Help me repair a TT850W PSU

[KAMENi]

Re: Help me repair a TT850W PSU

i keep it on when there is no load.
once i connect anything to 5v wire, it shuts down after 1 or 2 seconds top

[i4004]

Re: Help me repair a TT850W PSU

so without load you have 5v there?
you should check the protection ic(if it has one) or main pwm ic to see why is it shutting down.
could be faulty ic itself, or some passive component near it.

but what bothers me is the fact other voltages are fine. this can suggest transformer is faulty, because it's the only thing separating output voltages.
if the primary side is working ok(and if you have(for exmple) 12v ok it probably is)
i really see no way of 5v being faulty other than trafo.

anyhow if you don't succeed with repair you can send it to me, so i'll try to see what's happening.....if you wish.
if i find something i won't charge you anything(but i won't be buying comeponents to repair it either...i would send it back to you), if i don't, to trash bin it goes...before that i'll remove the trafo and then dismantle it, as final post-mortem..hehe...

[KAMENi]

Re: Help me repair a TT850W PSU

i doubt it can be the transformer because if it were malfunctioning it wouldnt give out -5v either
i will replace the ic1 ASAP

[KAMENi]

Re: Help me repair a TT850W PSU

i have unsoldered the ic1. it's a cm6800g.

i'll post the results when i replace it.
if anyone has any other ideas, you're all welcome to share them

[KAMENi]

Re: Help me repair a TT850W PSU

of course, I cant find this IC anywhere in Zagreb.

i have asked a shop if they can order it, but they'll let me know on monday, tuesday.

[Gianni]

Re: Help me repair a TT850W PSU

once i connect anything to 5v wire, it shuts down after 1 or 2 seconds top

I agree with i4004.

Now you have removed the IC so I think you can't do more test.

Have you tried to load only the output +3.3V or +12V without loading +5V?
If you can do this test, maybe using some power resistor, and the PSU stays on, than the problem is only on +5V.
I think it is possible that, due to parasitic elements/coupled inductance/bad joints, the +5V rail is OK without load and as soon as you load it, the voltage drop and the PSU shut down for undervoltage protection.

This could be related to faulty transformer too.
I don't know how the transformer is built but if there is a cold/cracked joint inside it, that could explain the problem.
I had built (long time ago) for job some transformers, they had the secondary output made by a thin copper foil. To connect the output to the PCB I had to solder the wire on the foil and than I had to wrap it on the bobbin. If the solder is not good, when you wrap the foil on the bobbin you can get some crack between wire and foil.
So if you have a bad joint inside the transformer you can have the right voltage as long as you don't sink current, when you try to get current than the voltage drop due to the high resistance between foil/wire.
A scope and an active load would be useful to test it but I know they are no "common tools" and moreover they are expensive.

I hope you can get the IC because it is difficult to find some kind of components.

Keep us informed
Gianni

[KAMENi]

Re: Help me repair a TT850W PSU

i have tried loading only 12v and it works normal (i connected old cpu fan). you know what load on 5v does. i didn't try on 3.3v because i didnt think the fan would start working on so low voltage.

do you think i can check the transformer myself? if i find it malfunctioning, i can get it rewound so that wouldnt be a problem


p.s. i can still solder the IC back on while i'm waiting for the shop to let me know if they can get it

[linuxguru]

Re: Help me repair a TT850W PSU

Put it in a DIP socket - it will simplify future maintenance. Use machined sockets with cylindrical gold-plated pin-sockets.

[KAMENi]

Re: Help me repair a TT850W PSU

i didnt exactly unsolder the IC itself, i unsoldered the whole PCB board with it. it's 1 in x 1 in

[Gianni]

Re: Help me repair a TT850W PSU

do you think i can check the transformer myself? if i find it malfunctioning, i can get it rewound so that wouldnt be a problem

Well... you can try to check but the best thing would be to check the waveform on the +5V diode to see what's happening when you load it.
Since you don't have a scope, you can only check the resistance between the +5V winding respect to ground, it must be very low in the order of few mohm.
I don't think you can rewound it because probably ferrite are glued and you must break them to free the bobbin and winding.
To replace them you need the exactly type (core material/shape/air gap if present) and very often they are not standard/commercial ferrite.


Gianni

[KAMENi]

Re: Help me repair a TT850W PSU

all 3 secundary points have 0.2 ohm to ground

[Gianni]

Re: Help me repair a TT850W PSU

all 3 secundary points have 0.2 ohm to ground

So the secondary windings are not open; I have measured a secondary winding of a trafo with Agilent 34401A DMM, it's a 6½ digit, and the resistance was 6mohm. I think the 200mohm should be OK if you are using a common DMM, probably if you short the DMM probe you still have 70~100mohm due to the DMM probe cables. It's hard to say if the remaining 100~130mohm are due to DMM limit/accuracy or if they are real.
In case they are real, it would justify the voltage drop under high load: by Ohm's law if you apply 10A to a 130mohm resistor you obtain 1.3V, so if you lose 1.3V on 5V, you have 3.7V as output voltage.

At this point I really don't know what to tell more to help you ; I would like to have the PSU here in my lab to understand what's going on.

I was also thinking about a problem with overload protection, but in this case, when it is triggering not only +5V must drop but also other rails.

i have tried loading only 12v and it works normal (i connected old cpu fan).

This is a very light load, I think it can consume around 160~200mA, and it is very low compared to MOBO current consumption.
If you don't have power resistors to check PSU with higher load, you could use some car's lamps, they are rated for 12V and can draw much current.
You can use them also on +5V and +3V output, you can connect some lamps in parallel to increase the current.
The only problem with lamps is that they have a very low resistance when they are off, so maybe they can overload the PSU at start up and until they are in steady state condition.
Anyway I would give it a try, lamps are not so expensive and you can keep them as spare parts for your car.
If you use them, protect your eyes from the light and place them on something that doesn't burn, they become very hot when are on.

I would consider the offer made by i4004, if you can send the PSU to him maybe he is able to fix it.

Ciao
Gianni

[i4004]

Re: Help me repair a TT850W PSU

and if i can't i'll send it to gianni: god damn, machine shouldn't win!
hehe...

anyhow, i do have a scope.

[KAMENi]

Re: Help me repair a TT850W PSU

i'll try to replace the IC if they can order one. if not, i will send it to i4004

thanks for your support guys

[KAMENi]

Re: Help me repair a TT850W PSU

unfortunately, i didn't find this IC anywhere in Zagreb, and no shop can order one.

i4004, thank You very much for the best intentions, but i'll have to say no to your offer. it's not my PSU and my cousin doesn't want to send it

thank You anyways


if someone should happen to run into a store that has this IC in its offer, please tell me their website, or contact via e-mail so i can order it


once again, thanx for all the help and support from both of You, Gianni and i4004

[Gianni]

Re: Help me repair a TT850W PSU

I'm sorry our help didn't solve the problem but at least you have done your best.
I don't know where to look for the IC but I doubt you can find it easily.
I can't check RS because is down right now, I serched in Digikey but I don't find it.

Good luck and don't give up till you know there's nothing more to do.
Gianni

[PCBONEZ]

Re: Help me repair a TT850W PSU

Check around for dead PSU's that might happen to have one.
.

[KAMENi]

Re: Help me repair a TT850W PSU

i will surely check every psu that comes into my hands for that IC

if anything new should happen, i'll let you know

thanx again