Jentec AF1205-E

[sebr]

This is a 5V / 2A PSU from a D-Link Switch. The outpult voltage is oscillating and causes only the switch power led to blink. The switch itself is fine since I tested with another PSU.
A picture of the PSU can be found here
All caps tested fine in terms of capacitance and ESR. I'm clueless - what could be the problem here?

[kaboom]

Re: Jentec AF1205-E

Load it with a 2.5 ohm, 10W or better resistor. Yes, a 10W will become rather hot, but it's only for a short testing period.

Replace the two small black caps by the opto.

[sebr]

Re: Jentec AF1205-E

As I plan to keep this switch I did a full recap (excluding the main cap) on this PSU and re-soldered the whole board but still no luck, same symptoms. Fuse wasn't / isn't blown.

[stj]

Re: Jentec AF1205-E

try replacing the opto isolator and the current-source.
that's the thing that looks like a transistor and has a part number probably ending in "431"

[momaka]

Re: Jentec AF1205-E

Also check primary side auxiliary rail diode - this is what gives power to the switch/PWM-FET/controller on the primary side. If not sure where that is, post pictures of the bottom side of the power supply. Please attache them to badcaps.net vs. going to off-site hosting service.

[sebr]

Re: Jentec AF1205-E

Well I have tested all the diodes and none of them is shorted or open in both directions.

[momaka]

Re: Jentec AF1205-E

Check the voltage on the cathode of D3 - does it oscillate too? (Also, keep in mind this is the primary side, so that voltage measurement will have to be done with respect to primary ground). If the voltage is oscillating there too, replace those two small caps by the optocoupler, regardless of what their ESR shows, especially if they are of some unknown or bad brand.

Also check ZD1 and ZD2 are not shorted. And replace the 431 shunt if not - those are fairly common parts and should be easy to find.

[sebr]

Re: Jentec AF1205-E

Thanks for your quick reply. Well as I wrote before I already did a complete recap (except pimary cap) so I gonna skip measuring on primary side.
ZD1 and ZD2 are not shorted either. The resistors on secondary measure fine as well.

[momaka]

Re: Jentec AF1205-E

I guess that doesn't leave much stuff to check now.

Thanks for your quick reply. Well as I wrote before I already did a complete recap (except pimary cap)

Okay, I was just making sure. I've just seen people mention they do a full recap and then forgo to replace some of the important small caps. So you are good there .

so I gonna skip measuring on primary side.

Well, actually do it anyways. That voltage on the cathode of D3 needs to be stable for that small chip to function.
After that, you only have solder joints and that 431 shunt to worry about.
Perhaps also check the transformer windings. They will appear as a short circuit, but that's fine. We just want to check none are open. On the primary side of the traffo, there are two windings, but they should be separate from each other.

[Agent24]

Re: Jentec AF1205-E

Is the primary controller a Fairchild PowerSwitch IC with integrated FET?

I had a power cycling PSU in a network switch, after checking *everything* else, it turned out the FPS IC was bad. Replacing it fixed the PSU. I had to use a newer, RoHS version as the original was EOL. Datasheets were practically identical otherwise.

[kaboom]

Re: Jentec AF1205-E

Is the primary controller a Fairchild PowerSwitch IC with integrated FET?

That teeny IC, IC1, on the back of the board, is the SMPS controller. It's like a super-tiny 3842, but otherwise works/does the same.

[Agent24]

Re: Jentec AF1205-E

Ahh! On a quick look I did not notice it...

[Behemot]

Re: Jentec AF1205-E

If everyting else was checked or replaced, that really only leaves the driving IC or transformer.