Hi,

In need of advise for Repairing PSU for POS terminal? Input is 230V AC
and Output: DC 5V/24V/12V/-12V/-19V with backlight inverter.

PSU could be powered on but produces flickering empty display and has
backlight.

Thanks.

SeventhPrince七少爷 wrote:
Hi,

In need of advise for Repairing PSU for POS terminal? Input is 230V AC
and Output: DC 5V/24V/12V/-12V/-19V with backlight inverter.

PSU could be powered on but produces flickering empty display and has
backlight.

Thanks.



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

SeventhPrince七少爷 wrote:
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.

"SeventhPrince???" wrote in message
oups.com...
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