"Dave" wrote in message
...
On Jun 29, 4:55 pm, "Arfa Daily" wrote:
"Dave" wrote in message

...





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 pushing 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" wrote in message

Did you locate the standby switching line from the system control micro,
and
determine if it is toggling, as I suggested previously, and are you
getting
a standby light ?

Arfa- Hide quoted text -

- Show quoted text -

I hate to sound stupid, but How do I find the switched line from the
control micro to see if it toggles? Didn't I do this when I checked
all of the power supply pins going to my video board?
I do have a glowing LED on the front of the set which turns on the
minute I plug the set in, but it is continually on, and does not
blink. I also tested every pin on my power supply board and found them
to be dead, except for the 5 volt stand by voltage. I did notice when
measuring the five volt line on the power switch it dropped to 0 volts
when I pushed the switch. Am I misunderstanding anything your
saying?



OK. The fact that you are getting a standby light shows that the 5v you are
seeing at the power supply, is definitely an output from it, not an input
from elsewhere, that is not moving. Seeing the 5v at the set's standby
pushbutton, again verifies that the 5v is reaching the main board, and is
also getting to the system control micro (to which the standby button will
almost certainly be connected). In response to that line being dropped from
5v to zero, by pushing the standby button, the system control micro should
issue a "power on" signal to the power supply. This is the usual
arrangement. It is highly unlikely that the standby pushbutton, is connected
directly to the power supply.

So, assuming that the standby switching line's pin is not marked at the
power supply, and that you do not have a set of schematics (or even a board
interconnect diagram will do) then the only way that you can proceed, is to
go looking for the standby switching line at the power supply, by looking at
every external connector pin that doesn't have any legend to tell you what
it does, with a 'scope set to DC coupling. As you move to each pin, you need
to push the standby button, and wait a couple of seconds to see if the DC
level on the pin you are checking, moves, or even if a pulse is produced.
Wait ten seconds before moving on to the next pin to test, to allow any
'problem' that the system control micro may have detected, to reset to the
'try again' condition.

If you find a pin that's toggling, then the chances are that it is not your
mainboard that's primarily at fault (as such) and that the problem lies
either with the power supply, or one of the loads that it's feeding. If
there is no pin that's toggling, then the system control is not issuing a
"power on" signal, and trouble shooting the power supply, is not the right
path.

Unfortunately, these things are not straightforward like in the 'good ol'
days', and can be very difficult to fault-find on, without a lot of
experience, and spare boards to try in them. Let us know if you manage to
find the standby switching line, and we might be able to proceed a bit
further, from there. Do you know what chassis is fitted by any chance ? It's
not a Vestel is it ? Are there any numbers / letters at the corner of the
PSU ?

Arfa