Dell PS-5251-2DFS won't start - no 5VSB

[pfrcom]

As per title, Dell PS-5251-2DFS won't start

Measuring its 5VSB (at ATX connector, between purple & black wires) shows ~0.02V

It uses OST caps mainly, with Taicon for some of the tiny ones - none obviously faulty

No hits searching this site for PS-5251-2DFS, but Google for PS-5251-2DS (from its PCB) found this

Would appreciate some advice from everell which "small pc board soldered to the main pc board" has the A916 transistor

I desoldered components from Q251 & IC511 on the small PCB next to the secondary caps - they're A965 (Q251) & TL431 (IC511)

Hoping A916 isn't on other small PCB which is beneath a heat sink on other side

[everell]

Re: Dell PS-5251-2DFS won't start - no 5VSB

The A916 transistor problem has something to do with the supervisory function whereas your problem is a dead 5vsb problem. Two totally different problems. The 5vsb components will be located between the two heat sinks.

Make sure you have about 330 volts DC across the two big "mains" capacitors. Replacing the "critical" capacitor in a two transistor circuit will usually fix the problem. It will be one of the small capacitors located near the smaller transformer between the two heat sinks.

Always wear safety glasses when working on these circuits inside the metal case. Sometimes those 5vsb circuits decide to explode and send parts flying!

[Longbow]

Re: Dell PS-5251-2DFS won't start - no 5VSB

Also remember that Dell has its own proprietary arrangement of the power plug. Be sure you have the Dell power plug diagram.

[pfrcom]

Re: Dell PS-5251-2DFS won't start - no 5VSB

Also remember that Dell has its own proprietary arrangement of the power plug

Thanks for feedback, but I don't think that's been true since the mid-noughties

[Behemot]

Re: Dell PS-5251-2DFS won't start - no 5VSB

Dell often has some crazy shit (though it is not *always* the case). Just recently I found some not-that-old systems (think DX980 or something like that) use quite large custom PSUs (where ordinary ATX PSU will fit) with some kind of SFX (=smaller) Main ATX connectors. And even that with different pinout. So they still do this.

[momaka]

Re: Dell PS-5251-2DFS won't start - no 5VSB

As per title, Dell PS-5251-2DFS won't start

Measuring its 5VSB (at ATX connector, between purple & black wires) shows ~0.02V

It uses OST caps mainly, with Taicon for some of the tiny ones - none obviously faulty

That's a Lite-On unit, given the "PS-" model number (probably also says "LiteON on the main transformer too).

Lite-On PSUs of that era have two problems in general: bad caps and the organic tan glue that goes brown with heat and becomes conductive (and corrosive).

Because Lite-On splurged so much of this glue everywhere, it is hard to do any work on the PSU. Thus, the first step I do with units like this is remove as much of the glue as possible - particularly from the primary side, as that's where it can cause the most problems (and the most damage when it goes more conductive). This sometimes involves removing even some large components to get to it. But once you start removing things, you can also get to your problem more easily.

I find the easiest way to remove the glue is to poke it and pry it with a small (but strong) flathead screwdriver, then pull it away with tweezers and pliers. When I say "easiest", though, I don't mean to say that this would be an easy task at all - or at least certainly not quick. Removing this glue takes a lot of time and patience, so just be prepared for that.

Glue aside, since you have no 5VSB, the first step would be to remove PSU board from its case and check several things:
- input line fuse and NTC inrush thermistor should read low resistance.
- 5VSB rectifier diode (close to 5VSB transformer) should not be shorted.

If these two pass, chances are you either have bad caps on the 5VSB circuit (either the output caps on 5VSB or the start-up cap for the offline-switch IC), failed component on the 5VSB primary auxiliary rail, open resistor that supplies HV to the offline-swtich IC, or bad offline-switch IC itself.

Just because those OST and Taicon caps don't look bad doesn't mean that they aren't - especially the small ones. OST, in particular, is well-known to go bad without bulging. So if you don't have an ESR meter, it's best to replace any small start-up caps on the primary side and the 5VSB output caps first (in addition to removing any brown crunchy glue).

Let us know what you find.

[R_J]

Re: Dell PS-5251-2DFS won't start - no 5VSB

Does it use a TNY277P ic for standby?

[pfrcom]

Re: Dell PS-5251-2DFS won't start - no 5VSB

first step would be to remove PSU board from its case and check several things:
- input line fuse and NTC inrush thermistor should read low resistance.
- 5VSB rectifier diode (close to 5VSB transformer) should not be shorted

Now it's warming up here, I can get back to this

Unfortunately my expertise doesn't include what you're referring to - I don't even know which is 5VSB transformer, much less rectifier diode or inrush thermistor

Although I can advise input line fuse reads 0.2 Ω (same as touching probes together)

And a couple of new pics, first shows where I've replaced some small caps (shotgun approach) to no avail

Does it use a TNY277P ic for standby?

I can see a TNY267P near the small transformer shown in the first new pic (is that the 5VSB transformer?)

[Behemot]

Re: Dell PS-5251-2DFS won't start - no 5VSB

Yes, it is a second power supply of its own.

Clean all that yellow stuff, it can get corrosive and conductive, and check whether it has not corroded thrugh some resistors already. It's pretty difficult to do and mostly could be removed only mechanically, I use combination of dental explorer and small screwdriver. Some ppl claim that some solvents could soften that a bit, haven't tried (or it did not work, don't rememer ).

[momaka]

Re: Dell PS-5251-2DFS won't start - no 5VSB

Although I can advise input line fuse reads 0.2 Ω (same as touching probes together)

And a couple of new pics, first shows where I’ve replaced some small caps (shotgun approach) to no avail

It's a good thing you replaced them anyways.

I have another slightly newer LiteOn unit from a Dell PC again, and in that one, almost every Ltec cap was reading very high ESR (bad). And from the OST caps, about half of the big ones were bad too (but only one showed a bulge).

I can see a TNY267P near the small transformer shown in the first new pic (is that the 5VSB transformer?)

Yup, that's the 5VSB transformer, and the TNY267P is what is used for generating the 5VSB rail. This is the main area of focus right now - once we get the 5VSB running, then we worry about the rest of the PSU.

Since the fuse is good, the first step would be to check for 330-350V DC across the two big 200V caps (each one should have about 160-175V DC across it). To measure, you will probably have to have the PSU removed from the metal case and hooked to your house power with alligator clips. Just be extremely *CAREFUL* when you take those readings. This is the primary side, and you could get a nasty shock if you touch something live by accident.

The way I do it is I hold both probes by one hand and use the other to plug in the PSU in the wall. Any wrong move or shock, and I disconnect the PSU from the wall instantly just from the shock/panic (though that hasn't happened yet ).

Report back if you do or don't get those voltages, and preferably what they are.

Clean all that yellow stuff, it can get corrosive and conductive, and check whether it has not corroded thrugh some resistors already.

I agree. That yellow glue will be trouble, especially in spots that it has turned brown or black.

However, focus on cleaning only the areas on the primary-side of the power supply for now. That would be the side with the two big 200V electrolytic caps. And also try cleaning any of the glue that has stuck to the TNY267P chip. The cleaned areas don't have to be 100% free from the glue - just enough to remove any potential conductive paths, particularly where the glue has turned dark.

The best way to remove this glue, IMO, is with brute force. I use a set of small flat-heat screwdrivers for this to poke and pry it away. A pair of good tweezers can help too. It's a very slow process, but it works.

[Behemot]

Re: Dell PS-5251-2DFS won't start - no 5VSB

I'd clean it on any feedback components too, even resistance of megaohms applied in parallel to some components can sometimes make the driving circuitry broken.