Emerson LF501EM4 Intermittant Power On

[Version 7]

Only after unplugging the set for an hour or so, then plugging back in, will the television power on. Once on, it will run for at least an hour (that's all I tested for). You can tell when it will power on because you will hear the power plug arch a bit when plugging it in.

The problem appears to be in the backlight circuit; protect3 (.45v), backlight-sw (.21v) and backlight-adj (.18v). Reset, P-on-h2, al+3.3v, amp+13v and p-on+21v are all the correct voltage.

I flipped the power board and I have proper voltages to #1 OVP, #2 UVLO and #14 VCC of each of the led drivers. I also removed the lcd and tested each of the LED rails and all worked. Also tested all capacitors with an ESR tester and all passed.

I'm thinking it's still in the driver circuit (page 41 of attached pdf) but unsure what to test next. Any help appreciated.

[Version 7]

Re: Emerson LF501EM4 Intermittant Power On

Anyone up to helping me out on this one?

[budm]

Re: Emerson LF501EM4 Intermittant Power On

"backlight-sw (.21v) and backlight-adj (.18v)" These two signals are from the main board, you are not getting the signal to turn on the backlights circuit, also very low dimming signal.
PROTECT3 is also not normal, it should be around 2~3VDC but since the backlights are not being turned on by the command signal then the PROTECT3 will wrong (notes: I am not 100% sure, I need to look at the diagram again).
Check all those 3 signals when the TV is working.

https://www.badcaps.net/forum/showth...rotect3&page=3

[Version 7]

Re: Emerson LF501EM4 Intermittant Power On

Thank you for looking at this - I've been working on this off and on for over a week and it's driving nuts... Looking at the block diagram, it gives me the impression that the logic board needs to see protect3 voltage from the power board before it will send backlight-sw voltage back to the led drivers - is that correct or am I thinking about this backwards?

[budm]

Re: Emerson LF501EM4 Intermittant Power On

Easy thing to do is to force on the backlights, I believe the instruction is on that thread.

[dick_barton]

Re: Emerson LF501EM4 Intermittant Power On

I would tend to agree with Budm. Without the Backlight-SW being at the correct voltage of approx 2.9V then the led backlight drivers will remain off.
The Backlight-SW is a control voltage from the main board.
I'm not 100% certain but the Fail Detect output of the Led Backlight Drivers are normally at 3.3V but taken to 0V (at Pin 7 internal mosfet switch) because it does not see the Backlight-SW voltage.

I would suggest you carry out the tests as given by Budm in the above post.