Help Needed With PC Switchmode Power Supplies

Well, it would take a fair amount of time to do it right. If i were to guess, bad solder joints and capacitors are the primary suspects. The bad solder joints are pretty straight forward if you know what to look for. The caps would take a ESR meter at minimum to check for series resistance. This might involve taking them out of circuit in some cases.

If one was going deep into this, you would have to reverse engineer a schematic along with data sheets to figure out what's really going on inside. And that's highly time consuming. You got the funds to to that?

There is always the possibility that there is some latent failure of the power transformers or such. Those parts will NOT be readily available if they can be found at all. A 494 is a pretty common switcher control IC. Used commonly in car stereo amps.

Bob

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494 IC is found in some generic LCD for lamp inverter.

Cheers, Wizard

Hi!

Please define "the funds to do that". Hundreds? Thousands?

The way these supplies seem to be dropping out, I may spend the money one of the two ways.

I just don't know where the trouble lies. These supplies die silently and so far there is only one documented case of one actually blowing a transformer...which was a rather smoky, fuse-blowing event as I'm told.

More than anything else, I really want to know what is killing them. I have many other PS/2 systems operating with other supplies that don't run in anywhere near as pleasant conditions. Many don't cool themselves as well as these Delta 335/400 watt units do, nor do they have as much output power. As an example, I have some 94 watt supplies used in the PS/2 Model 50(z) that have traces which have delaminated from the circuit board and even some sign of having been *very* hot at one time or another. These also operate under rather heavy load with fancy video cards, 386 upgrades, memory cards and network cards. Yet not a one of these has ever given up and quit working, even under the odd bad set of line conditions.

William

The usual killers of switchmode supplies aside from the fan failing and overheating it are cracked solder joints and dried up electrolytic capacitors. Probably 90% of failures are due to one or more of those initial problems.

Read time=money. IN other words, can you afford the time to trace out the schematic?

The compounding issue is that there is nothing that requires the power supplies to use the same design. So you trace one, and when the matter comes up again, you may have to trace again. This is not the same as tracing a piece of consumer equipment that you will come across multiple times but the circuitry will remain the same, or you can trade your finished schematic to others to get a higher return on the work.

Michael

Hi!

I don't know...depends upon what people are asking for their time and how badly I really want to know why these things died. (And I want to know that pretty badly. Obviously any proposal to look at it has to be reasonable...but beyond that...I just want to know why they died.)

For most power supplies I'd agree with you, at least to an extent. However, the PS/2 Model 95 line used this same Delta 335 watt supply across almost the entire model line. I've seen a lot of them, and all are the same inside. Even the 400 watt unit is largely the same inside...key differences I see? The fan speed on those varies in operation, circuit boards are a different color and the heatsinks might be a bit thicker, but that was all I noticed right away. The point is that these were a custom-built, standardized design. As far as I can tell they did not change the design much--if at all--during the lifetime of the PS/2 Model 95.

I've only ever seen one that was *not* made by Delta Electronics.

I might disagree with you on the frequency that this scenario will occur. I know others who have power supplies just like this that have died. This is what is so alarming to me...most of these machines just run and run. The Model 95 and its 335 watt supply seem to be the exception to this rule. I've got at least two or three here now that have all failed with the same symptoms--they're just dead.

William

On Wed, 26 Oct 2005 04:57:27 GMT, "William R. Walsh" put finger to keyboard and composed:

I'd be keen to have a look at one, but I'm on the other side of the world. Judging by Sam G's diagnosis, I expect that the PSU is either shutting down for reasons of overvoltage or overcurrent. IME the overvoltage protection is designed to latch on so that it can only be reset by removing power from the PSU. Being a latch, the OV condition should be visible on a DMM after the event has occurred. I don't know if an overcurrent condition would be similarly trapped though.

If the fault is due to an OV, then I'd be looking around pins 1 and 2 of the TL494. Perhaps one of the feedback resistors has gone open??? Could there be a dirty trimpot???

If it's an overcurrent fault, then I'd be looking at shorted or leaky diodes on the load side or, in the worst case, a shorted turn in the switchmode transformer. Maybe a leaky chopper transistor could behave this way, too???

-- Franc Zabkar

Please remove one 'i' from my address when replying by email.

Hi!

Well, I could try to do that. These PSUs are fairly heavy (probably 15,

20 pounds) so shipping wouldn't be cheap...but given some time to get the $ I could likely send it your way.

I can be contacted off the group using wct walshcomptech if you'd like to let me know where in the world you are so I can find out how bad the shipping would be.

William

"Franc Zabkar" bravely wrote to "All" (26 Oct 05 17:29:05) --- on the heady topic of "Re: Help Needed With PC Switchmode Power Supplies"

FZ> From: Franc Zabkar FZ> Xref: core-easynews sci.electronics.repair:346288

FZ> On Wed, 26 Oct 2005 04:57:27 GMT, "William R. Walsh" FZ> put finger to keyboard and FZ> composed:

FZ> I'd be keen to have a look at one, but I'm on the other side of the FZ> world. Judging by Sam G's diagnosis, I expect that the PSU is either FZ> shutting down for reasons of overvoltage or overcurrent. IME the FZ> overvoltage protection is designed to latch on so that it can only be FZ> reset by removing power from the PSU. Being a latch, the OV condition FZ> should be visible on a DMM after the event has occurred. I don't know FZ> if an overcurrent condition would be similarly trapped though.

FZ> If the fault is due to an OV, then I'd be looking around pins 1 and 2 FZ> of the TL494. Perhaps one of the feedback resistors has gone open??? FZ> Could there be a dirty trimpot???

FZ> If it's an overcurrent fault, then I'd be looking at shorted or leaky FZ> diodes on the load side or, in the worst case, a shorted turn in the FZ> switchmode transformer. Maybe a leaky chopper transistor could behave FZ> this way, too???

Franc,

The high value base bias resistors on the output transistors have a habit of going open circuit. I suspect it is due to line transients. These are now typically thin film types with a lower voltage tolerance than the old carbon type. Replace these resistors by 2 of equivalent series value to double the voltage tolerance. Be sure to mount them standing off the circuit board by at least 1 diameter. Perhaps the better remedy for transients is a power line filter between the line socket and the fuse. Some mfgrs skimp on the line filter while others include the fancy shielded modular type with the line socket built in.

A*s*i*m*o*v

... Wasted power is current squared times the resistance.

On Wed, 26 Oct 2005 14:02:33 GMT, "Asimov" put finger to keyboard and composed:

Thanks for the info. I think the OP should now be able to find a local person to follow up on your advice. Perhaps Sam could have another look?

-- Franc Zabkar

Please remove one 'i' from my address when replying by email.

Hi!

I found some other repair shops that might be willing to look at it. For now I have to see about contacting them.

Local repair shops...let's not even go there. I'm more than a little disheartened with the seeming sad state of repair shops these days...

Sam offered to have another look but said that he wasn't sure when he might be able to get to it, if ever. I'm very glad he was willing to look at it to begin with and feel that bothering him further about it might not be the most appropriate thing to do. I'd reckon that Sam has a busy enough life without my bothering him. :-)

William

In message , wm snipped-for-privacy@hotmail.com writes

Found the 92F0051 for $55, is this more than you would pay someone to repair one?

--
Clint Sharp

Hi!

Buying them from resellers has been a hit and miss for me. I bought a few of them recently and three of four were dead in the box! (Well, okay, only one was *truly* dead. Another was intermittent under load and the third one had been very hot at the power plugs. That might be fixable.)

Even if they do run when I get them, the chance seems very good that they will be dead soon. So I see three things here--one, it is getting very expensive to buy them, even at $15 or $20 per. Every one I have to buy pushes the "acceptable margin" of a repair cost that much higher. Second, there's no sign of catastrophic failure--so *I* think the odds of whatever is going wrong being fixable must be pretty good. Finally, it's environmentally irresponsible to just throw them away and move on to a new one.

At the rate I'm going, $200 or $300 for someone to fix one wouldn't be entirely unreasonable. So the answer to your question is "no"...I'd certainly pay more to know why they died or to have one repaired.

William

The two most common problems that I have seen are bad capacitors (high ESR) or open startup resistors. That should not be too hard to track down.

Where are you located? If you're not in a big rush, I might be willing to take a look at one. How much do these weigh?

-

----------------------------------------------- Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA

-----------------------------------------------

Hi!

I'm not in any big hurry. I'm located in eastern Central Illinois. I'm not sure of an exact weight for these power supplies--would guess about 10~15 pounds.

If you'd like to, please send me an e-mail: wct walshcomptech com

William