I have an arcade machine that developed a dead powersupply. There was no smoke or signs of failure. It just wouldn't turn on one day. It is a peter chou supply which is nearly identical to an AT style computer power supply. While I could just substitute one of those into the metal enclosure, I decided to try and learn something fixing this broken power supply.
First thing I did was replace all the caps. The caps that I removed all tested good, but since this machine has a lot of time on it I figured it was a good idea to replace them anyway. However the supply was still dead. I went through and checked for shorted transistors, schottkys, etc. None were apparent. I checked the big resistors and did find one bad (open) one near the fillter caps. I struggled along for a while and wasn't getting anywhere. I then moved onto voltage testing. The first thing I checked was the TL494CN ic. There was no voltage present between pins 12 & 7 (Vcc & gnd). This could be an issue.
Now here is where it gets weird. I pulled out my scope meter (not really knowing what to do with it) and started probing around. Somewhere along the way the fan that I had hooked up tuned on!! The 5v, 12v, and -5v rails were all reading normal. I figured out that if I touched pin 11 of the TL494CN, the supply would start. I was able to unplug and repeat this several times! So now I tried re-soldering that ic, but that didn't fix anything. Then I had the bright idea to swap the TL494CN with one from another old powersupply. So I soldered in a socket and put the donor one in. It still didn't work and now the trick of probing pin 11 is not working either. And to my dismay, popping the original TL494CN back in did not help either. The pin 11 trick was still not working.
So I am guessing that there is something very minuscule preventing the supply from starting normally. Any ideas on how to proceed?
First thing I did was replace all the caps. The caps that I removed all tested good, but since this machine has a lot of time on it I figured it was a good idea to replace them anyway. However the supply was still dead. I went through and checked for shorted transistors, schottkys, etc. None were apparent. I checked the big resistors and did find one bad (open) one near the fillter caps. I struggled along for a while and wasn't getting anywhere. I then moved onto voltage testing. The first thing I checked was the TL494CN ic. There was no voltage present between pins 12 & 7 (Vcc & gnd). This could be an issue.
Now here is where it gets weird. I pulled out my scope meter (not really knowing what to do with it) and started probing around. Somewhere along the way the fan that I had hooked up tuned on!! The 5v, 12v, and -5v rails were all reading normal. I figured out that if I touched pin 11 of the TL494CN, the supply would start. I was able to unplug and repeat this several times! So now I tried re-soldering that ic, but that didn't fix anything. Then I had the bright idea to swap the TL494CN with one from another old powersupply. So I soldered in a socket and put the donor one in. It still didn't work and now the trick of probing pin 11 is not working either. And to my dismay, popping the original TL494CN back in did not help either. The pin 11 trick was still not working.
So I am guessing that there is something very minuscule preventing the supply from starting normally. Any ideas on how to proceed?
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