"mike" wrote in message
...
On 5/18/2013 3:03 AM, Arfa Daily wrote:


"mike" wrote in message
...
I picked up a free LG W2252TQ-TF.
lights up for a second then quits.
Figgered bad caps, but I pulled and tested 'em all.
All good. No bulges.

Checked all the diodes. Other semiconductors aren't
shorted. Two fets in the backlight supply probably
have the highest stress level, but I thought I'd
ask for ideas before trying to get them out.

It's gonna be difficult to power it up disassembled.

Ideas on other possible high failure rate parts?
Schematic?

Thanks, mike


Backlight tube itself ? Most of the lamp inverters that I've seen, have
current monitoring. If a 'wrong' current caused by a defective tube is
detected, the inverter just shuts down. It makes them hard to
troubleshoot without an appropriate load. You can check that the supply
voltage stays up, and that the 'on' signal to the inverter stays
asserted. I've had a couple of the high voltage tube coupling caps fail
over the years, and also transformers. You can make sure that the
backlight itself has quit, and it's not just that the screen has gone
black, by shining a powerful light at the front of the screen, at an
angle. Usually, you will be able to see at lest some vestige of what
should be being displayed, if it *is* that the backlight has gone out.

Arfa

Good call. One of the backlight secondaries is open.
Thought I'd take a shot at trying to fix it.

Hypothesizing that the connection to the pin might be open, I measured
some capacitances to attempt to determine which end.

The good secondary measures 2.68pF from primary to either end of the
secondary.
The bad secondary measures 2.68pF from primary to one end of the secondary
and 10pF to the other end.

Now, I'm confused. I expected to have the sum of the two measurements
on the open secondary to add up to about 2.68pF.

Another thing I'd never seen before was that each end of the winding has
two turns of what looks like 5 or so wires twisted together headed for the
connector pin. The wire to the pin is potted, so can't see the actual pin
connection.

Can't get my head around how or why they did that???

The winding is in the clear, looks like a single layer, so
if I can find the break, I can probably fix it.

Sod's law of course, says that it will be at the inside of the winding ...
:-\

Failing that, you might be able to cull a suitable transformer from a scrap
board. Quite a few use the same or very similar transformers. It might even
be worth Googling for a replacement, as some of these are available.
Likewise, complete inverter boards for some models, are available at quite
reasonable prices.

i.e.
http://www.shopjimmy.com/lg-eay38280...t-inverter.htm

and

http://www.ebay.com/itm/New-replacem...-/400219921779

or

http://www.aliexpress.com/store/prod...735816735.html

Arfa