I am having a problem in a ATX supply. The problem is when I plug PSU to 230 V AC the PSU won't start thus I opened and tested 5vSB not present on wire and also not on diode (IN5822). Thus started measuring input side, 300 V DC present at BIG caps so traced the 5 V section (hot side). HOT side

OC1,2,3 are optocouplers also the S8050 transistor is a non standard transistor (some values missing in circuit). Also I have marked 5 V on secondary side on transformer kindly ignore it. Later I am able to start the standby section I will explain later, but shorting green and black starts the fan for a second and shuts.

Now how I able to start. Let me explain. While plugged in to live I connected multi-meter lead to auxiliary and found no volts then I connect lead to collector of C5027S and observed that multi meter making little noise while connecting and 300 V on it and then checked the standby section output it have 5 V. Checked green wire found 3.5-3.8 V shorted to ground still fan spin and stop.

Later just testing random resistances and diodes and just connected on one lead to one pin OC1 and fan starts but till I connected the lead as soon as I lift the lead it shuts (that pin goes to 3 no. pin of supervisor IC (LP7510) . Can’t figure out the problem but I have suspicion about C5027S. I am not able to find its replacements but on this website mentioned "2SC945 it's possible to replace with BC639. KSC5027 is NOT 2SC5027. On his place it's possible to use MJE13007"

Kindly help me can't figure why it doesn't start and if started then why no fan shuts. Reply if any additional requirement. Find some replacement MJE13005, also can't figure out 2SC5027 is same or not.

Back side PCB: LP7510_marked

similar circuit: similar_circuit

  • \$\begingroup\$ My suggestion is to check the voltage across each of the 430k resistors. It is not uncommon for them to fail. These are used to leak enough current to start the psu. If they are open circuit, the circuit will not start. Probing with a multimeter can cause the circuit to begin working. The little electrolytic capacitor drawn above the 1.5k resistor is also a common failure point. The simplest way to check is to replace it. It’s value is not too critical but it must be in good condition. \$\endgroup\$
    – Kartman
    Sep 5, 2023 at 12:19
  • \$\begingroup\$ Thanks @Kartman for reply i have checked those resistor their values tested good (tested out of circuit) after first resistor around 124v and after second milivolts (multmeter not accurate for milivolts :) also capacitor mentioned is not bulged, physical apperance seems good anyway i will replace it. i am suspecting that switching is not happening until i test it with multimeter. \$\endgroup\$ Sep 5, 2023 at 12:37

1 Answer 1


About the replacement for 5027, I read in another forum 13005 can be used as it's just for +5Vsb.

  • 1
    \$\begingroup\$ megan russel - Hi, (a) Stack Exchange is a Q&A site, not a forum. Please see the site rules in the tour & help center as they differ from those of a typical forum. (b) It's not allowed to ask new questions in an answer, as you did, so I have removed that part of your post. (c) Questions asking for shopping recommendations are off-topic as they are too localised and quickly become obsolete, so they don't meet the goals of this site. Therefore this is not the place to ask which Indian shops to buy things, sorry. \$\endgroup\$
    – SamGibson
    Sep 12, 2023 at 18:31

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