On Monday, November 25, 2019 at 7:02:20 AM UTC-5, wrote:
Hey All,
I know a few of you work on repairing LCD tv's and before I toss this set
out (it's personal, not customer), I was wondering if anyone ran across a
problem where for lack of a better explanation, the backlight for the LCD
panel is drawing excess current, even though there is no visual problem with
the backlight. The set still has a bright picture, isn't shading one side
over the other (or is it top and bottom?) the color temp seems normal all
the way around.
It's about a 10 year old Samsung SyncMaster P2570HD.
The long story, over the summer I noticed the smell of "something buring" in
the office the set was parked in but couldn't tell where it was coming from.
A couple weeks later, about 20 minutes after turning the tv on, the picture
went out, the set was still on, still had audio but solid black.
After searching around for disassembly help (snaps mostly, no screws) the
problem was obvious, a (probable 2W) resistor was charcol black along with
the circuit board and foil traces underneath. The resistor measured around
400K still and as with other Samsung monitors/tv's, the board was populated
with those 820uf/25V caps that always seem to fail.
So I replaced all the caps, took a guess at 390K for the resistor, patched
the foil traces and fired it up. Worked perfectly.
But, in about 15~20 minutes, the smell came back. The resistor was
overheating again.
Off to search for a schematic, which does seem to exist at serveral of the
sites, but none of them match. The power supply module is a IP-58155A but all
the schematics I found for that part number is actually for a IP-54155B, not
the same animal.
But they do show R301 (the resistor that overheats) as a 390K.
Anyway, in the search for more schematics one hit returned an Ebay auction
for the exact power supply module (IP-58155A), $35 with free shipping, sold
american.
The module came a few days later and appeared to be brand new, really. No
indication it was ever used or repaired.
Put it in and as you can guess, 15~20 minutes later, that burning smell came
back.
Looking at the foil traces, one side of it (R301) is directly connected to
the connectors for the lamp feed, where neither of the online schematics
shows that. With the two connectors for the lamp assembly disconnected, the
resistor is cool as a cucumber with the set running (duh).
So that is what leads me to asking if the lamps can possibly be drawing
excessive current, even though they seem to be fine with brightness and
function?
I mean I know the repair would require replacing the whole panel which isn't
worth it but I'm curious if there is anything else in the panel besides the
lamps that can cause that resistor to overheat.
Normally something that large (2W) would require a dead short (or close to
it) for it to turn to charcoal like the original.
Any guesses?
-bruce
Bad lamps. Most CCFL inverters shut down when a defective lamp is encountered and I am very surprised this one doesn't. It won't be a shorted lamp but a lamp(s) that are weak and require more voltage to deliver the current the inverter transformer is designed to deliver.
That board has a single output transformer, but it has two secondaries. A way to check this is to turn the TV on and put your scope probe *NEAR* the output harnesses or connector and watch the waveforms. Use a high vertical input setting and a fast horiz setting. Make sure to place the probe in the same relative position to the connector on both sides as the physical placement of the probe changes the amplitude greatly.
Don't connect the probe electrically to the inverter unless you were planning to buy a new scope anyway...
Now, one of the two waveforms will be much higher than the other, maybe twice as high. The actual amplitude isn't important because you're only looking for differences between the two sides, not taking p-p measurements.
Mark which side is higher, then see if you can switch the two harnesses and try again. You'll probably have to flip the board over to do this so make sure you put down an insulator between the board and TV frame because those heatsinks could be at hot ground potential.
If the side that was higher is now lower and vice versa, you have an open lamp(s) in the display. You might be able to find CCFL tubes on line to replace them.