DIYbanter - View Single Post - polarity of power cord

zirath

Arfa Daily wrote:
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Tim wrote:
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Arfa Daily wrote:
"zirath" wrote in message
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We recently got a gateway fpd1520 (15" lcd flatscreen) monitor
from ebay. It didn't have a power cord. It says it's a 12v dc
2.5a but it doesn't give the polarity. A person from gateway
said he thought it wouldn't hurt the monitor if it got plugged
in backwards but I'd rather not try it.

Would appreciate any help.
If it has got any external metal parts such as RCA (phono)
connector outers, BNC connector outers, 'D' connector surround
etc, or possibly screws for a stand, as these often go right
through the plastic case, and into the internal chassis, then try
measuring from any such metal to each of the DC power input
connector's terminals in turn, using an ohm meter. Chances are
you'll find a direct connection, and that will be your DC ground
( "-" ) connection. The other will then be the "+". Assuming that
it's a 'standard' co-axial DC connector, on most modern
equipment, 'pin' is "+" and side contact is "-" although that's
not cast in stone. Be aware when you are obtaining a replacement
PSU, that the plug is often a slightly abnormal size, being a
little larger than those you typically find on 'general' power
supplies. Also, make sure that you get one well rated for the
job, as these monitors do draw quite a lot of current, and may
well surge up close to the quoted 2.5 amps at startup, as the LCD
backlights first fire up before settling to their run current.

As to whether it would be safe to reverse connect it, I wouldn't
like to say. Some equipment is perfectly well protected against
such 'consumer antics', but it is by no means guaranteed, and if
it is not adequately protected, the result is often an item
that's fried beyond repair, for no other reason than unobtainable
power supply devices, as many previous posts on this subject over
the years, will attest ...

Arfa

The ohm meter showed something like a capacitor on the center pin
(it went to 0 and then back towards the center). The side showed
no resistance.

The monitor went on briefly and then went black when plugged into
the power. Seems like it's a dud. Not sure if it's worth trying to
repair it or just return it.

Is there no little pic of the connector around where it give the
info about power? Usually there is a universal icon used that
indicates polarity, kinda like a big C around a dot. Most, but not
all, coaxial power connectors have the minus side on the outside,
that way if it touches a grounded side, it will not destroy the
adapter. If the adaptor cannot start the monitor because it is not
powerful enough, it will sometimes just flash the backlight then go
off.

- Tim -
The adapter symbol shows center + and side -. It is labeled 12vdc
4a. The monitor is 12vdc 2.5a so the adapter should have enough
power.
Before condemning the monitor, I would feel inclined to just try a
different power supply (bearing in mind that the one you are using
now is not an 'original'). The reason that I say this is that there
can be quite a high initial pulse of current demand, as the
backlights strike, and it just might be that the power supply sags a
little when hit with this, even though it *says* that it is rated for
4A ...

Arfa

Thinking about it again, were you trying it with a signal going in ?
LCD monitors, like their CRT counterparts in many cases, don't just
'idle' when there is no signal. They do just like you say - that is
power up long enough to check if they can detect an input signal, and
if they can't, go back to an inert condition.

Arfa
I tried it in a few different conditions: with the monitor on 1st and
then the computer - it showed the computer booting for a few seconds
and then went black - then it came on again after the boot was finished
for a few seconds and then went black again; with the computer on and
then the monitor; with the monitor disconnected from the computer (the
manual says it's supposed to bring up a diagnostic screen in this
condition but it didn't). The screen stayed black the whole time in
subsequent attempts.

The power led would go from yellow to green when the computer started
booting but the screen stayed black.

I also noticed that the screen was not completely flush in the
housing - like someone had tried to, or did, open it and didn't close
it completely.
The fact that the light stays green, is a good sign. What you next need
to do, is to shine a strong desklamp or hand flashlamp at the screen at
an angle, after it has gone back to black. If you can then see an image
on the screen, that indicates that the backlamp tubes are not staying
alight. This could well be because either the inverter is faulty, or
that a worn tube is making the inverter shut down. Either is quite a
common fault condition.

Arfa

I was able to see the image on the screen - but the monitor remained
black the whole time (it didn't go back to black).
(I shined a bright light on the monitor at an angle).

OK. Well that tells us that the whole monitor part is working, and the
failure is in the LCD screen backlighting circuit. It is generally a bit
fiddly, but it is repairable if you are that way inclined. Unless you are
able (or want to) just throw it back where you got it from.

Arfa

I'm into trying to repair it.

Is there info somewhere on how to do it?

Thanks for your help.