Advise for Repairing PSU for POS terminal

Sounds like a classic failure of electrolytic capacitors on the output side.

I have checked with Ohmmeter for all the caps and they turn out good. There's only 2 caps with the value of 1uF to 47uF with a voltage rating of 25 to 50v. I have changed only one of them (47microF 50v) as I am yet to source for the other (10microF 50V).

Thanks

You can't test caps with an ohm meter, you need an ESR meter. Have you measured the outputs of the PSU?

ok. not yet. will measure now. Thanks

Did the measurement. All the rated voltages come out good.

This should have been your first check, as it would seem to indicate that the supply is good, however, with a switch mode power supply, you can't toatally declare that to be the case, without one further check.

Whilst the voltages may APPEAR to be correct, when measured on a multimeter ( anolog or digital ) they must also be checked for switching hash using an oscilloscope. It is common to see peak to peak noise as big as the DC level of the rail, caused by poor electrolytics on the secondary side. Microcontrollers, and other circuit sections, will not tolerate this sort of disturbance on their rails and reset lines.

Judging by the observations you have made regarding low value / voltage electrolytics on the primary side of the supply, you have been trying to make use of the advice that I offered another poster further down the list, who was having trouble with an external tape drive PSU. The advice that I was giving there, related to a dead switcher with no fuse blows. You will also have noticed that I said to check the electrolytics with an ESR meter, as James has also said.

If you do not have an ESR meter, or do not know what one is, I would suggest that you Google the subject, and acquaint yourself. If you intend to go on trying to repair switch mode power supplies without one, you will consistently fail. It is the single most useful item of test equipment that you can have in your workshop for this type of work, and will save you hours of diagnostic time. Look up Bob Parker in Australia. He produces a very good ESR meter at very reasonable cost, and distributes world-wide. His meter has an unambiguous digital display, and I have been using one daily, for a number of years now.

Arfa

Arfa

Or just replace all the capacitors, unless you repair a lot of things like I do, an ESR meter is not always a great investment, capacitors are cheap.

Bob Parker's meter retails in the US, in kit form, for around $60. You're not going to buy too many quality caps for switchers for that money, so I reckon that it represents a good investment for anyone hoping to make money by repairing stuff with switchers in it. Also bear in mind that if you're doing it for money, your time should be worth at least $15 an hour, so changing all the caps as opposed to spending 5 minutes finding and replacing the one that's really faulty, has got to be worth the investment in a very short time, hasn't it ?

Arfa

Very true, however I was under the impression this guy is just trying to fix his own unit, if he's hoping to do it to make money, or even do it regularly then yes the ESR meter is a wise investment, I love mine but I fix a lot of stuff in my hobbies.

The amount of posts that he puts in here asking questions, then I reckon that if he's not trying to do it commercially, he must have a lot of buddies with dud gear ...

Otherwise, then yes, I agree with you, just change all those pesky caps. Actually, there is a school of thought that replaces them all anyway - witness the repair kits for say satellite receiver PSUs that contain every cap, but I've always had a foot firmly in the other camp that says if it ain't faulty, don't replace it !

Arfa

I hadn't noticed that he was posting frequently.

But if you look at his original post here, it doesn't indicate anything but a beginner. It would seem he didn't even measure the voltages out of the power supply, before jumping to the conclusion that the PS was faulty.

Michael

But if you look at this group regularly, you will see quite a lot of posts from him over a considerable time, and in much the same vein so, if he is ever going to advance beyond beginner, he's gonna need some proper test equipment to go with his enthusiasm.

I not suggesting that he goes out and spends thousands on a scope, like we professionals have to, just a $60 christmas present to himself that he can have fun building and the satisfaction of using, for years to come.

All I'm saying is that electrolytics give so much trouble in all areas of modern electronic equipment, that an ESR meter is as useful to have around as a multimeter. Trying to work without one is like trying to remove your engine's spark plugs with an adjustable wrench ...

Arfa