Tip & How-To about Televison & Video
Hi.
Before proceeding I assume you have technical training along with the multimeter. A full
testing must be done with power on, and for safety reason must not be
done by an untrained user.
The tip contains only general indications. The reader is supposed to have the training to apply in practice the testing method below.
If there is no power at all, then the responsible is almost always the PSU.
When there is no power, test AC in to PSU. If there is no AC in (read
volts, or read ohms on power cable), that means that the power cord
must be replaced.
If there is AC in, then test the PSU section near to AC input. You
always find a resistor or a fuse in there . Check if it has blown.
After that you test the rest of PSU component. If there is no power at
all , and problem is not power cord, it is often possible to spot at a
glance a leaky or bulged capacitor.
If there is power, but the monitor stays in standby, then te PSU is
forced to low voltage only by controller board. This happens when a
fault had been detected.
In this case testing the PSU becomes more complicate. The technician
must locate the control lines from controller board. This control lines
operate at lower PSU voltage (often +5V), and turn on higher voltages
on PSU board.
The inverter may use for example a +20V line from PSU. The +20V line is
turned on by the controller board, closing a +5V control line to PSU.
When PSU is in standby mode, the technician testing the PSU must bridge
the +5V line with the PSU low +5V circuit, and test if the +20V line to
the inverter gets turned on.
As explained this kind of test must not be done by the unexperienced
user, as the chance to waste the unit beyond repair is high, and there
are also many safety concerns.
After testing the PSU, next part to test will be the inverter board.
The inverter is tested reading DC voltage in from controller board, and AC out to back light system.
The inverter is an high voltage board, remember, safety first when testing AC circuit.
If the inverter has no DC from controller, then test PSU, if PSU is OK,
and inverter has still no DC in when PSU line to inverter is on, then
replace controller board.
If the inverter has DC in, and no AC out, then replace the inverter.
If the inverter has DC in and AC out when higher DC voltage to inverter
is turned on, then test at component level the main controller board,
and if present test LCD controller.
Regards.
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