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LCD backlight problems

K

Kirk S.

Jan 1, 1970
0
Hi,

Got a KOGI L4BX monitor with a picture and no backlight. Can I simply use a
VOM to test the output from the inverter? Is it typically worth replacing
the bulbs? The inverters seem to be about half the cost of the unit so
replacement of that part isn't really financially purdent (or is it?).

Thanks for any help!

Kirk S.
 
No, you cannot simply use a VOM, you need an oscilloscope to check the
output.
Also be aware that the high frequency and relatively high backlight
voltage will require some additional care and safety when measuring.

The inverters are usually very easy to repair for a well experienced
technician with lots of smps type of knowledge.
 
J

James Sweet

Jan 1, 1970
0
Kirk said:
Hi,

Got a KOGI L4BX monitor with a picture and no backlight. Can I simply use a
VOM to test the output from the inverter? Is it typically worth replacing
the bulbs? The inverters seem to be about half the cost of the unit so
replacement of that part isn't really financially purdent (or is it?).

Thanks for any help!

Kirk S.


Not really, you'd probably just blow the DMM. You can get CCFL lamps
very cheaply and just wire one of them to the inverter for testing,
otherwise the inverters can often be repaired or replaced with something
cheap and generic.
 
You wil be suprised by how many CCFL inverters I have repaired by
simply replacing the marginal ESR capacitors that caused excessive
start up current that blew one of the surface mount fuses.

Very rarely do I see one with any output device damage unless the fuse
had been replaced without fixing the capacitor problems first.
 
K

Kirk S.

Jan 1, 1970
0
Thanks for the information...

I do have a scope however I'm not very experienced using it. Can I simply
use the 1000x probe on the output and check for a waveform? It isn't like
checking the chopper on the hot side of a SMPS.

Everything is surface mount on the inverter. Do the ceramic caps go bad?

Kirk S.
 
Not all inverters have electrolytic capacitors on them, but most do
have some square capacitors that are not unmarked ceramic surface
mounts but usually tantalum or electrolytic surface mounts.

Usually the scope probe does not even need connected, just place it
near the inverter transformer and if it is running it will be clearly
visible by a waveform that is not the usualy 60Hz noise waveform. I
use this to confirm flybacks and sometimes horizontal coils to see if
something is running or not. It does not tell me if it is running
correctly though.

Start by checking the fuses, should be a main power in fuse and
probably one on each output transformer and transistor feed. If any of
the fuses are open, check for obvious shorted components. If no shorts
are found, you can chance replacing the fuse, but there still may be a
problem. Without a full schematic or lots of experience, it will be
difficult to determine what caused the fuse to blow before replacing
it.

If it seems to work with the new fuse, replace the lamps, they might be
causing some excessive current draw due to age.
 
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