Hello,
I picked up a free Sharp 36k-s400 (36") CRT TV when a neighbor replaced it with a new plasma set.
The set randomly shuts down and it won't come back on until I pull the power cord. I can leave it unplugged for 20-30 seconds then it will power up again after that. Sometimes when I turn the set on it will shut down in a minute or two, sometimes it will run a few hours, sometimes it will run all night. There's no particular pattern to it.
I don't think it's overheating because the house is pretty cool, plus the fact that sometimes it shuts down in minutes of powering up.
When I got the set I removed the back and blew it out with an air compressor, it was pretty gunked up. The area of the PCB where the flyback is mounted looks like it's been overheated in the past as it's dark brown compared to the rest of the PCB. (I assume it *was* overheating because of the dust and gunk)
I think that some components are bad or going bad due to previous overheating. I may be wrong but I'm guessing.
Seconds before the set shuts down the picture goes out of whack and begins to jitter and squeeze down. I can also hear a high pitched squealing that oscillates in relation to the screen jitter. This lasts 10-30 seconds until the picture collapes into a bright white horizontal band in the middle of the screen that then winks out and I can hear (I assume) a relay shutting the power off.
At this point the set is locked up and will not respond to the remote or front panel and the only way to recover is to pull the power cord for a short time. Once you cycle the power that way it may run again all night long without problem.
I have basic electronics experience but this is not my forte. I have the service manual for this set with full schematics, I have a Tektronix 100mhz dual trace scope and a Fluke 77 DVOM. I do *not* have a high voltage test meter or high voltage probes for the scope. I bought the scope to help me repair computer monitors but never got around to learning how to use it yet (I'm lazy)..
Can anyone offer insight or direction? I'm pretty sure it's power related. I inspected the capacitors but none were swollen. Maybe a bad resistor or diode? I HOPE it's not the flyback as I have no idea how to test a flyback. If it is though, I'm pretty sure I could unsolder and replace it.
I love this set, it has a magnificent picture and I've never had such a huge TV before. I want to fix this thing and I sure could use some guidance because I am not a TV repairman but I can fix anything if I can nail down the problem.
Thanks!
Sharp 36" TV shuts down randomly
Sat, 02/10/2007 - 13:53
#1
Sharp 36" TV shuts down randomly
Ok, first thing you need to do is NEVER connect up the ociliscope to the TV chassis or any monitor with out using an isolation transformer. You will blow up the sets power supply if you do. One with your problem. You could have a few differant things wrong with your TV set. The most common one for this set is bad connections on the Printed circuit board at or around the flyback transformer, the sweep power supply, the main power supply or in the horizontal or vertical drive circuits. You should be able to see if the connections look dried out and not shiney. Be carefull when you resolder the connections and inspect all of your work you did very carefully before plugging in the set. You could have a bad capacitor in the vertical output circuit also. Good Luck, and let us know the progress of this repair.
Thank you for responding!
And I will be certain not to use the scope, I can't afford to ruin it or the set!
I will get out a magnifier and inspect the PCB carefully.
When I took the cover off the other day it was on the floor and I didn't have my glasses on but I could clearly see the PCB was discolored from overheating in the area around the flyback and several components were also discolored.
My first thought when I saw that was that the values of some components may have been altered by over heating.
I powered it up there on the floor and put an old satellite receiver (in demo mode) on it to throw something on the screen then left it running over night, next morning it was still running so I figured it was OK and buttoned it up and called it good.
I should have known better.
I wonder if some of the solder pads may have separated from the PCB from heating?
I've seen that happen before too. One thing I did not do was inspect the solder side of the PCB.
This is going to be a real challenge due to the size and weight of this thing, I have to recruit a neighbor to help me move it around.
Boy I wish I had tested it for a few days longer!
Thanks!
Dont give up. I am sure if you persist, you will find the fault.