Dell PS-5251-2DF2 missing -12V line

[rogfanther]

Hello,

Good morning to all. Been frequenting here mostly for motherboard recapping, since those cheap chinese ones started developing this "plague".


I´ve started messing around with power supplies, trying to learn how they work and how to fix them.

Got this Dell one, that would not start. The fuse was burnt, power MOSFET shorted, and two caps bulged ( Ltec ones, 470 x 10 and 100 x 35 ) . After replacing them ( the original MOSFET was rated at 900V 9A, I´m testing with a 500V - 14A one, while the compatible is on its way ) , most of the lines work ok , with exception of the -12V one.

5VSB gives 4.9-5.1V, Power Good ok, 5V, 3.3V and 12V all are at the right voltage or near enough ( like, 4.9 or 11.8 ) .

The board uses a 7912 regulator for the -12V ( removed it, tested good ) . Measuring at the board, there is 0.5V at its input pin. Following this pin circuit, both caps in it tested good . Will replace them as well, just in case. The circuit goes to one big toroidal inductor, with the other voltages as well, but I could not find the other pin of what I suppose is a -12V coil.

Trying to understand, this toroid mixes all the voltages to somehow equalize them, or remove intereference ?

Is it normal/possible for this to fail, or should I search for other failed componentes, even in far places of the board, that could be causing this ?

Or, maybe, could Dell have made a power supply that will not put out -12V when not connected to a motherboard ?

Thanks for the help.

[everell]

Re: Dell PS-5251-2DF2 missing -12V line

I worked on a dell same model a while back. Seems like I remember it having problems with the 3.3 volt line. Turned out to be a bad A916 transistor. I drew out the supervisory circuit to try to understand what was going on. I will include it and a picture of the power supply.

[rogfanther]

Re: Dell PS-5251-2DF2 missing -12V line

Thanks, that photo is exactly my psu. Had a lot of dirt inside, and the fan was stuck also.

Tomorrow I will probably remove the big toroidal inductor, and test continuity in it. I understand I need to read more about the theory of ATX psus, but would you mind explaining if that toroid is critical to working, or if it can be substituted with one take from other model of psu ? If necessary, I can rebuild it, just would like to learn and understand the whys and hows.

And just for testing, would it be very bad if I somehow disconnected the -12v from it, and conected them with a piece of wire for testing the rest of the -12V ?

Also, would you mind elaborating a little about the importance of the supervisory circuit ? Or directing me to some reading about its functions...

BTW, I tried to update the user profile, but the board would say I was not allowed to access the administrative interface. For the sake of politeness, as I think I should have made my introduction, I´m currently from Brazil .

Thanks for your help.

[momaka]

Re: Dell PS-5251-2DF2 missing -12V line

Tomorrow I will probably remove the big toroidal inductor, and test continuity in it. I understand I need to read more about the theory of ATX psus, but would you mind explaining if that toroid is critical to working, or if it can be substituted with one take from other model of psu ?

Yes, that toroid IS absolutely critical to the function of this power supply. Do NOT remove it.
I don't encourage you to replace it with a different one, either, unless of course, the original one is burned.

While I haven't worked with this particular PSU, I would suggest you check the rectifiers for the -12v rail. To get to them, start at the input of the 7912 regulator and go back towards the toroidal coil, then make note of the wire that's connected to it. Next, use a multimeter on continuity check and probe the other wires on the toroidal coil until you find the other end of the wire (when you find the two wire sets for the -12v rail, your multimeter will show a very low resistance/short circuit). After that, you should be able to find the -12v rectifiers. Most likely they will be regular axial diodes. Checking them in circuit is a bit tricky, so I suggest to remove them and test them out of circuit with your multimeter on diode/continuity test function.

Feel free to post some pictures too, if you like.

[rogfanther]

Re: Dell PS-5251-2DF2 missing -12V line

Hi,

I will try to find a place with good ilumination to take the pictures. The camera I have is a little crappy in the macro photos.

I need to draw the my schematic in a presentable form, but the -12V line goes to the pin Everell mentioned in the supervisory board, and to a 7912 regulator. I removed that and it tested good ( feeding with a -16V supply, gave -9v in its output pin ) .

The input pin of the 7912 goes to a leg of the toroid. Problem is that no other pin of the toroid gives continuity to it. I have tried to identify the other coils, as they have different color / wire diameter, and they seem to be ok ( at least have continuity between at least two ). Just the -12V wire seem to not give continuity anywhere. I will remove it from the board to be right.

Also, removeing the 7912 from the board and measuring at its input pin, gives just 0.5V. I believe it should be something like -15 or -18V, right ?

Thanks.

[everell]

Re: Dell PS-5251-2DF2 missing -12V line

Over the past few years, I have fixed at least three of these Dell power supplies. In every case the A916 transistor was either shorted or leaky. I have never been able to figure out just how that circuit works, but it is tied to the -12 volt line, and thru other components also ties to the +12 volt line and the 3.3 volt regulator. So that transistor can cause some strange problems. I would suggest that you remove and test the A916 transistor, or just replace it with a new one. Save a few headaches later.

That transistor is located on the small pc board soldered to the main pc board.

Also a couple more pictures.

[Longbow]

Re: Dell PS-5251-2DF2 missing -12V line

Good find on that leaky transistor. I believe what we're looking at is a circuit that makes the +12v track with the -12v. Interesting that they had to do it with discreet parts in a messy circuit. Since all the other voltages are good, you know that the main power supply functions are all fine. If that PNP transistor shorts, then finding it is easy. If leaking, it might show some strange numbers when doing the normal diode tests with the part desoldered.

Since this circuit is a known trouble maker, please take a voltage reading on the IC terminal from a known working supply and put it in the stickies.

[rogfanther]

Re: Dell PS-5251-2DF2 missing -12V line

Hi Everell,

Thanks, will search for that or an equivalent ( our electronic stores seem to store just the more common parts ) .Will take some photos in Monday, and also try to draw part of the circuit.

And I am still curious, the coils from that toroid must present some low resistance, right ? Is it possible, that the same thing that made the mosfet to shortcircuit could somehow have damaged that ? Also, all in the name of science and research, provided that I be careful, rewinding that with the same wire gauge and same number of turns of every coil, should probably work ?

[rogfanther]

Re: Dell PS-5251-2DF2 missing -12V line

Hi LongBow,
A friend says they have a bunch of these, with various defects , in a backroom at his work. He promised to try to get some for me. But when they come, I would like to already have a good understanding of this one... Currently asking for some friends that deal with used computer gear, and the like, but they seem to just have those very small Dell supplies.

After getting one that works ok, will try to make a drawing with important voltages, to at least contribute something useful for the forum.

Thanks.

[momaka]

Re: Dell PS-5251-2DF2 missing -12V line

And I am still curious, the coils from that toroid must present some low resistance, right ? Is it possible, that the same thing that made the mosfet to shortcircuit could somehow have damaged that ?

If the wire for the -12v was very thin, it's possible.
In any case, check the -12v rectifier. Based on everell's pictures, it looks like the -12v rectifier is right behind the big inductor, close to the small ceramic capacitor C206.

Also, all in the name of science and research, provided that I be careful, rewinding that with the same wire gauge and same number of turns of every coil, should probably work ?

Yes.
Although if the 5v and the 12v rails are working fine, then the wires on the toroid for the 5v and 12v rails are fine too and you don't need to rewind them.
If you can't find which other wire connects to the -12v, then just trace out the 5v and 12v rails and see which sets of wires they use. Then see which wires they don't use, and that should be the -12v rail. As for the 3.3v rail, it uses that other smaller toroid.

Over the past few years, I have fixed at least three of these Dell power supplies. In every case the A916 transistor was either shorted or leaky. I have never been able to figure out just how that circuit works, but it is tied to the -12 volt line, and thru other components also ties to the +12 volt line and the 3.3 volt regulator. So that transistor can cause some strange problems.

Sounds complex. Then again, this is a Lite-ON power supply .

[everell]

Re: Dell PS-5251-2DF2 missing -12V line

Here is a data sheet for the A916. Since this is a trouble maker for several power supplies I fixed, I ordered a handful from Digikey.

[Longbow]

Re: Dell PS-5251-2DF2 missing -12V line

Interesting problem and very helpful advice given. It is always good to find a problem that affects hundreds or thousands of units so they don't have to be tossed. Transistor data sheets - this particular part seems like a general purpose PNP, but the votage rating is 120 Volts, which is much higher than the typical. It looks to me like the circuit deals with + and - 12 volts, so the selection of this part seems confusing. Because of the failure rate of the part, I would be inclined to use a different transistor here. Sometimes during startup, various parts may be subjected to much higher voltages than their normal operating voltage, and this may be an explanation.

As far as looking at the resistance of the switching transformer windings, this method will not yield useful information. The d.c. resistance should be low, by design. Hint: the most often replaced part in a SMPS is the transformer. The part that fails least often is the transformer. (A possible exception is a CFL inverter, where a faulty switching device can fry the transformer, which is usually very tiny.)

[rogfanther]

Re: Dell PS-5251-2DF2 missing -12V line

As far as looking at the resistance of the switching transformer windings, this method will not yield useful information. The d.c. resistance should be low, by design. Hint: the most often replaced part in a SMPS is the transformer. The part that fails least often is the transformer. (A possible exception is a CFL inverter, where a faulty switching device can fry the transformer, which is usually very tiny.)

Hi,

I will post some photos, so that I can explai myself better, but the problem ( I believe ) is not in the switching transformer. All the other voltages are ok ( less than 5% off their nominal values ) . The only that is wrong is the -12V, measuring 0.72 V. The transformer I believe is involved in the problem is the toroidal one, that couples all the voltages before they go out of the psu.

I´m still reading and studying to learn what it does and how it works, but in a bunch of trashed boards I have, I could always find at least two terminals for each voltage, and in this problematic one, the -12V doesn´t appear to connect to other pins, so probably I will remove it to measure correctly. The wire is different gauge and color from the others, so I believe it should not be so hard to identify.

Thanks.

[rogfanther]

Re: Dell PS-5251-2DF2 missing -12V line

Hi,

Here are some photos.

Measuring continuity in the toroid coils, I got 0 ohms between pins 3, 4 and 6 ( thick yellow wire ) . Also 0 ohms between 2 and 5 (thick red wire ) . But between 1 and 7, the pins for the -12V, I measure approximately 12 kohm.

Desoldering the thin wire from the toroid pins, one leg of it shows a short to the 2-5 coil, and the other leg seems to not be connected anywhere.

I still have to find the a916 transistor or an equivalent, but will solder the toroid back ( with the -12v coil disconnected ) to measure if there is voltage in the input side.

The red line in the foto is the path of the -12V, with the 7912 in the upper side. Input comes from the toroid, output goes to the supervisory board.

[Longbow]

Re: Dell PS-5251-2DF2 missing -12V line

Well, it would be interesting to see a general schematic showing where this part appears in the circuit. This is a transformer. It might be your standby power supply transformer, which usually provides standby +5 volts. But it is obvious that you have 2 main windings, one with heavy wire and one with smaller wire. The only thing you can do is to check the resistance between the larger wire and the smaller wire - it should be open (infinite). Anything less than that would indicate a primary/secondary short. If it appears fine, it probably is. I don't see any burn marks. Put it back in.

[rogfanther]

Re: Dell PS-5251-2DF2 missing -12V line

Three main windings, one with thick golden wire, one with thick red wire, one with thin wire. ( the thin wire is connected to the -12V line ) .

I will recap the output of it, just for getting rid of those OST and Ltec caps.

[momaka]

Re: Dell PS-5251-2DF2 missing -12V line

This is a transformer. It might be your standby power supply transformer, which usually provides standby +5 volts.

No.
This is the main filtering toroid/inductor for the 5v, 12v, and -12v rails. Depeding on the PSU design (and age), some manufacturers will have a winding even for the -5v rail.

Measuring continuity in the toroid coils, I got 0 ohms between pins 3, 4 and 6 ( thick yellow wire ) . Also 0 ohms between 2 and 5 (thick red wire ) .

Those are normal readings, so those windings are good.
The red wire on the toroid is for the 12v rail, and the two yellow wires are for the 5v rail.

But between 1 and 7, the pins for the -12V, I measure approximately 12 kohm.

Desoldering the thin wire from the toroid pins, one leg of it shows a short to the 2-5 coil, and the other leg seems to not be connected anywhere.

Not good. You should have continuity just like for the 5v and 12v windings.
My guess would be that when the small transistor shorted, it overheated the -12v winding on the toroid and caused it to melt.
Since one leg of it does show a short to pins 2 and 5 (the 12v winding), then you should rewind the toroid's 12v and -12v windings.
Better yet, if you have any other junk PSUs, find a toroid of the same size and rewind it so that it has the same number of turns for each rail as this one.

Also, I may sound like a scratched CD, repeating myself in an annoying way, but again: check the -12v rectifier! Should be 1 or 2 axial diodes.
Based on the underside pictures you posted, the -12v rectifier should be right next to the toroid, close to the edge of the board and that small yellow ceramic cap C206.

[Toasty]

Re: Dell PS-5251-2DF2 missing -12V line

Please-
+12v present?

1) Load PSU and test.
2) Measure from Pin1 to Pin2 on 7912, -not- from system ground. Voltage?

Circuit uses +12v as an inverted supply where +12 is ground.

[rogfanther]

Re: Dell PS-5251-2DF2 missing -12V line

Also, I may sound like a scratched CD, repeating myself in an annoying way, but again: check the -12v rectifier! Should be 1 or 2 axial diodes.
Based on the underside pictures you posted, the -12v rectifier should be right next to the toroid, close to the edge of the board and that small yellow ceramic cap C206.

Thanks Momaka, I will remove and test then again. Onboard, with the DMM diode test, they worked ok. One of them I have removed, but it teste ok, so I put it back. Better, I will remove and change them ( I believe there are three, will post a photo later ) .

Please-
+12v present?

1) Load PSU and test.
2) Measure from Pin1 to Pin2 on 7912, -not- from system ground. Voltage?

Circuit uses +12v as an inverted supply where +12 is ground.

Thanks Toasty, will do. Even with the toroid transformer in this strange condition? Well, I will change some caps from the 3.3V output line and ressolder the toroid & 7912 to take those measures. Lets see if this psu comes back to life .

[rogfanther]

Re: Dell PS-5251-2DF2 missing -12V line

Diode test ok ( UG2D 0432 ) , put a 1F4007 in its place.
Pin 1-2, 0.55V
Pin 1-3, 0.72V
Pin 2-3, 0.18V

Also changed two 2200 x 6.3 caps in the 3.3V line, a 470 x 25, a 1000 x 16 and 47 x 50. After removing all the glue, will change the transistor.