Over the past few months I've received a few cheapo LCD TVs with blown
power supplies that could not be repaired, the reason being that replacement
parts (or whole boards) were simply not available. Another one just came in
with the same problem, and this time an idea occurred to me.
The power supplies in these sets are pretty simple, at least in terms of
outputs and connections. They usually consist of a 5VDC standby supply and
switchable 12 and 24VDV supplies. The standby voltage is always derived from
a separate switching transformer, and it's usually the 12/24 volt circuit
that goes bad. The connections are usually GND,5V,12v,24v, and power on
signal.
So the obvious question is - can an external power supply be added to the
set? I once tried this but my homemade power supply wasn't filtered or
regulated well enough to provide satisfactory operation. Looking on Ebay, I
see that low-cost switching power supplies are readily available, in a wide
assortment of voltages and amperages. I thought I might buy some and try
creating external power supplies for these TVs, but first I'd like to know
just how much current is required for a 27 to 32" TV. The 24 and 12V power
supplies are available in current ratings of 2,3,5,10 amps and higher. A
relay circuit would have to be added so that the backlights etc. wouldn't
stay on when in power off mode.
Adding an external power supply might not be as practical as replacing the
internal one, but it's far cheaper and in some cases may be the only way to
save the set from the landfill.

Chris F. wrote:
The power supplies in these sets are pretty simple, at least in terms of
outputs and connections. They usually consist of a 5VDC standby supply and
switchable 12 and 24VDV supplies. The standby voltage is always derived from
a separate switching transformer, and it's usually the 12/24 volt circuit
that goes bad. The connections are usually GND,5V,12v,24v, and power on
signal.
So the obvious question is - can an external power supply be added to the
set? I once tried this but my homemade power supply wasn't filtered or
regulated well enough to provide satisfactory operation. Looking on Ebay, I
see that low-cost switching power supplies are readily available, in a wide
assortment of voltages and amperages. I thought I might buy some and try
creating external power supplies for these TVs, but first I'd like to know
just how much current is required for a 27 to 32" TV. The 24 and 12V power
supplies are available in current ratings of 2,3,5,10 amps and higher. A
relay circuit would have to be added so that the backlights etc. wouldn't
stay on when in power off mode.
Adding an external power supply might not be as practical as replacing the
internal one, but it's far cheaper and in some cases may be the only way to
save the set from the landfill.

Before you get excited... : think about *why* the supply
died. Sure, it's possible that it just "gave up the ghost".
But, it is also possible that some aspect of the *load*
has changed in A Bad Way. Unless you fix the root cause,
you stand a good chance of toasting your *new* supply... *or*
something else in the set when it finds "enough" power available
for it to cook itself without taking out the "semiconductor fuses"
(otherwise known as transistors : ) in the power supply.

"D Yuniskis" wrote in message
...
Chris F. wrote:
The power supplies in these sets are pretty simple, at least in terms
of outputs and connections. They usually consist of a 5VDC standby supply
and switchable 12 and 24VDV supplies. The standby voltage is always
derived from a separate switching transformer, and it's usually the 12/24
volt circuit that goes bad. The connections are usually GND,5V,12v,24v,
and power on signal.
So the obvious question is - can an external power supply be added to
the set? I once tried this but my homemade power supply wasn't filtered
or regulated well enough to provide satisfactory operation. Looking on
Ebay, I see that low-cost switching power supplies are readily available,
in a wide assortment of voltages and amperages. I thought I might buy
some and try creating external power supplies for these TVs, but first
I'd like to know just how much current is required for a 27 to 32" TV.
The 24 and 12V power supplies are available in current ratings of
2,3,5,10 amps and higher. A relay circuit would have to be added so that
the backlights etc. wouldn't stay on when in power off mode.
Adding an external power supply might not be as practical as replacing
the internal one, but it's far cheaper and in some cases may be the only
way to save the set from the landfill.

Before you get excited... : think about *why* the supply
died. Sure, it's possible that it just "gave up the ghost".
But, it is also possible that some aspect of the *load*
has changed in A Bad Way. Unless you fix the root cause,
you stand a good chance of toasting your *new* supply... *or*
something else in the set when it finds "enough" power available
for it to cook itself without taking out the "semiconductor fuses"
(otherwise known as transistors : ) in the power supply.

In my experience, most LCD TV internal power supplies fail for no other
reason than that they just fail. Also, it is fairly rare that I find that
the failure is on the secondary side, particularly in the 24v circuit, which
normally feeds primarily the backlights, and sometimes the sound. For sure
the 24v circuit may not appear to be working, but this is more often than
not, because the backlight inverter has failed, and caused the supply to
shut down. And whilst we are talking shutdown, it is very rare for faults
external to the PSU to cause PSU damage. The switchers fitted to LCD TVs are
absolutely *covered* in monitoring circuitry, and any overload condition on
any rail, will bring about an immediate shutdown of all secondary supplies,
and often, will place the standby supply into a cycling startup mode as
well.

Most of the failures that I come across involve either the standby supply,
or the PFC supply, both of which are primary-side failures. There are many
many 'service kits' available to deal with most of these failures, and they
are usually quite reasonably priced. They contain all of the components
which fail, and if you just go ahead and replace all the parts that they
give you, irrespective of whether they appear to be faulty or not, you will
usually end up with a working supply. Before totally writing off the
existing supplies, you should probably try to diagnose your problems a
little deeper, and then see if a kit is available.

As to what the demand is on the various rails, they are all quite light
except for the 24v rail, where the demand is typically around 4 amps. I use
two 12v 50watt halogen lamps in series as a test load for the 24v rail.

With regard to trying to use an 'alternative' supply, wired in externally, I
personally would not consider doing this as I feel that you may be opening
yourself up to all manner of potential safety issues, and certainly would be
compromising any safety approvals that were already in place for the
equipment. This could bring a whole legal ****storm down on your head, if
anyone received any injury as a result ...

Far better, I think, that the existing power supplies are repaired. Although
diagnosing these supplies is a bit of a black art, contrary to what you
believe, most are repairable, and most parts are available, if you look hard
enough.

Arfa

On Tue, 24 Nov 2009 14:46:58 -0000, "Arfa Daily"
wrote:


"D Yuniskis" wrote in message
...
Chris F. wrote:
The power supplies in these sets are pretty simple, at least in terms
of outputs and connections. They usually consist of a 5VDC standby supply
and switchable 12 and 24VDV supplies. The standby voltage is always
derived from a separate switching transformer, and it's usually the 12/24
volt circuit that goes bad. The connections are usually GND,5V,12v,24v,
and power on signal.
So the obvious question is - can an external power supply be added to
the set? I once tried this but my homemade power supply wasn't filtered
or regulated well enough to provide satisfactory operation. Looking on
Ebay, I see that low-cost switching power supplies are readily available,
in a wide assortment of voltages and amperages. I thought I might buy
some and try creating external power supplies for these TVs, but first
I'd like to know just how much current is required for a 27 to 32" TV.
The 24 and 12V power supplies are available in current ratings of
2,3,5,10 amps and higher. A relay circuit would have to be added so that
the backlights etc. wouldn't stay on when in power off mode.
Adding an external power supply might not be as practical as replacing
the internal one, but it's far cheaper and in some cases may be the only
way to save the set from the landfill.

Before you get excited... : think about *why* the supply
died. Sure, it's possible that it just "gave up the ghost".
But, it is also possible that some aspect of the *load*
has changed in A Bad Way. Unless you fix the root cause,
you stand a good chance of toasting your *new* supply... *or*
something else in the set when it finds "enough" power available
for it to cook itself without taking out the "semiconductor fuses"
(otherwise known as transistors : ) in the power supply.

In my experience, most LCD TV internal power supplies fail for no other
reason than that they just fail. Also, it is fairly rare that I find that
the failure is on the secondary side, particularly in the 24v circuit, which
normally feeds primarily the backlights, and sometimes the sound. For sure
the 24v circuit may not appear to be working, but this is more often than
not, because the backlight inverter has failed, and caused the supply to
shut down. And whilst we are talking shutdown, it is very rare for faults
external to the PSU to cause PSU damage. The switchers fitted to LCD TVs are
absolutely *covered* in monitoring circuitry, and any overload condition on
any rail, will bring about an immediate shutdown of all secondary supplies,
and often, will place the standby supply into a cycling startup mode as
well.

Most of the failures that I come across involve either the standby supply,
or the PFC supply, both of which are primary-side failures. There are many
many 'service kits' available to deal with most of these failures, and they
are usually quite reasonably priced. They contain all of the components
which fail, and if you just go ahead and replace all the parts that they
give you, irrespective of whether they appear to be faulty or not, you will
usually end up with a working supply. Before totally writing off the
existing supplies, you should probably try to diagnose your problems a
little deeper, and then see if a kit is available.

As to what the demand is on the various rails, they are all quite light
except for the 24v rail, where the demand is typically around 4 amps. I use
two 12v 50watt halogen lamps in series as a test load for the 24v rail.

With regard to trying to use an 'alternative' supply, wired in externally, I
personally would not consider doing this as I feel that you may be opening
yourself up to all manner of potential safety issues, and certainly would be
compromising any safety approvals that were already in place for the
equipment. This could bring a whole legal ****storm down on your head, if
anyone received any injury as a result ...

Far better, I think, that the existing power supplies are repaired. Although
diagnosing these supplies is a bit of a black art, contrary to what you
believe, most are repairable, and most parts are available, if you look hard
enough.

Arfa

There are a few things I would add. Examining schematics of a few LCD
TV power supplies show a great deal of cross coupling in the
regulation. Using 5V, 12V, and 24 V supplies and swtiching them on
without proper care in the sequencing almost certainly will not work.

Parts availablity can be a problem unless you can obtain a parts kit.
Parts are not stocked at the local Radio Shack and major suppliers
such as Mouser or Digikey may not heve them.

PlainBill

Parts availablity can be a problem unless you can obtain a parts kit.
Parts are not stocked at the local Radio Shack and major suppliers
such as Mouser or Digikey may not heve them.

PlainBill

Can someone link to a SMPS repair parts kit? I've not seen these.

Thanks.

"Fester Bestertester" wrote in message
...
Parts availablity can be a problem unless you can obtain a parts kit.
Parts are not stocked at the local Radio Shack and major suppliers
such as Mouser or Digikey may not heve them.

PlainBill

Can someone link to a SMPS repair parts kit? I've not seen these.

Thanks.


For instance

http://cgi.ebay.co.uk/LCD-PSU-REPAIR...13178002r28571

UK e-bay, but I'm sure a bit of googling or e-bay research would turn up
others in the U.S. or wherever

Arfa

Far better, I think, that the existing power supplies are repaired.
Although
diagnosing these supplies is a bit of a black art, contrary to what you
believe, most are repairable, and most parts are available, if you look
hard
enough.

Arfa

There are a few things I would add. Examining schematics of a few LCD
TV power supplies show a great deal of cross coupling in the
regulation. Using 5V, 12V, and 24 V supplies and swtiching them on
without proper care in the sequencing almost certainly will not work.

Parts availablity can be a problem unless you can obtain a parts kit.
Parts are not stocked at the local Radio Shack and major suppliers
such as Mouser or Digikey may not heve them.

PlainBill

Interestingly, whole power supplies are available here, sometimes at
ridiculously low prices. For instance, many sets use a Vestel chassis, and
some of these PSUs are being sold brand new for as little as 25UKP, although
you can also find the same PSU at the same vendor for 3 times that, but
listed as being for a different make / model, so it's often advantageous to
know what power supply is used in multiple different manufacturer's sets.
Anyway, the point I was going to make is that a friend of mine whose shop
specialises in repair of LCD TV sets, often buys in these power supplies at
this price, and fits them into sets that they were never intended to go in,
and apparently without any problem. As long as he can find the turn on
signal from the system control micro, and there is physically room to fit a
Vestel supply in the set in question, he just goes ahead and does it, if
necessary, soldering the wires directly to the output connectors.

To take your point about cross-coupled regulation, I have not found this to
be a problem. I repair a lot of Vestel supplies for the trade, and they are
happy to regulate with any load from nothing to full, on any rail, all
rails, or no rails at all. As for the rails coming up in any specific order,
I think all of the ones that I've seen have all of the switched rails come
up together. Certainly, my friend doesn't seem to have ever had an issue
with the actual working of an internal supply substitute.

Arfa

On Tue, 24 Nov 2009 19:42:34 -0000, "Arfa Daily"
wrote:


Far better, I think, that the existing power supplies are repaired.
Although
diagnosing these supplies is a bit of a black art, contrary to what you
believe, most are repairable, and most parts are available, if you look
hard
enough.

Arfa

There are a few things I would add. Examining schematics of a few LCD
TV power supplies show a great deal of cross coupling in the
regulation. Using 5V, 12V, and 24 V supplies and swtiching them on
without proper care in the sequencing almost certainly will not work.

Parts availablity can be a problem unless you can obtain a parts kit.
Parts are not stocked at the local Radio Shack and major suppliers
such as Mouser or Digikey may not heve them.

PlainBill

Interestingly, whole power supplies are available here, sometimes at
ridiculously low prices. For instance, many sets use a Vestel chassis, and
some of these PSUs are being sold brand new for as little as 25UKP, although
you can also find the same PSU at the same vendor for 3 times that, but
listed as being for a different make / model, so it's often advantageous to
know what power supply is used in multiple different manufacturer's sets.
Anyway, the point I was going to make is that a friend of mine whose shop
specialises in repair of LCD TV sets, often buys in these power supplies at
this price, and fits them into sets that they were never intended to go in,
and apparently without any problem. As long as he can find the turn on
signal from the system control micro, and there is physically room to fit a
Vestel supply in the set in question, he just goes ahead and does it, if
necessary, soldering the wires directly to the output connectors.

To take your point about cross-coupled regulation, I have not found this to
be a problem. I repair a lot of Vestel supplies for the trade, and they are
happy to regulate with any load from nothing to full, on any rail, all
rails, or no rails at all. As for the rails coming up in any specific order,
I think all of the ones that I've seen have all of the switched rails come
up together. Certainly, my friend doesn't seem to have ever had an issue
with the actual working of an internal supply substitute.

Arfa

I think we were failing to communicate properly here. I interpreted
the plan as to buy three separate power supplies (5V, 12V, 24V) and
use them to replace an integrated 5V, 12V, 24V supply. I do not think
that will work well.

You appear to be discussing using a 5V, 12V, 24V supply designed for
an XYZ brand of TV in a QED TV. I would have to agree, if dimensional
and current ratings are compatible, that should work.

From a personal perspective, I 'grew up' with analog power supplies
(often using vacuum tube rectifiers). These were inefficient, but
easy to repair. The switch to SMPS supplies required more
sophisticated techniques, but in the end was not that much more
difficult, even for the multiple output supplies found in PCs.

At this point I am finding the supplies in LCD TVs to be extremely
difficult to troubleshoot safely and effectively.

PlainBill

snip


From a personal perspective, I 'grew up' with analog power supplies
(often using vacuum tube rectifiers). These were inefficient, but
easy to repair. The switch to SMPS supplies required more
sophisticated techniques, but in the end was not that much more
difficult, even for the multiple output supplies found in PCs.

At this point I am finding the supplies in LCD TVs to be extremely
difficult to troubleshoot safely and effectively.

PlainBill

Agreed. The key to it seems to be obtaining schematics. I offer a trade
repair service on a particular LCD power supply board, but I am only able to
do that as a result of hours of painstaking research to find schematics to
cover all the variants. I then spent a long time analysing just what made
this supply tick (figuratively, not literally !! ) and then sat down and
designed a test jig and a set of adaptor harnesses to allow the different
variants to all be repaired using that jig. I can pretty much crack most
problems on them in fairly short order now, but still get the occasional one
that gets the better of me. Just yesterday, actually ... I have one at the
moment that has a really odd fault. The PFC supply won't start up at full
supply volts. Turn it down on a variac just 10 volts, and it will start
every time. If you then turn back up, it keeps running, but the PFC supply
starts to squegg, and the 390v rail drops to around 340v. At the same time,
the 24v secondary side rail drops to around 23v, but curiously, all the
other rails stay spot on. That might be the clue to what's going wrong, as
the 24v rail is normally *very* tightly regulated. I would add that the
problem occurs whether there is a load on any of the rails or not. It'll be
an interesting one to eventually find when I have the time, but in the
meantime to get my customer out of trouble with his, I've had to do him a
swapout.

Arfa

I should have tested more thoroughly before posting. Turns out the power
supply is good - all three output voltages are normal. The problem must be
in the mainboard - the power light comes on but nothing happens. So it looks
like a dead end for this set.
What I might try to do is fit the power supply from this set into another
one, rather than building an external supply....

"Arfa Daily" wrote in message
...

snip


From a personal perspective, I 'grew up' with analog power supplies
(often using vacuum tube rectifiers). These were inefficient, but
easy to repair. The switch to SMPS supplies required more
sophisticated techniques, but in the end was not that much more
difficult, even for the multiple output supplies found in PCs.

At this point I am finding the supplies in LCD TVs to be extremely
difficult to troubleshoot safely and effectively.

PlainBill

Agreed. The key to it seems to be obtaining schematics. I offer a trade
repair service on a particular LCD power supply board, but I am only able
to do that as a result of hours of painstaking research to find schematics
to cover all the variants. I then spent a long time analysing just what
made this supply tick (figuratively, not literally !! ) and then sat down
and designed a test jig and a set of adaptor harnesses to allow the
different variants to all be repaired using that jig. I can pretty much
crack most problems on them in fairly short order now, but still get the
occasional one that gets the better of me. Just yesterday, actually ... I
have one at the moment that has a really odd fault. The PFC supply won't
start up at full supply volts. Turn it down on a variac just 10 volts, and
it will start every time. If you then turn back up, it keeps running, but
the PFC supply starts to squegg, and the 390v rail drops to around 340v.
At the same time, the 24v secondary side rail drops to around 23v, but
curiously, all the other rails stay spot on. That might be the clue to
what's going wrong, as the 24v rail is normally *very* tightly regulated.
I would add that the problem occurs whether there is a load on any of the
rails or not. It'll be an interesting one to eventually find when I have
the time, but in the meantime to get my customer out of trouble with his,
I've had to do him a swapout.

Arfa

Can I just make a little observation, Chris ? The common convention on
newsgroups, is to post replies to the bottom of the existing text. This
generally makes it easier to follow the chronology of the thread, and makes
it easier to see exactly what points you are replying to. Some newsgroups
are really pedantic about this, and you will get royally castigated by their
regulars, if you top post ...

Replying to individual points within the body of the existing text is also
generally accepted as being OK.

Arfa