Old Compaq Armada internal PSU repair

Leave the primary capacitor alone. Measure voltage on startup capacitor that you replaced. If (whatever voltage) but consistent, you have a dead PWM controller on the primary. Since that PS is so old it is likely a jellybean part, see what the numbers are on it. Replace it.

If (whatever voltage) but pulsing, it could be either the controller or the resistor feeding the startup cap. More commonly the resistor in this situation. See the numbers or color bands on the resistor, take 1 leg out of resistor (make sure to discharge that big capacitor again before you do that!), measure resistor - if higher value replace resistor. If correct replace PWM controller. Should run then.

[yuuki47]

Re: Old Compaq Armada internal PSU repair

If correct replace PWM controller. Should run then.

Yeah I'm guessing the PWM controller is inside those black-coated PCBs, in which case, I'd have no idea what chip to replace it with. If its the PWM controller, I'm guessing its impossible to revive that PSU.

I'll do those measurements today and tell ya, have to re-assemble the PSU though.

[yuuki47]

Re: Old Compaq Armada internal PSU repair

Huh, you were right! The voltage in the startup cap stays at a constant 11.3 volts. But that also seems low. It's a 32v capacitor, why would they use something this high for 11k volts?

But I also noticed that upon disconnecting the PSU from the wall, that voltage starts to fluctuate between 11.3 volts and 9 volts before it settles at 9 volts.

Weird... I'll have to look for that resistor you talked about.

It looks like this PSU is toast though if the PWM controller is dead. I don't think those black-coated PCBs are repairable, unless I can remove that black paint somehow?

[Th3_uN1Qu3]

Re: Old Compaq Armada internal PSU repair

But I also noticed that upon disconnecting the PSU from the wall, that voltage starts to fluctuate between 11.3 volts and 9 volts before it settles at 9 volts.

11.3 is too low. The PWM controllers these use start up at about 13-15 volts. Either the controller is dead, or there's not enough current feeding it. I suspect the former.

Weird... I'll have to look for that resistor you talked about.

It looks like this PSU is toast though if the PWM controller is dead. I don't think those black-coated PCBs are repairable, unless I can remove that black paint somehow?

Worth a check. And yes, that black paint is removable somehow, either chemically, by heat, mechanically (scraping/sanding) or a combination of the three methods. But if I were you I'd do something easier.

I'd take a perfboard and wire in a UC3842 in a basic configuration. Or wait, make it a UC3843, that one starts up at 8.3 volts. Go download the datasheet and it is not important to understand the circuit, just the connections, which part goes where and the inputs and outputs to and from the rest of the power supply.

Lift the gate pin of the power MOSFET and the two primary side pins of the optoisolator off the board and also connect these to your board. The source pin and current limiting does not really matter for now - use the series lightbulb trick for testing as this power supply does not have PFC.

See if you can get any output this way. You should. If it works, we can elaborate about making it permanent with current sense/limiting working and everything.

[stj]

Re: Old Compaq Armada internal PSU repair

if the black is smooth then it's epoxy and you can use heat
if it's rough then it's cement and i have read that oven cleaner with disolve it - not confirmed that though.

[yuuki47]

Re: Old Compaq Armada internal PSU repair

So you're saying that I should get a UC3843 and wire it up in a prototype board with the basic configuration the datasheet has. Ok, I can do that, I've made prototype boards before for a Raspberry Pi Modem Project.

But uh, this is the part I don't get quite understand:

"Lift the gate pin of the power MOSFET and the two primary side pins of the optoisolator off the board and also connect these to your board. The source pin and current limiting does not really matter for now - use the series lightbulb trick for testing as this power supply does not have PFC."

Where is the optoisolator? What even is that? You'll have to excuse me... I'm completely self-taught and a total amatuer, this is just a hobby for me so I'm not well-versed. I would really appreciate it if you could explain this in a bit more detail.

[Th3_uN1Qu3]

Re: Old Compaq Armada internal PSU repair

Where is the optoisolator? What even is that? You'll have to excuse me... I'm completely self-taught and a total amatuer, this is just a hobby for me so I'm not well-versed. I would really appreciate it if you could explain this in a bit more detail.

Little 4 pin rectangular black box the size of half a DIP package bridging primary and secondary, also called an optocoupler. Most common one is the EL817, if you see 817 anywhere that is it. Although a power brick that old may have different optos maybe in white DIP packages, some are 6 or 8 pins.

AT and ATX power supplies with bipolar transistors use current drive transformers, the EE-19 ones in the middle between the 5VSB transformer and the main transformer are what drive the bridge and also provide current monitoring and limiting. Your power brick, although old, should be a flyback topology using an opto so go look for it.

[stj]

Re: Old Compaq Armada internal PSU repair

optoisolator looks like a 4pin chip - it's on the edge next to a big diode

[yuuki47]

Re: Old Compaq Armada internal PSU repair

Well, okay, I'll have to order more parts. I'll post back when I get around to building the PWM prototype.