Repair SXJ2010 power supply board for learning

[mikey5791]

Hi,
Got this junk brand ATX power supply board extracted from a dead power supply. There is only a label of SXJ2010 V1.4 from underneath the pcb. It only has 5v standby power.
I removed the original 5A glass fuse and soldered on a 60w incandescent bulb as current limiter. Upon power on to mains 230v ac, the bulb lighted brightly and then shuts off. By using a multimeter, i got about 100vdc on each of the main cap of 470uf, 200v. There is no other voltage on the secondary side apart from the 5vsb purple wire.
My question is it safe to take measurement of dc voltage from the three transformer on the primary side? I guess my negative probe should be on the negative side of main cap while positive probe should place on the primary transformer leg. I am using this cheap board only for learning purpose and not to power any other electronic item.

[stj]

Re: Repair SXJ2010 power supply board for learning

check the 4 rectifier diodes, you should have about 160v on each cap

[mikey5791]

Re: Repair SXJ2010 power supply board for learning

check the 4 rectifier diodes, you should have about 160v on each cap

Changed one of the partially open IN5408 rectifier diode and got reading of 148vdc on each of the main cap. Still no output voltage except the 5vsb.

[lotas]

Re: Repair SXJ2010 power supply board for learning

In order for the remaining voltages to appear at the output, you need to connect the green wire (PC-ON) to the black one (GND).

[mikey5791]

Re: Repair SXJ2010 power supply board for learning

In order for the remaining voltages to appear at the output, you need to connect the green wire (PC-ON) to the black one (GND).

Fyi, i had connected the green wire to the black one using a metal paper clip. There is still no other output such as 3.3v(orange), 5v(red) or 12v(yellow) except 5vsb on the purple wire.

[stj]

Re: Repair SXJ2010 power supply board for learning

check / replace any small electrolytics.
by small i mean under 100uf

[Per Hansson]

Re: Repair SXJ2010 power supply board for learning

Fyi, i had connected the green wire to the black one using a metal paper clip. There is still no other output such as 3.3v(orange), 5v(red) or 12v(yellow) except 5vsb on the purple wire.

Most power supplies will require a small load to work, try connecting an old 3.5" hard drive to it for example.

As for your question about probing please read this thread first:
https://www.badcaps.net/forum/showthread.php?t=96508

[stj]

Re: Repair SXJ2010 power supply board for learning

this one has load resistors to help kill the output caps ;

[mikey5791]

Re: Repair SXJ2010 power supply board for learning

check / replace any small electrolytics.
by small i mean under 100uf

Fyi, earlier on i had replaced those small caps under 100uf especially those hiding under the primary side heatsink like c6 and c5(both are 10uf50v type). Then there is c16(10uf50v) near to TL494 pwm which i kept as is since the esr is less than 0.5 ohm.
Also there are two small caps positioned near LM339 voltage quad comparator at c22 which is originally 1uf50v but esr is high at 3.5 ohm. Too bad i ran out of 1uf50v so i replaced with 10uf50v. Another cap at c24 which is originally 2.2uf50v but esr is high at 3.1 ohm. This i too replaced with 10uf50v.
The last cap under 100uf is at c29 (near PG grey wire) which i replaced the exact replacement of 1uf50v.
Fyi, under the second heatsink at c14 the original cap is 100uf50v but esr reads 0.13 ohm. So i replaced with 470uf25v new cap due to out of stock for the original spec. In addition, i also unsoldered all the secondary caps and tested for capacitance and esr and replaced those with esr higher than 0.33 ohm.

[mikey5791]

Re: Repair SXJ2010 power supply board for learning

Most power supplies will require a small load to work, try connecting an old 3.5" hard drive to it for example.

As for your question about probing please read this thread first:
https://www.badcaps.net/forum/showthread.php?t=96508

Fyi, i did connected an old 3.5" hard drive as load but still no other output except the 5vsb. I had read through the thread on danger of touching the heat sink during mains power on. Will remember your warning and the extra caution while doing any live probing.

[mikey5791]

Re: Repair SXJ2010 power supply board for learning

Will appreciate your kind assistance in getting the required output. Thanking in advance.

[lotas]

Re: Repair SXJ2010 power supply board for learning

Well, now try to remove the lamp and put the fuse, and also connect green to black.

[stj]

Re: Repair SXJ2010 power supply board for learning

try changing the cap at the 494, esr is not always the important thing - for that cap it's the storage / ripple current
now i know the chips - i'll see if i can find any similar schematics

[lotas]

Re: Repair SXJ2010 power supply board for learning

Here the scheme is something like this.

[mikey5791]

Re: Repair SXJ2010 power supply board for learning

Well, now try to remove the lamp and put the fuse, and also connect green to black.

Done removing the lamp and put in the 5A glass fuse, connected green to black wire. After power on mains, there is still no other output except the 5vsb.

[harp]

Re: Repair SXJ2010 power supply board for learning

When device is not connected in main, do you have any short on output, like resistance lower of 10 ohm on any rail ,12v, 5v, -5v, 3.3v? If you have the APFC chip on it, connect toaster, not bulb.

When device is connected to main, try observe gray wire (PG have to be 5v if ok), and other output, is they climb, unstable, faling, briefly show ok...

Measureing in DC voltage is safe (for equipment) if your measure equipment is not connected to main, that means it work on batteries.

I will look up for testing any output diodes/mosfet and optocoupler

[mikey5791]

Re: Repair SXJ2010 power supply board for learning

try changing the cap at the 494, esr is not always the important thing - for that cap it's the storage / ripple current
now i know the chips - i'll see if i can find any similar schematics

The cap at the TL494 pwm is located at c16(10uf50v) now i replaced with a new 10uf100v. Upon mains power on, there is still no other output except the 5vsb. Any idea or places to check for further diagnosis will be appreciated.

[mikey5791]

Re: Repair SXJ2010 power supply board for learning

When device is not connected in main, do you have any short on output, like resistance lower of 10 ohm on any rail ,12v, 5v, -5v, 3.3v? If you have the APFC chip on it, connect toaster, not bulb.

When device is connected to main, try observe gray wire (PG have to be 5v if ok), and other output, is they climb, unstable, faling, briefly show ok.
I will look up for testing any output diodes/mosfet and optocoupler

May i know how to measure short on output (10 ohm on any rail)? Do i put black negative probe of dmm on ground and red positive probe on the output rail, then set dmm to ohm resistance scale? Kindly advise.
In addition, when the psu is connected to main, there is no output on gray wire (not even 5v). Only 5vsb from purple wire is detected.

[R_J]

Re: Repair SXJ2010 power supply board for learning

Check if you are getting VCC (pin 12) to the TL494 ic, this voltage is also supplied by the standby circuit, then check if you have voltage supplying the drive transformer (T2 in schematic), you may have an open resistor like R13 or R10 (likely marked different on your board)

[mikey5791]

Re: Repair SXJ2010 power supply board for learning

Check if you are getting VCC (pin 12) to the TL494 ic, this voltage is also supplied by the standby circuit, then check if you have voltage supplying the drive transformer (T2 in schematic), you may have an open resistor like R13 or R10 (likely marked different on your board)

On pin 12 vcc TL494 ic, i got +13.7vdc stable.

Sorry i am not able to identify and relate the drive transformer (T2 in schematic) as i tried to trace each of the transformer pin to the schematic one. It is strange the pin connection to components on board also do not match what was on the schematic.
But voltage supplying to the middle transformer marked YW EE-16-10(primary side 5 pins and secondary 3 pins) is less than 5vdc.
While voltage supplying to the biggest transformer marked DX EC-35-10-02 (primary side 2 pins and secondary 5 pins) also measured less than 5vdc. Is this normal or something not right?