Sylvania LC420SS8 power problem

[beanboy33]

I am trying to fix a sylvania LC420SS8 for my father inlaw. He told me that for the past couple of weeks he would try to turn it on. The green light would blink twice then shut off. He would keep on trying to turn it on, after 20 or so try's it would finally stay on. But now it is slowly taking more and more tries to keep it on. I opened the back and looked for some bulging caps ( thinking its turning off due to high ripple). But I have found nothing. I connected my meter to TP500 (the 345 supply) and when I turn on the tv it reads 345v and then quickly drops when the tv turns off. Coming out of the D500 rectifier , if I remember correctly is about 150v @ 1.5 ripple (do to no filtering). I'm just wondering if I should just shotgun the caps and replace them all? The schematics for this tv is located in this thread (https://www.badcaps.net/forum/attach...1&d=1354250862)

Any idea are appreciated.

[beanboy33]

Re: Sylvania LC420SS8 power problem

I am going to work on it some more after work today. Any ideas are welcomed.

[beanboy33]

Re: Sylvania LC420SS8 power problem

The AL 15 volts is present. But when it try to turn it on it drops. Removed the 345 volt cable that goes from the power supply to the inverter. The 345volt supply still reads around 160 on standby and when the tv is turned on it goes from around 345 then drops to around 160

[budm]

Re: Sylvania LC420SS8 power problem

"The AL 15 volts is present. But when it try to turn it on it drops" How much does it drop to?
Check the voltages on those pin of the standby power supply and report the reading, use chassis ground as the ground ref. point.
The 345VDC is generated by the PFC Voltage booster, it will only go up from 160vdc to 345VDC when the main board send out the PS_ON signal to the power supply board. Right now the TV is sensing something wrong so it goes into shutdown mode, as you can see from the diagram, there are many sensing circuits (protect1, 2...) for protection.

[beanboy33]

Re: Sylvania LC420SS8 power problem

Budm, thanks for your reply.

AL 15 is 14.25 and while tv is trying to turned on it drops to 9.8.

6v reads 3.87
4v reads 1.67
2.7v reads 2.66
13v reads 6.83

I took a heat gun and warmed up the area between the D500 heatsink and the Q600 heatsink. The tv stated up fine. I took some freeze spray and started to cool small areas down as I turned the tv off and on. I kept on working. But if I let is sit for a few minutes unplugged it will stop working. Because the cold spray didn't produce a failure I don't think it is a component with a thermal problem. I think it's a bad solder joint somewhere and the extreme heat and the extreme cold expands and contracts the joint enough for it to work. But at room temp the joint fails.

What do you think.

[beanboy33]

Re: Sylvania LC420SS8 power problem

Maybe the area that the heat gun gets warm is from D500 to T600. It's hard to tell.

[beanboy33]

Re: Sylvania LC420SS8 power problem

I will work on it more tonight while waiting for the Bruins game to start.

[beanboy33]

Re: Sylvania LC420SS8 power problem

Still trying to figure it out

[beanboy33]

Re: Sylvania LC420SS8 power problem

found cracked etch. problem fixed

[budm]

Re: Sylvania LC420SS8 power problem

Great job finding cold solder joints.

[zvaniel4o]

Re: Sylvania LC420SS8 power problem

found cracked etch. problem fixed

Where exactly was the etch? I'm having the same TV with the same problem, tried with the hot gun - no result... I mind as well solder it, but there was no change as well...

[zvaniel4o]

Re: Sylvania LC420SS8 power problem

It was not bad solder joints... it was faulty smt and tht diodes... replaced - works like charm!