Yes, I shorted the green wire with one black wire & then only I get +7- +8v on 5v rail. and it runs a motor well.
what maybe the reason that +5v is working but +12v not.
can you suggest me how to look for the cause of the problem?
Hello again,
One of the reasons power supplies like these fail is because the electrolytic caps dry out or for some other reason they fail. So one thing you could do is look for high ESR (effective series resistance) in all the electrolytic caps and see if any have very high resistance. I think there are testers for this, but what i did was i pulled all mine out of the circuit board and tested them with a frequency generator and scope. When the ESR goes high it is also probably that the capacitance goes down quite a bit, so you could test for that too.
Alternately if you dont have the test equipment, you can try replacing all the electrolytic caps, especially all the ones that handle significant power like the output caps and the two large converter caps. The output caps are the worst ones. It could be that changing them alone will fix the problem. This kind of problem came about even more when some cheap low quality caps crept into the market some time ago and ended up in some power supplies.
Because you are getting +7v or more out of the +5v line it sounds like the converter part of the circuit is working, but the regulation part is not. This could mean a lot of things like the 12v line is shorted by a bad component (like output cap). The LM339 is often used to detect overvoltage and undervoltage and some other things.
You should note here though that without having the power supply right in front of me to do tests on i can not be sure what is wrong with it. If you do own an oscilloscope however then we can do some tests if you like, but you have to be warned that there are very dangerous voltages inside the power supply case in certain places so there is always risk of a bad shock which could lead to death. There's always that risk so if you've never worked on anything like this it is better that you do not attempt it.
How many watts is the power supply rated for? Maybe you can get a new one for cheap on the web somewhere. Various places sell these things quite cheap these days if they are not too high in wattage. You might find one as low as 15 dollars (USD) somewhere for example.
Do you intend to convert this into a hobby power supply too?