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ATX Power Supply Problem???

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Who is your answer to sawwa7?

Do you know what the -5v actually does?
I found another several year old PSU that also does not have the -5v today.
 

Something must be backwards or miswired for this to be happening. Are the -5V and -12V that you are measuring well-regulated? How many outputs are you supposed to have and what are their voltages supposed to be? Is the connector wired incorrectly? Is it going to the correct pins on the power supply output. I think this might be a physical error rather than a circuit error.
 

VSMVDD is right,don't waste your time trying to repair it.but maybe this will help
 

Oh come on guys, it's not a waste of time.
PC PSU's fail all the time & the experience gained in trying to repair or even figure out what has blown in one can be very helpful.

Other wise what happens to the PSU? it ends up being thrown away and making more landfill.

I don't know about the country that you live in but in the UK we are running out of landfill sites/places to bury our rubbish and a PC PSU will take many many years to decompose.

The repair may only cost a few pounds to replace a few transistors or may even be as simple as a bad joint which costs nothing but a bit of his time & if he "wants" to spend that time and learn more then surely it costs us nothing to help rather than be snobs & tell him not to "waste his time" and the PSU end up as waste.
 

one of the snobs updated his post to help him :)
regards
BB
 

Power diodes that have gone short circuit - caused by overvoltage - is a
common fault in switched mode PSUs.
Measure them all - especially the TO220 ones with heatsinks.
 

listen man
carefully

if you try to measure raw voltage from it without dealing with the sense line

that most atx and also most AT types need a signal supplied

the psu will sit there and NOT supply some rails

other than this the lack of both +12v and +5v is pretty impossible

this would mean than two outputs from the switching transformer have failed

the +12v sometimes comes from a diode to220 package
the +5v also comes from a similar package

- voltage comes from 79 style regulators

however sometimes have there own transformer
also using a single fet driver stage

so you could find the main set of chopper driver fets have failed
check these for shorts Source to Drain {on beep range}
there WILL be a low resistance Gate to Drain this is normal
if you see two transformers this will be the case

also there will be some 1uf electrolytic caps can dry up
check ones near to heatsinks esp the lo value ones

most psu have .047uf electrolytic caps in the pwm stages {around the lm723 ua494 etc oscillator regulation driver chip}

i would change all the low value caps and test the fets etc as described

what to do is take a picture of it
from above a good quality one
and post it

the eagle eyes amoungst us will spot ANY problems from this

there are billions of designs ive seen atleast 200 different psu's
circuits for this
but + voltage comes from the diode packs and not regulators

it is unlikely these have failed or there would be a bad smell from it

the first thing i do is take the board out and sniff round it

if you know the smell of fried semi's youll find the problem

payes to have a good sniff around
{after you brush it out of dust and take the pcb out the case}

dont forget to use your DVM on beep range
to beep test for short caps
+ - meter probes contacts one way beeeeep the other beep
it's an 'ok' cap or cool enough

if you find a short you can track it to the exact component using the lowest resistance range {usualy 10 or 20R} look for the smallest resistace
above this is the fried part
put the negative probe to the ground of the voltage output side {any black wire}

and test backwards using the pos probe to find shorts in psu rails if any


most often if both +5 and +12 have failed
there is a sense problem

or the usual lm723 regulator chip {14pin} has failed

this wouldnt nessisary effector the - voltage as this prob isnt regulated by the 723

sometime its a lm723 sometimes a ua723 etc

there are a few varient chips used by chinese makers

try these steps
and let us know


im sorry i put you off just trying to save you some time


but if you want to learn pictures in electronics
speek louder than 1billion words of texts


:D

Added after 11 minutes:

cyberrat said:
Oh come on guys, it's not a waste of time.
PC PSU's fail all the time & the experience gained in trying to repair or even figure out what has blown in one can be very helpful.

Other wise what happens to the PSU? it ends up being thrown away and making more landfill.

I don't know about the country that you live in but in the UK we are running out of landfill sites/places to bury our rubbish and a PC PSU will take many many years to decompose.

The repair may only cost a few pounds to replace a few transistors or may even be as simple as a bad joint which costs nothing but a bit of his time & if he "wants" to spend that time and learn more then surely it costs us nothing to help rather than be snobs & tell him not to "waste his time" and the PSU end up as waste.


dont worry by the time landfill disappears

london will be gone also
so we can use it

this isnt a problem anyway

as its made of metal you can be double shure some worker or a magnet will pluck it from the trash squash it up
and youll be driving in it next year

its the cars that are the problem
by the time the gas runs out there will be no energy to change them into something else anyway
 

agreed. if you pull the power supply from the computer and just try to run it on the bench it doesn't work the same. you need to bias the +5V supply (red wire) with a 10 ohm 10Watt resistor. also, you must short the +3.3V (brown, but sometimes orange) SENSE wire to the 3.3V supply orange wire. typically the sense wire is slightly smaller gauge wire (look at the AWG printed on wire insulation) or the sense wire is brown or there are 2 wires going to one spot on the main connector (connector with most number of pins). these 2 things must be accomplished for your atx power supply to work correct 1) bias 5V rail, 2) solder sense wire to 3.3V rail

goodluck!

Mr.Cool
 

i think the problem is fet
or inversion is possible
kindly check all the joints
and fixed them
regards
 

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