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