In the process of trying to determin if my power supply is defective,
or the video board on an LCD TV. I checked the voltage output pins and
found the 5 volts stand by voltage was working fine, but nevertheless
the set does not turn on.
Some one suggested I put my oscilloscope probe on the switched outputs
while pu****ng the power button to see if the other outputs attempt to
come on before going in to shut down.
Is this a valid test? Can I really expect to see the other output pins
briefly come up if something on the video board is causing my power
supply to go in shut down?
Testing the switched outputs revealed there was not even a spike, so
my hunch is I have a bad power supply.
Thanks in advance, and thanks much for the previous suggestions.
"Dave" <dav...@[EMAIL PROTECTED]
> wrote in message
news:3651c4bf-6211-4ba9-
ad8d-7ed66fecdc87@[EMAIL PROTECTED]
> I'm wanting to determin if my power supply voltages are correct on my
> dead LCD TV. On my power supply board the output voltages are printed,
> but since the set doesn't turn on, the only voltage I see present is 5
> volts on the stand power supply pin.
> I realize the power supply isn't completely turned on until the
> microprocessor on my video board sends it the correct turn on pulse,
> but isn't there more I can do aside from assuming the video board is
> likely defective. I checked ESR on all capacitors, looked for shorted
> diodes and transistors, burnt resistors, etc. What is a high failure
> item on the video board which might cause this? The set doesn't come
> on for even a second.
> This is a Westinghouse model SK32H240S
> Thanks in advance.
On many, although not all, LCD TV sets that I've had the 'pleasure' to
work
on (!) the standby supply produces +5v and +3v3. Normally, when you
have one
that won't come out of standby (do you actually even get a standby
light
showing?) the first thing to do is to locate the standby switching
line to
see if that is toggling. If it is, you then need to look at each
individual
output rail from the power supply to see if they start to try to come
up.
Often, they will, but fall over again so quickly that you don't see
any
activity at all in terms of the set coming to life. The rails are
normally
something like +3v3 switched, +5v switched, +12v, +24v high current
(for the
backlight inverter) and +33v.
If the power supply is trying to start, then the next thing to do is
to
unplug the backlight inverter board. It is common for these to fail,
and if
they impose a heavy load on the 24v rail, the power supply will shut
back
down before it ever really gets going. Another common failure for a
heavily
loaded rail, is a failed audio IC.
In general, LCD TV power supplies are *very* heavily self protecting,
and
they will either shut back down very quickly, or not even come up at
all (as
far as you can see) if they detect the slightest thing wrong with any
of
their rails - thats over-voltage, under-voltage, or excess load.
It can be very tricky to get to the bottom of what's causing an
apparently
dead LCD TV. I have also had system control micros fail, and also
corrupted
EEPROMs causing startup problems.
Arfa
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