Dell NPS-250KB caps

[kc8adu]

Re: Dell NPS-250KB caps

no big deal.
hammer out the dent and replace cap.
its worth the effort as its a decent unit.

[stevo1210]

Re: Dell NPS-250KB caps

When I replace the damaged cap, I have to replace the other cap next to it with the same value cap right??. Is this because they are in parallel??

I desoldered the Tosin 470uf 200V caps from a dead PSU, they are ready for installation into this PSU tomorrow.

I want to make sure that I'm getting this right...... Most importantly the polarity... I better remember which side is + and -.... though there are makings on the PCB....

Worse thing that could happen is that it explodes and triggers the safety switch or the circuit breaker on the panel....

Oh, and i'll stand real far from it when the power is plugged in.... just have to find where that 25 metre power extension cord has gone off to....

Thanks.

[Rainbow]

Re: Dell NPS-250KB caps

The caps are connected in series. They're rated 200V each but about 320V is across them - and the voltage needs to be divided (about) equally. That's why the caps must be equal. And the resistors too. If they're not, voltage on one of them can rise above 200V and it will blow.

[stevo1210]

Re: Dell NPS-250KB caps

Hello Everyone,

I just finished the recap on this PSU yesterday night. I don't know If I should turn it on yet because I have a strange feeling that I may have missed something. The polarity is correct, the capcitance on the two "new" caps is only 470uf though. Do the resistors near the caps have to be changed too?? because these caps are of a lower capacitance.

Before the recap of the primary caps:



After the recap, using Nippon Chemicon 200V 470uf caps. The Tosin caps which were going to be used was 250V 470uf and it was too tall and touched the heatsinks. And the 250V voltage was higher than the normal caps so I didn't want to risk it.





In the image below, do the resistors need to be replaced because I installed caps with different capacitances??




Thanks.

[stevo1210]

Re: Dell NPS-250KB caps

I started the PSU with the load of a CD drive and a fan. It fired up immediately when i connected the green to black cable.
Looks like my recap job worked after all....

[stevo1210]

Re: Dell NPS-250KB caps

I'm just wondering about the amount of rust around the fan on the casing of this PSU. There's rust all around the fan area inside the PSU. Is there a way to like get rid of the rust to paint some silver over it??
I think that If I paint some silver spraypaint over the rust, it will still rust underneath. I remember a long time ago there was a white liquid in a bottle which my dad used to cure the rust around the rear windscreen of our Holden Commodore, then fill up the holes with silicone to stop water dripping onto the back seats etc. as it rained.
The liquid looked similar to Liquid paper, and this stuff would kill the rust and stop the rusting.
I'm looking for something similar to that stuff, but I have no idea what it's called.

Thanks.

[stevo1210]

Re: Dell NPS-250KB caps

OK, after some benchmarking and burn-in testing, I think I have some form of problem. I have been running burn-in software etc. to test this PSU for problems and I've found none so far. My PC is still stable and I don't see any problems.

But i can hear a high pitched hissing noise coming from the PSU.
It isn't loud, but I can hear it. I am not sure if it's normal to have a hissing noise come from this PSU either.
I also disconnected the motherboard fans and the HDD etc. and the high pitched noise is still there and it's definitely coming from the PSU. I inspected the caps and I have found no problems with them and also the PSU is not hot either, it stays cool throughout the burn-in and PC benchmarking process.

Right now I am using an Intel SE-440BX-2 motherboard for the test, along with a Pentium II 350MHz CPU, 64MB SDRAM and an S3 1MB Trio PCI Graphics card. Is this hissing noise telling me something bad?? like an explosion waiting to happen??

I don't know if it was like this before the recap, because I never booted it up before I did the recap due to the damaged 200V primary cap.

Thanks.

[starfury1]

Re: Dell NPS-250KB caps

You posted further while I was doing this one...yeah hissing noise...dont know but not sure I like the sound of that
hight pitch wine type noise might be transformer coils
KC8 I think and a few others here are very familiar with these so possibly could answer this a lot better then me



==================================

The reason I am saying the following is because maybe why the first cap went was because of an open circuit resistor and the same thing is liable to happen if this was so.

I got a feeling we went through this in another post but as Rainbow
said.
There should be 2 resistors possibly 1/2 watt or 1 watt across (parallel) each capacitor 330K?
(or somewhere round this mark in value)

If one of them has gone open circuit the voltage across the caps is then down to the characteristics of the cap

Since in Australia the caps will be in series across the mains
(240), it is "important" they are there and in OK working order if you want the caps to last a long time.

The reason they are there is to force equal voltage (sharing) across each cap
other wise it will depend on the characteristics of each cap how much voltage end up across it
(which is why they tend to be same type, same values so the specs are close...also why you should replace both if ones blown)
One will mostly become the work horse doing most of the work.
(yeah they do help to discharge them)

(the psu I posted in the psu photo thread with the blown Rubycon had NO resistors, it had seen action for about a year)

Mind you one still will probably be more favored but the resistors will help equal this out

I am not sure what they would be value wise
( but it would be in hundreds of K)
or what they would be marked as circuit wise on that PCB.

all you need to do is check the resistance (with the power OFF! and the psu not plugged in ) of them, I think you Got a digital multimeter
(make sure they are connected to each caps too, like a pcb track hasn't be come open circuit)

So what happens is 320 Volts DC gets divided equally across the caps....they get 160 Volts across each of them.
This also ensures their voltage rating in not exceeded.

The circuit is known as a "voltage divider", it just 2 resistors in series of the same value.

320V-----[resistor 1]--------- 160V--------[Resistor 2]------------GND


320V-------+-[] |-------------160V----------+-[] |---------------GND

Each cap is in parallel with one resistor
(caps are in series so at the 160v point its all joined together)
(it a bit hard to show it in ASCII with the post format)

Anyway thats my understanding of the situation with the main I/P caps

When testing SMPSU the old TV trick is to use a lamp in series with the psu
The idea being if there is a short on the mains side (in this case) the lamp will light up protecting the psu from KABOOM (well thats the theory)

If the psu is likely to be ok it should go bright for a second or so then dimm or out (the circuit of cause is live!)
The quick flash is the inrush current as the caps charge up


I have posted about this lamp setup before.
Ideally a 1 to 1 (240VAC/240VAC) transformers is handy to have as it isolates the psu from the mains.

Humm as to the rust yeah well possibly see if you can get some anti rust spray paint and spray the area.
You could spray the case but make sure that you keep grounds points not sprayed and also make sure there is good earth contact when its bolted in the computer case (so I wouldn't spray were the pcb's bolts down and mask of the screw mounting holes where it goes into the case, so they don't fill with paint ...granted the screws will probably bite through it but still better to keep em clear)

Someone might have a better idea here thought

Anyway hope this makes sense, helps and good to see it does fire up

[starfury1]

Re: Dell NPS-250KB caps

oh ok high pitched ...it probably ok but I'll leave it to others to comment on

like I said sometimes you can hear the wine of the psu...provided its not LOUD

Your young and probably have good hearing...compared to other supplies have you ever slightly noticed it with them ?

The Delta is a little older too so maybe this accounts for it being a little more noticeable

[starfury1]

Re: Dell NPS-250KB caps

Ok here's a link to an ATX 200W old psu schematic
(I have used this before)

Anyway the 2 resistors I am talking about above "on this schematic" are R2 R3 220K (1 Watt)

You will see them across C5 C6 which would be like the 2 caps above

The Mains Front end of this (on older supplies) is more really what should be there unlike Deer and other El cheapo's that throw away half of the front end
(sure it doesn't need to be there for it to work ...just throw away safety and spew RF etc all over the place...it will still work)

Newer psu's Passive PFC and Active PFC would be somewhat different, so its not really applicable (I think) to the supply's you buy today except in the fact safety and RF etc standards must still be adhered to in some manner, plus now power factor correction must be done as well
(this means correcting for the phase shift between voltage and current, both should peak and trough at the same point in time..in phase.)
Capacitance or Inductance will cause a phase shift in current and voltage


yeah, slightly muffed the first post put a line or 3 in the wrong place while editing so if it reads a bit wrong thats why.

BTW stevo going to 250V is quite ok (and possibly better idea really) but touching the heat sink not a good idea...so yeah 200V it is then

[stevo1210]

Re: Dell NPS-250KB caps

The reason I am saying the following is because maybe why the first cap went was because of an open circuit resistor and the same thing is liable to happen if this was so.

The first set of nichicon caps were actually damaged physically. The PSU was dented very heavily on one side and the case is also a little warped because the corner of a computer fell onto this PSU while in transit.

I got a feeling we went through this in another post but as Rainbow
said.
There should be 2 resistors possibly 1/2 watt or 1 watt across (parallel) each capacitor 330K?
(or somewhere round this mark in value)

There are resistors next to each primary capacitor. I have no idea how to read the colour values of the stripes.... normally my father does that for me because I have heriditary colour blindness which makes it very hard to tell the differences between red and green stripes....

Humm as to the rust yeah well possibly see if you can get some anti rust spray paint and spray the area.
You could spray the case but make sure that you keep grounds points not sprayed and also make sure there is good earth contact when its bolted in the computer case (so I wouldn't spray were the pcb's bolts down and mask of the screw mounting holes where it goes into the case, so they don't fill with paint ...granted the screws will probably bite through it but still better to keep em clear)

Someone might have a better idea here thought

Anyway hope this makes sense, helps and good to see it does fire up

I'm actually planning to only paint around the fan area where it's rusted alot. As the rest of the PSU is fine so I won't bother painting those sections, only the fan grill needs some paint.
Looks like i'll have to use paper and masking tape to cover the other areas, especially where the bolt for the ground point is.... unfortunately, it's very close to the fan which is going to be a bit hard in covering....
As for anti rust sprays, I'll have to try Bunnings warehouse to see what they have down there.... Most likely they will have something to stop rust etc.

Hopefully everything i've written makes sense.... well, it is after all 2.06AM here in Sydney, AU.... I must love staying up late on Sunday mornings....

Thanks.

[starfury1]

Re: Dell NPS-250KB caps

Better to look at the bottom then the top and see if you can see 2 that are in series and also connect to the caps.
They may not be near one another either
looking at the pics, I suspect possibly R3,R4, R5 hiding at the back if two look the same to you see if they connect to each other...then the caps
(one has to be the odd man out thought)


I have a delta down the back so will have a look and get back to you tomorrow...not the same model but may well be close enough.

Cheers mate and yeah what am I doing here...I came to look at MB reviews in the P35 chipset hours ago.... LOL

[linuxguru]

Re: Dell NPS-250KB caps

Look for Permatex anti-rust coating or similar at any auto spares shop. There's one that comes in a black aerosol can, sprays a clear spray and cures quickly to form a hard, translucent plastic layer above the rust. The brown rust turns into a black layer below the plastic and stays stable in the plastic matrix.

Make sure that you spray this stuff only outdoors - I'm not sure what it contains, but it helps to keep it out of indoor living spaces.

[stevo1210]

Re: Dell NPS-250KB caps

I just came up with an idea!!

What if I was to sand away the rust with sand paper or something similar and then paint over the rusting area and the use compressed air or a hairdryer to blow away all the rust residues??
Will that idea work or will the rust just regrow again from where I sand down and paint over??
It could save me money because I won't have to buy any anti-rust spray?? Last time I checked a a store around here, it was $12 for a 175grm spray can of anti-rust. $12 is even more than what I paid for this PSU....

Thanks.

[linuxguru]

Re: Dell NPS-250KB caps

You need very little of the stuff - one can of anti-rust lasts for years, and can be used for lots of patchwork - cars, boats, ladders, etc.

[999999999]

Re: Dell NPS-250KB caps

How bad is the rust? This PSU may be several years old already, do we know what kind of environment it was in when it rusted?

If the rust isn't flaking off, you might just take a dry paper towel and rub off any loose particles and spray any paint you have on it, or even spray nothing on it just leaving it be. It's not as though it has to survive being wet outside and does it really matter if it looks perfect? If it has to look perfect you'll end up sanding down, priming, and painting the entire thing to get an even color. Since it gets fairly warm I suggest an engine or stove paint that is baked on (in your oven, electronics removed from the metal casing of course), but personally I would not bother, to me it would be overkill to paint it.

[gonzo0815]

Re: Dell NPS-250KB caps

You can soak the whole case in Coca Cola, the Phosphoric acid will chemically alter the iron oxide.

Then scrub off the lose particles and put on whatever you have handy.
But to be honest, who cares about PSU cases?

The high pitched noise is quite usual for PSU`s, as long as it is not loud.
A loud hissing and whining is always a sing of overload.
it is very typical, so i think its not that important.
Any way, it would not hurt to inspect all solder joints for cracks etc.

[stevo1210]

Re: Dell NPS-250KB caps

You can soak the whole case in Coca Cola, the Phosphoric acid will chemically alter the iron oxide.

Then scrub off the lose particles and put on whatever you have handy.
But to be honest, who cares about PSU cases?

The high pitched noise is quite usual for PSU`s, as long as it is not loud.
A loud hissing and whining is always a sing of overload.
it is very typical, so i think its not that important.
Any way, it would not hurt to inspect all solder joints for cracks etc.

Hmm.... I should have a try with some Coca Cola and then sand the residues away.
About the hissing noise, it isn't loud and it isn't small sounding, but it's there. I can hear it when standing a meter or so away from the unit.
I have a similar unit powering my Media Center PC and it doesn't hiss at all. Only this unit hisses.
It still hisses even without any load, with light load (CDROM and fan only) and heavy load (MB, CPU,RAM, HD etc.) and the tone of the hissing noise doesn't change at all even with or without any load.
The noise is coming from the section close to where the primary caps are. I have no idea if this is dangerous or not, but i'm afraid of it blowing up or something similar.
Everything else seems fine.... PC boots up, stable 5V and 3.3V rails, no problems there. The safety switch and the fuses in my house haven't tripped yet.... So I'm guessing that I'm safe for now??

Thanks.

[999999999]

Re: Dell NPS-250KB caps

The hissing noise is due to some inductive element resonating. Usually it is a transformer, but anything with coils could cause it in worst case scenario. Considering that a (plastic) straw is non-conductive, if you are really serious about this noise you could put that straw to your ear and probe around, though at some point I have to wonder how much time an aged 200W PSU is really worth. I agree with others that a Newton PSU, which is a rebranded Delta, is generally a good PSU per wattage rating, but it's time passes, and your time is spend, and ultimately it's value is low and your time invested at some point has to be accepted as more than the value of the PSU.

Why the fixation on rust? If it's not flaking off and thus a potential sorting problem, such a cosmetic issue on an old PSU is not worth bothering with. If you have a chisel or whatever, that can scrape off scalar rust, whatever remains is good enough to get a reasonable life out of this PSU. If it were a '55 Chevy convertible, maybe more restoration is worthwhile, but it is not, this is a vanilla PSU and while pjretty good for it's era, is not of much value relative to the cost and time to keep it working. IOW, given some time looking for a deal on a replacement PSU, most people (globally) could find a suitable replacement for under $25 USD, and that replacement would have a good expected lifespan remainging instead of wondering what the next part to fail is going to be (mainly capacitors, as they age it can't be cost-effective to replace all of them and yet all are aging).

IOW, it was a 200W full PS2 sized PSU. This is easily replaced, even most came-free-with-case junk new psu may have as long a viable lifespan as the remaining life in this several years old, still being worked on, PSU.

IN summary what I'm saying is, cosmetics are not so important, on a very old low wattage PSU it may be better to accept that if it can't be fixed quickly and at lowest cost, it may be time to abandon it.

[stevo1210]

Re: Dell NPS-250KB caps

The hissing noise is due to some inductive element resonating. Usually it is a transformer, but anything with coils could cause it in worst case scenario. Considering that a (plastic) straw is non-conductive, if you are really serious about this noise you could put that straw to your ear and probe around, though at some point I have to wonder how much time an aged 200W PSU is really worth. I agree with others that a Newton PSU, which is a rebranded Delta, is generally a good PSU per wattage rating, but it's time passes, and your time is spend, and ultimately it's value is low and your time invested at some point has to be accepted as more than the value of the PSU.

Why the fixation on rust? If it's not flaking off and thus a potential sorting problem, such a cosmetic issue on an old PSU is not worth bothering with. If you have a chisel or whatever, that can scrape off scalar rust, whatever remains is good enough to get a reasonable life out of this PSU. If it were a '55 Chevy convertible, maybe more restoration is worthwhile, but it is not, this is a vanilla PSU and while pjretty good for it's era, is not of much value relative to the cost and time to keep it working. IOW, given some time looking for a deal on a replacement PSU, most people (globally) could find a suitable replacement for under $25 USD, and that replacement would have a good expected lifespan remainging instead of wondering what the next part to fail is going to be (mainly capacitors, as they age it can't be cost-effective to replace all of them and yet all are aging).

IOW, it was a 200W full PS2 sized PSU. This is easily replaced, even most came-free-with-case junk new psu may have as long a viable lifespan as the remaining life in this several years old, still being worked on, PSU.

IN summary what I'm saying is, cosmetics are not so important, on a very old low wattage PSU it may be better to accept that if it can't be fixed quickly and at lowest cost, it may be time to abandon it.

Well, I actually want to respray this PSU to make it look a bit more presentable. I also think that the rust is starting to prove as a hazard as it's quite sharp and has cut my fingers on some occasions. I know I could file it down, but I have a can of silver spray paint that could be put to some usefulness.

I'm looking forward to using this PSU in my media center PC because I know that it won't kill my motherboard etc.
Also this PSU is very quiet and I can't hear it running either.

I currently use a DEER rebranded PSU for my media center PC and it really has to go before it does any serious damage.
I also went looking for a decent PSU but all of them are more than $50AUD.... which is roughly $40USD.
For $20USD (around $35AUD) I could only get a Deer or an L&C PSU.

Thanks.