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

[Toasty]

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

Diode 1F4 is not a correct replacement.

1F4 = 1A, 25A surge, 150ns reverse recovery (trr), Fast recovery diode

UG2D = 2A, 80A surge, 25ns reverse recovery (trr), Ultra-fast recovery diode

Unless UG2D was reading open/shorted/leaky, it's likely okay.

Again, +12v present? Other voltages?

Trace 7912 pin2 (input) back and find which component has 12 to 25 volts present on other side of it to find problem. Use pin1 as your ground(+)

Toast

[rogfanther]

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

I will put it back, and search for a more adequate replacement later.
Actually, as I have to buy wire, will go out and search for it now...

All the other voltages were present. 5V, 12V, 3.3V, 5VSB, all normal.

Going back from 7912 pin 2, it goes to the pin of the questionable toroid.

Now, as I´m trying to learn from this psu, and the good sense says something in the toroid should be bad ( the coil for that -12V read open, and shorted to the 5V line, some resistance between the 5 and 12V lines ) , I went and tried to rewind that coil. After removing the wire, it still showed some resistance between 5 and 12, but I made the experience anyway.

The wire I had here is a little thicker, so was not a good fit, and I will rewind better later, after seeing if it makes some difference.

Well, now I have -12V. That is, all voltages are somewhat wrong now :

12 -> 12.6V
5 -> 4.7V
3.3V -> 1.35 V
-12V -> -10.5V

As the 3.3V was reasonably good before I interfering with the toroid, I believe a good toroid, or maybe just rewinding it all, would improve things here.

What do you think, is it normal, that is, problems in the toroid could make the 3.3V regulation goes crazy like this ?

[rogfanther]

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

Well, got another diode, just in case. UF5408, 3A, 150A reverse, trr 50ns.

Been reading about the tricks to hand-wind toroidal transforms, will need to practice a little before trying to do this one...

[momaka]

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

What do you think, is it normal, that is, problems in the toroid could make the 3.3V regulation goes crazy like this ?

Yes.
Check that none of the windings for each rail shows a resistance to any of the windings for the other rails. If this is not it, rewind the toroid with a few extra turns for each winding, especially the 12v rail, since it's a bit higher.
Might be a good idea to save the original wire from the toroid, too, if it's not too late already.

Also, what capacitors did you use for replacement (brand and series)? This could definitely cause a problem if the ESR of the new caps is either too low or much higher than the originals.

Well, got another diode, just in case. UF5408, 3A, 150A reverse, trr 50ns.

If the original -12v diode wasn't bad/shorted, just put it back in. The UF5408 would also work, though.

[PCBONEZ]

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

Well, now I have -12V. That is, all voltages are somewhat wrong now :

12 -> 12.6V
5 -> 4.7V
3.3V -> 1.35 V
-12V -> -10.5V

As the 3.3V was reasonably good before I interfering with the toroid, I believe a good toroid, or maybe just rewinding it all, would improve things here.

What do you think, is it normal, that is, problems in the toroid could make the 3.3V regulation goes crazy like this ?

If you don't have some load on it then the regulation will be erratic.
[Some won't even start.]
An old hard drive is usually enough.
.

[rogfanther]

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

[QUOTE=momaka;171442]Yes.
Check that none of the windings for each rail shows a resistance to any of the windings for the other rails. If this is not it, rewind the toroid with a few extra turns for each winding, especially the 12v rail, since it's a bit higher.
Might be a good idea to save the original wire from the toroid, too, if it's not too late already.

Also, what capacitors did you use for replacement (brand and series)? This could definitely cause a problem if the ESR of the new caps is either too low or much higher than the originals.

[QUOTE]
Thanks,

I believe they show some resistance, yes. Will get another DMM and some new batteries just to be sure. The wire for the -12V, that is the only I messed with thus far, is still here.

The original OST ones measured the capacitance ok. I substituted them with some new Hitanos we had here, more to see if it would make some diference. But the voltages kept the same, with the OST and with the Hitano. They changed only after I messed with the toroid coils. Still don´t have an ESR meter, but I`m working on getting / building one.

I´m waiting for a shipment of Panasonic FC, as I understood from the forum that they could be a good substitute for most caps in a PSU. After fixing the other more pressing problems of this psu, will change most , if not all, with them.

PCBonez,
The voltages were correct even without load ( excepting that pesky -12V ) , before I messed with the toroid . Now, they are wrong without load, and when I put a load ( an old hard disk we found in a drawer ) , the psu won´t start, and I need to disconnect it and remove the jumper in the atx connector for it to start again. Will get another, known good hard disk, so that I not be mislead for a maybe shorted "load".

Thanks.

[everell]

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

I ran across the notes I made at the time I was fixing my psu like yours. Looks like the A916 was not the only bad component. I don't remember looking at the -12 volt rail, but the problem did cause problems with the 3.3 volt rail. So take a look and see what you think!

[momaka]

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

I ran across the notes I made at the time I was fixing my psu like yours. Looks like the A916 was not the only bad component. I don't remember looking at the -12 volt rail, but the problem did cause problems with the 3.3 volt rail. So take a look and see what you think!

Is it me, or does this PSU look stupidly over-engineered?
I was looking at the TSM111 datasheet and noticed this on the Pin Description:
ADJ (pin 4) - "Adjustment pin for 3.3V OVP. This pin is to be used for an OVP other than 3.3V (eg for uC power supply = 2.7V). When not in use, this pin should be grounded. When in use, VS33 should not be connected."
But according to everell's schematic of the supervisory board on the previous page, VS33 IS connected. So what the hell did Lite-ON do here? ADJ is obviously not grounded and if anything, seems to be related to the 12v rail and that weird circuit which contained the burned A916 transistor.
Also, from everell's supervisory board schematic, you can see that UV (pin 11) on the TSM111 IC is connected through a 100 ohm and another resistor in parallel to pin 3 on the supervisory PCB. Now if you trace pin 3 back on the underside of the PSU, you will see that it goes to that small orange capacitor C206, which in turn is connected to the AC output of the -12v rail. The description for UV (pin 11) of the TSM111 says: "Undervoltage detection, control and detect main AC voltage failure".
But, pin 3 on the supervisory board is also connected to ZD381, a resistor, 2 2222A transistors, and ADJ on the TSM111 IC.

So, I wonder if this circuit could be simplified...
Would have been helpful if the TSM111 datasheet (both SGS Thomson and ST Microelectronics) was a little better. Whoever drew the sample circuits on those datasheets didn't do a very a good job. On that sample circuit, UV (pin 11) is connected to a zener diode (which seems to be bypassed by a wire?) and then is connected both to ground and the 3.3v rail at the same time. Seriously?

Anyways, perhaps if we ground ADJ and regulated UV to 1.26v, then maybe those complex circuits can be removed completely? Any comments on this? Because I really don't see what all of those complex circuits are doing there. I know the power supply was working fine before and I don't blame the design, but if many of these power supplies start failing due to that A916 transistor, I think the supervisory board should be modified (read: simplified). Both the sample circuit and the evaluation board seem much more simple.

[everell]

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

I couldn't understand the circuit either. I simply replaced the A916 transistor, the 431 regulator, and the other bad parts in drawing, and the psu started working fine. I spent much time repairing and analyzing that thing. Later, I repaired a couple more by replacing that A916 transistor on the supervisor board. I like the Bestecs much better. Easy to understand, and when fixed right, work well!

[rogfanther]

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

Well, after rewinding the -12v coil, and measuring the toroid transformer out of the pcb, there seem to be no shorts.

One bit of information : without the -12V coil connected , voltages of 5 and 12V are in spec ( 5.12 and 12.2-12.4 ) , but the 3.3V is strange ( 2.7V ) .

With the complete toroid coils, voltages go out of spec ( 4.7V, 12.7V, 0.94 to 1.7V , -10.6V ) . I changed another cap, a 220 x 25V one, but probably will ressolder the old caps, just to be sure it is not one of my new caps that is causing it.

One part of information : it seems that while I was thinking about the toroid, with too many people in the shop offering their "advice" [ to be honest, it was the boss ] , I tried the psu without that second coil to the left, that belongs to the 3.3V circuit. Could this have damaged something ? I know it was probably foolish, but well, he is the boss.

Here are the pics of the supervisory board. Most of that black area is glue. Will clean it all and test the resistors, also changing the transistor. Could not find an A1023 here, possibly the MJE350 is a suitable one ?

[everell]

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

Some of the glue on your supervisory board is yellow, and some almost black. Definitely one of your problems.

Kaboom.......where are you when we need you. Tell this man about glue on power supplies!!!

[momaka]

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

One part of information : it seems that while I was thinking about the toroid, with too many people in the shop offering their "advice" [ to be honest, it was the boss ] , I tried the psu without that second coil to the left, that belongs to the 3.3V circuit. Could this have damaged something ?

Damaged something - unlikely.
But that coil is critical to the operation and regulation of the 3.3v rail. Without it, you would likely get massive waves of ripple on the output and the voltage might be unstable too.

One bit of information : without the -12V coil connected , voltages of 5 and 12V are in spec ( 5.12 and 12.2-12.4 ) , but the 3.3V is strange ( 2.7V ) .

With the complete toroid coils, voltages go out of spec ( 4.7V, 12.7V, 0.94 to 1.7V , -10.6V ) .

Interesting.
I'd say change that transistor and all other parts everell mentioned, then test it again.

Kaboom.......where are you when we need you. Tell this man about glue on power supplies!!!

[rogfanther]

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

The coil is already back. I don´t blame him 100%, because I did not measure voltages again between putting it back and dessoldering the toroid for measuring its continuity. So maybe there were some problem with regulation / supervisory board that was somehow being avoided because the -12V wasn´t running.

Will change both transistors, the 431 and both resistors ( although they measure right ) just in case .

Stupid me that I forgot to take some resistance measurings in the blue before cleaning it out... It would be nice to document it with numbers and pictures, just for some unbeliever.... After this recent learning about power supplies, and the books I read, I´ve learned about this phenomenon of old glue turning conductive. I confess I had never thought of that before.

The transistor around the black part seems to be connected with the termistor / fan, so maybe it overheated because of the fan getting stuck ( too much dust in it ) .

Any comment about the substitution of A1023 with the MJE350 ? Searching for a datasheet for the A1023 still...

Thanks.

[momaka]

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

So maybe there were some problem with regulation / supervisory board that was somehow being avoided because the -12V wasn´t running.

Yes, it's possible.
That weird circuit that connects the 3.3v rail, 12v rail, and -12v rail goes to ADJ, so I woudln't be surprised if that was the case.

Any comment about the substitution of A1023 with the MJE350 ? Searching for a data sheet for the A1023 still...

I'm not that good with transistor replacements, but from what I can see from the data sheets, both A916 and A1023 are PNP transistors rated for -800mA, whereas the MJE350 is rated for -500mA.
Based on that, I don't think you can replace the A916/A1023 with the MJE350. But I could be wrong since I don't have that much experience with transistor replacements.
Of course if you really have nothing else available, it might be worth a try. The MJE350 has a mounting hole so maybe you can attach a heatsink to it (with thermal compound, of course) to make up for the greater heat dissipation due to the lower current rating.

[rogfanther]

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

Those I had around here. I thought as this is some kind of a control circuit, that it would not be going to all that 800mA, but thinking about the burnt glue I must revise my estimatives. But if the MJEs are good just for testing, I can source the correct parts without hurry.

I could not find a datasheet for the 1023, but will try to find something rated for maybe 1A with 100V...

[momaka]

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

Those I had around here. I thought as this is some kind of a control circuit, that it would not be going to all that 800mA

Yes, I was thinking the same thing, which is why I thought the MJE350 with a heatsink might work fine too.

I could not find a datasheet for the 1023

Try looking for KTA1023 on google
Here's what I found:
http://www.alldatasheet.com/datashee...C/KTA1023.html

[rogfanther]

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

I´ve got some B772s. Rated for 3A. The maximum voltage is 40V, but I believe in the superv. board it would not be over 12V, isn´t it ?
As I was in it, changed the fan also.
The resistors measure ok, but I will change them also.
3.3V continues low ( 0.9 - 1.2 V ) , and I´ve observed that the 12V seems to creep up slowly. Starting at 12.23, then going up. Last I looked, it was at 12.67. Then I shut it down to rethink my strategies.

Soldered back the original caps from the 3.3V line, without noticeable effect. Seems I need to rethink my approach of this problem, or at least understand better from does the 3.3V voltage comes and why it is influenced from the -12V coil in the toroid being connected.

[Longbow]

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

I'm sure there must be a sticky on this subject, but the photos of your supply show one important thing that must be fixed. For years, manufacturers would use yellow hot glue to make sure large parts didn't work loose, or that tack-ons would not short out something. After a while, everyone discovered that the yellow glue turns brown with age and heat. After it is brown (maybe even before) you have a conductor wherever the brown glue happens to touch. It must be chipped off carefully with a pick. Sometimes freezing the glue will make it brittle and easier to chip off. Note: Brown glue: Great conductor. Get all of it off the board before you waste any more time with it. I wouldn't be surprised if the glue were actually the cause of the failures.

[Longbow]

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

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.

Thanks. I'd like to see a schematic of a toroid filter transformer used in such a circuit. I can't think of why the individual output lines from a switcher would need to be coupled to each other through a transformer, but admittedly there's always more to learn. Perhaps we are dealing with a class D supply? I looked at the data sheet for TSM111 supervisory chip, which can be seen here:

http://pdf1.alldatasheet.com/datashe...CS/TSM111.html

In the photo the transformer appears to be very small, considering the current requirements of a typical computer PSU. In any case, shorted windings would obviously have to be addressed first. The fact that symptoms are changing every time something is removed and replaced suggests to me that there is burned glue pc board contamination and not a bad transformer.

[PCBONEZ]

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

Not used as a transformer.
It's coupled inductors.

Inductors oppose current change.
The coupling allows the field in each rail to oppose current change in all the rails.
[Took me a while to understand that. Folks here taught me...]
.