dead arcade (peter chou) power supply

[bluto]

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?

[ChaoZ]

verify that the voltage after the 2 big caps, outputting approximately 300vdc. (Careful)

And you can check the nearby caps providing power to the tl494.

there are caps which die without bloating.

[bluto]

Yea I replaced all of them with Panny's just in case.

[stj]

i'v not seen that model, what's the output amperage on the 5 & 12?

i have schematics for the 7amp model.

[bluto]

The model on the housing says USP-11-150x. The amperage's are: +5v/16A, -5v/0.5A, +12v/6a. The number on the board is UK11 U-SAN 150 V3. I think this is the schematic found here: http://home.comcast.net/~mtpacifico/...ir%20Guide.pdf

[stj]

in your second photo it looks like it may have sparked between 2 resistors!

[bluto]

The two black dots just underneath the two big transistors are from a sharpie. I like to make the polarity of the e-caps before I replace them. I found a couple of old AT PSUs that have nearly identical components. I am thinking about trying to swap out components individually (the small transistors, diodes, etc.) near the controller IC. Any ideas on which ones to focus on?

[japlytic]

TL494 pins 9 (Emitter 1) and 10 (Emitter 2) are most likely to be tied to GND.
So try checking the transistor connected to Pin 11 (usually has base pulled up to +12V)
It pays to replace both drive transistors and diodes connected to these two transistors, along with any electrolytic capacitors connected to the centre-tap of the control transformer and between the emitter of the drive transistors and GND.
Attached is a typical TL494 driver output section.

[stj]

there's an australian site with some psu repair stuff - i'll try & find it.

[stj]

my mistake - it was canadian

http://www.2coinsperplay.com/
http://www.2coinsperplay.com/Tech/Po...ply/Power.html

[bluto]

Thanks for the links. There is some great information there. I think I can fix this thing in due time. It has to be something with the start-up circuit and/or the TL494 (since I was able to make it start by probing with the scopemeter). I will keep at it and update with any progress.

[ratdude747]

find another AT psu and retrofit it?

[stj]

get another arcade supply if it was me, they are cheap.
probably well under $30

[seanc]

What's that cross vent cap?

[bluto]

The cross vent cap is a Jimax 4700uf. It is the only one that I didn't replace. It tested ok though. If all else fails, I will replace the PSU with a standard AT. But I am hoping to fix it for authenticity sake. I also want to try an make a learning experience out of it. I might be learning the hard way though.

[Krankshaft]

Check the startup circuit for fried resisitors or diodes.

There is a chance it took out the PWM IC if a power surge took out the startup circuit.

One IC is the PWM while the other is probably a supervisory IC that shuts the PSU down due to fault conditions.

[robert]

Cant be all that impossible to repair such a PSU.
Check every transistor and diode (best if you have/have access to a curve tracer, but standard test should do ususally), then replace the 494.
Also test all the larger resistors on the primary side, would not be the first time some increased their resistance or opened up without showing it, preventing the supply from starting.
As you said that it could run, i would rule out shorted diodes on the secondary side, also faulty switching transistors.

Oh, i just see the 494 is socketed and responds to touching a lead?
Looks like that socket has a bad contact. Either install a new (precision machined-contact type) or solder the 494 (preferably a new one!) in directly.


If it starts, then also replace that 4700u cap.

[bluto]

Thanks for all the replies. I put the socket in for the 494. I did so to allow easy swapping of the part with ones I scavenged from other psus. I think the original one is dead/has issues. It didn't work when I swapped it into another psu. I probably take the socket out when I know that it is working properly. I will thoroughly check all of the diodes and resistors as you have suggested.

Also note that this psu just refused to come on one day. There was no catastrophic event leading up the the failure. No smoke or blown fuse either. So the theory that some resistor/diode failed due to age and is preventing startup fits well here. I will post again once this thing is fixed!

[stj]

if you replace the psu, make sure you get one with a 5v calibration pot on it.

http://www.happcontrols.com/powersupplies/80006400.htm

http://www.happcontrols.com/powersupplies/80021000.htm

http://www.lizardlick.com/pages/electrical.shtml

[stj]

btw, what game does it run?