RCA LED50B45RQ LCD Flatpanel powers off right away

[SurrealMustard]

I'm looking at this annoying 50" RCA LCD and am having a bit of trouble figuring out what's causing it problems. What happens is that when it's plugged in, I can power it up, but it shuts itself off immediately afterward. Once this happens, the whole thing completely shuts down and will not even let me try to power it up again unless power is cut and then reapplied.

I know there are at least three other threads on this, but I wanted to start fresh because not only did the problem not get solved in any one of them, but different conclusions were reached each time.

The only thing I've tried so far is replacing the mainboard with one purchased on eBay, since this is what the cause of the problem was for one other individual on the forum who was experiencing the exact same issue with his set. I'm not 100% convinced that the mainboard I purchased was good, but as always, the odds of two bad mainboards in a row are quite low, so I'm naturally skeptical.

Since the troubleshooting processes always inevitably tend to lead there, I guess I'll get it out of the way now: Is there any way to test the backlight system in this on a basic level without tearing its 50" panel apart? I'll attach some pictures of the topology for this set and the power supply board (borrowed from another thread because I forgot to take pictures when I was inside of it, but it is the same).

[dskall]

Re: RCA LED50B45RQ LCD Flatpanel powers off right away

Don't think it is a backlight issue. Unplug t-con. Does it stay on? What are you getting on screen? What is voltage on main cap when power button pushed?

[maxvalutech]

Re: RCA LED50B45RQ LCD Flatpanel powers off right away

Also can you disconnect main board and short ps-on to gnd to force power supply on as set out in a number of threads. There is a protection circuit to shutdown the tv when there is a problem.

[SurrealMustard]

Followup questions

Don't think it is a backlight issue. Unplug t-con. Does it stay on? What are you getting on screen?

I don't think I can unplug just the T-CON because this unit doesn't appear to have a dedicated module.

What is voltage on main cap when power button pushed?

Which one is the "main" one?

Also can you disconnect main board and short ps-on to gnd to force power supply on as set out in a number of threads. There is a protection circuit to shutdown the tv when there is a problem.

Tried that, got nothin' at all.

[maxvalutech]

Re: RCA LED50B45RQ LCD Flatpanel powers off right away

Because you cant force the power board on with main out of circuit then power board is prime suspect. Main cap/s just after power inlet on hot side and measured on legs with neg to neg pos to pos - measure immediately on button pushed (set up beforehand) and then see if it drops as set turns off due protection.

[SurrealMustard]

confirmation of diagnosis

Because you cant force the power board on with main out of circuit then power board is prime suspect. Main cap/s just after power inlet on hot side and measured on legs with neg to neg pos to pos - measure immediately on button pushed (set up beforehand) and then see if it drops as set turns off due protection.

So, the fact that I get no backlight at all whatsoever doing the PS-ON trick, even though they come on for a split second with the mainboard hooked up, means that the problem likely lies in the power supply and not in the backlights?

[dskall]

Re: RCA LED50B45RQ LCD Flatpanel powers off right away

So far we've seen no voltages at all. Image on screen etc? So it's hard to help

[SurrealMustard]

400, then levels off.

So far we've seen no voltages at all. Image on screen etc? So it's hard to help

That main capacitor gets to 400 briefly after the power key is pressed, then slowly levels off to 173 or so. As for screenshots, there wasn't much to see. It would show the normal on-screen dialogs before giving up.

[dskall]

Re: RCA LED50B45RQ LCD Flatpanel powers off right away

So if you unplug cable to panel board does it still shut down?

[maxvalutech]

Re: RCA LED50B45RQ LCD Flatpanel powers off right away

Voltage information is needed as tested at connector points when power board is isolated and forced on. Also is there a bl-on switch that needs to be forced on as well? The pics needed are of the boards not the screen.

[dick_barton]

Re: RCA LED50B45RQ LCD Flatpanel powers off right away

Disconnect the main board and jumper Stby to standby voltage, check that you have all the supplies up and running and if backlights are not on then jumper backlights to standby voltage. I believe from the other threads they said that the led voltage was 270V.

[SurrealMustard]

clarification and findings

So if you unplug cable to panel board does it still shut down?

With that cable going from the mainboard to the panel disconnected, the symptoms are the same.

Voltage information is needed as tested at connector points when power board is isolated and forced on. Also is there a bl-on switch that needs to be forced on as well? The pics needed are of the boards not the screen.

Here's a better picture of the power supply. Also, a pinout of the connector going off to the mainboard, if it helps.

Disconnect the main board and jumper Stby to standby voltage, check that you have all the supplies up and running and if backlights are not on then jumper backlights to standby voltage. I believe from the other threads they said that the led voltage was 270V.

So, I need to connect 5VSB to 5V?

[dick_barton]

Re: RCA LED50B45RQ LCD Flatpanel powers off right away

Sorry my error. +5VSTB to PS_ON using 1K jumper

[SurrealMustard]

a better way?

Sorry my error. +5VSTB to PS_ON using 1K jumper

I can try, but is there any easier or more efficient way to narrow the diagnosis down between backlights and the power supply board than having to decode and salvage a resistor from something else?

[dick_barton]

Re: RCA LED50B45RQ LCD Flatpanel powers off right away

It's a fairly simple test and only takes a few minutes which can isolate the power supply as being the problem if the power rails come up and backlights switch on?
Having some resistors handy for this task should form part of your test equipment.

[jetadm123]

Re: RCA LED50B45RQ LCD Flatpanel powers off right away

A 1K resistor was chosen because it's a common value and readily available. Any value between 470-1k will work. You're just looking for something to limit the current. Suggest you order a few 1k resistors the next time you purchase parts.

[dskall]

Re: RCA LED50B45RQ LCD Flatpanel powers off right away

Can't get much easier than jumping two terminals with a resistor

[tvtanner]

Re: a better way?

I can try, but is there any easier or more efficient way to narrow the diagnosis down between backlights and the power supply board than having to decode and salvage a resistor from something else?

I think you have some led backlights shorted out so when you turn set on the power supply shuts off. If you had led checker you could check your backlights by disconnecting plug to lights and checking each leg and looking through holes in back for light or you could remove display and check lights one by one or by each string of lights. Good luck.

[tvsteve2]

Re: RCA LED50B45RQ LCD Flatpanel powers off right away

tvtanner is right! Bad LEDs is the problem in these sets! I've done at least 10 all the leds are lousy and when new over driven at 100% I replace all the leds with hot air and reglue the lenses. make sure to reduce backlight settings to 75% they run cooler. I got into the service menu and looked for limiting but couldn't find it.

[102mm]

Re: RCA LED50B45RQ LCD Flatpanel powers off right away

I just repaired this set. it was the LEDs shorted for me. disconnecting them i was able to power on just fine. when i opened it up i found LEDs on a few strings shorted.