"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