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

Arfa Daily wrote:
"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




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.

"James Sweet" wrote in message
news:FUphf.6802$Dx3.4099@trnddc07...
Arfa Daily wrote:
"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


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

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.

"James Sweet" wrote in message
news:1Bthf.6820$Dx3.5242@trnddc07...


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

"Arfa Daily" ) writes:

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 ...

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

"Michael Black" wrote in message
...
"Arfa Daily" ) writes:

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 ...

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