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


news:3651c4bf-6211-4ba9-
...


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

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?

Let's put it this way... Your hand won't jerk back from a hot stove until it
comes in contact with the stove.

On Sun, 29 Jun 2008 11:29:51 -0700 (PDT), Dave put
finger to keyboard and composed:

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.

If a microprocessor has detected a fault that has caused it to shut
down the supply, then you may have to cycle power to the unit in order
for the uP to forget and try again.

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

"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

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?

"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

On Jun 30, 2:08*am, "Arfa Daily" wrote:
"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- Hide quoted text -

- Show quoted text -

Thanks Arfa,
There are a bunch of numbers on the power supply board, starting with
a VLT70039.50 LOGAH REV5 UA-4141-1-LF
I just recieved the service manual from Andrews on this set and its a
joke. The only page that shows a schematic is so blury its useless.

I did find what I believe to be the return line from the micro going
to my power supply board, It says "PWR ON" so I'm going to assume this
is the right one, but I get no voltage what so ever when I press my
power on button. Thanks again.
Any further suggestions would be appreciated.
Dave

- 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- Hide quoted text -

- Show quoted text -

Thanks Arfa,
There are a bunch of numbers on the power supply board, starting with
a VLT70039.50 LOGAH REV5 UA-4141-1-LF
I just recieved the service manual from Andrews on this set and its a
joke. The only page that shows a schematic is so blury its useless.

I did find what I believe to be the return line from the micro going
to my power supply board, It says "PWR ON" so I'm going to assume this
is the right one, but I get no voltage what so ever when I press my
power on button. Thanks again.
Any further suggestions would be appreciated.
Dave

OK Dave. That line is definitely the power switching line. If it has no
voltage on it at all (check this with a 'scope set to DC, in case it just
pulses too quickly to see on a meter) then there's a good chance that the
micro is not telling the power supply to turn on. If at all possible from
the manual, you want to trace this signal back at least to where it comes
off the mainboard, and preferably all the way back to a pin on the micro,
just to prove that there are no simple connector isues. Also, now that you
have the manual, try to see if you can verify that standby supplies are 5v
only on that set i.e. that you don't have say a 3v3 standby rail that's
missing. Also, try to verify that there is not a 3v3 regulator on the
mainboard that derives its input from the 5v standby rail, and that's not
working. After that, is where it starts to get hairy ...

If the "power on" line is low, never changes - even immediately after a
fully-unplugged-from-the wall-outlet repower - and the power supply is in
standby, then the chances are that the line should shift from low (0v) to
high (5v) in order to wake the power supply up. You can thus now verify the
power supply by disconnecting the line (usually you can easily pop the pin
from the connector shell to isolate any wire) and then connecting a low
value resistor - start with 100 ohms - between the 5v standby supply, and
the actual power supply pin where you have disconnected the wire from. If
this brings the power supply to life, then the chances are that you have a
bad mainboard, and you are unlikely to be able to fix that. You might try
looking on e-Bay for replacement boards. These often pop up this side of the
pond, at very reasonable prices, as sets often get broken up for spares as a
result of insurance write offs from LCD panel damage.

One word of caution, I am assuming that you have enough knowledge of switch
mode power supplies to realise that they are potentially *very* dangerous at
their front ends. The main filter cap on one that's not running, can remain
charged to a very high voltage, long after you disconnect it from line
power. Also, most of these LCD PSUs have a resonant PFC stage at the front
end, which will run, producing full front end rectified DC across the main
filter cap, with as little as 40v ac input applied to the supply.

Good luck. Let us know if you make any progress.

Arfa

On Jul 1, 1:50*am, "Arfa Daily" wrote:
- 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- Hide quoted text -

- Show quoted text -

Thanks Arfa,
There are a bunch of numbers on the power supply board, starting with
a VLT70039.50 * *LOGAH REV5 * UA-4141-1-LF
I just recieved the service manual from Andrews on this set and its a
joke. The only page that shows a schematic is so blury its useless.

I did find what I believe to be the return line from the micro going
to my power supply board, It says "PWR ON" so I'm going to assume this
is the right one, but I get no voltage what so ever when I press my
power on button. Thanks again.
Any further suggestions would be appreciated.
Dave

OK Dave. That line is definitely the power switching line. If it has no
voltage on it at all (check this with a 'scope set to DC, in case it just
pulses too quickly to see on a meter) then there's a good chance that the
micro is not telling the power supply to turn on. If at all possible from
the manual, you want to trace this signal back at least to where it comes
off the mainboard, and preferably all the way back to a pin on the micro,
just to prove that there are no simple connector isues. Also, now that you
have the manual, try to see if you can verify that standby supplies are 5v
only on that set i.e. that you don't have say a 3v3 standby rail that's
missing. Also, try to verify that there is not a 3v3 regulator on the
mainboard that derives its input from the 5v standby rail, and that's not
working. After that, is where it starts to get hairy ...

If the "power on" line is low, never changes - even immediately after a
fully-unplugged-from-the wall-outlet repower - and the power supply is in
standby, then the chances are that the line should shift from low (0v) to
high (5v) in order to wake the power supply up. You can thus now verify the
power supply by disconnecting the line (usually you can easily pop the pin
from the connector shell to isolate any wire) and then connecting a low
value resistor - start with 100 ohms - between the 5v standby supply, and
the actual power supply pin where you have disconnected the wire from. If
this brings the power supply to life, then the chances are that you have a
bad mainboard, and you are unlikely to be able to fix that. You might try
looking on e-Bay for replacement boards. These often pop up this side of the
pond, at very reasonable prices, as sets often get broken up for spares as a
result of insurance write offs from LCD panel damage.

One word of caution, I am assuming that you have enough knowledge of switch
mode power supplies to realise that they are potentially *very* dangerous at
their front ends. The main filter cap on one that's not running, can remain
charged to a very high voltage, long after you disconnect it from line
power. Also, most of these LCD PSUs have a resonant PFC stage at the front
end, which will run, producing full front end rectified DC across the main
filter cap, with as little as 40v ac input applied to the supply.

Good luck. Let us know if you make any progress.

Arfa- Hide quoted text -

- Show quoted text -

Even if I can't fix it I appreciate all your help in expanding my
understanding of LCD power supplies. I will let you know if I have any
luck.
Dave

"Dave" wrote in message
...
On Jul 1, 1:50 am, "Arfa Daily" wrote:
- 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- Hide quoted text -

- Show quoted text -

Thanks Arfa,
There are a bunch of numbers on the power supply board, starting with
a VLT70039.50 LOGAH REV5 UA-4141-1-LF
I just recieved the service manual from Andrews on this set and its a
joke. The only page that shows a schematic is so blury its useless.

I did find what I believe to be the return line from the micro going
to my power supply board, It says "PWR ON" so I'm going to assume this
is the right one, but I get no voltage what so ever when I press my
power on button. Thanks again.
Any further suggestions would be appreciated.
Dave

OK Dave. That line is definitely the power switching line. If it has no
voltage on it at all (check this with a 'scope set to DC, in case it just
pulses too quickly to see on a meter) then there's a good chance that the
micro is not telling the power supply to turn on. If at all possible from
the manual, you want to trace this signal back at least to where it comes
off the mainboard, and preferably all the way back to a pin on the micro,
just to prove that there are no simple connector isues. Also, now that you
have the manual, try to see if you can verify that standby supplies are 5v
only on that set i.e. that you don't have say a 3v3 standby rail that's
missing. Also, try to verify that there is not a 3v3 regulator on the
mainboard that derives its input from the 5v standby rail, and that's not
working. After that, is where it starts to get hairy ...

If the "power on" line is low, never changes - even immediately after a
fully-unplugged-from-the wall-outlet repower - and the power supply is in
standby, then the chances are that the line should shift from low (0v) to
high (5v) in order to wake the power supply up. You can thus now verify
the
power supply by disconnecting the line (usually you can easily pop the pin
from the connector shell to isolate any wire) and then connecting a low
value resistor - start with 100 ohms - between the 5v standby supply, and
the actual power supply pin where you have disconnected the wire from. If
this brings the power supply to life, then the chances are that you have a
bad mainboard, and you are unlikely to be able to fix that. You might try
looking on e-Bay for replacement boards. These often pop up this side of
the
pond, at very reasonable prices, as sets often get broken up for spares as
a
result of insurance write offs from LCD panel damage.

One word of caution, I am assuming that you have enough knowledge of
switch
mode power supplies to realise that they are potentially *very* dangerous
at
their front ends. The main filter cap on one that's not running, can
remain
charged to a very high voltage, long after you disconnect it from line
power. Also, most of these LCD PSUs have a resonant PFC stage at the front
end, which will run, producing full front end rectified DC across the main
filter cap, with as little as 40v ac input applied to the supply.

Good luck. Let us know if you make any progress.

Arfa- Hide quoted text -

- Show quoted text -

Even if I can't fix it I appreciate all your help in expanding my
understanding of LCD power supplies. I will let you know if I have any
luck.
Dave

No problem. There's something 'new' to challenge us every day ... d;~}

Arfa