RCA LED42C45RQ screen blinks, but will not stay on

[budm]

Re: RCA LED42C45RQ screen blinks, but will not stay on

The LED driver circuit is a simple power supply, no boost converter or anything fancy like that. The TV uses two LED strings, each LED string is controlled by a MOSFET, so there is MOSFET per LED string to sink the LED current.
Both MOSFET's GATES (both GATEs of the MOSFETs are tied together) are driven by single driver IC.
So you need to check the B+ on the filter caps (either Cap ES1, or ES3) to see if the B+ goes away or not when BL-ON is present and stays present.
There are some filter caps for those two ICs in the LED power supply/driver circuits that you should check or replace them.
You also need to check the DCV of thse two MOSFET Drain pin for the LED string, you may have only one working string right now.

[Pioneerx01]

Re: RCA LED42C45RQ screen blinks, but will not stay on

When checking the B+ voltage on the filter caps I get 104V+ which instantly starts to drop after LED's flash. Since my multimeter has no min/max option the voltage is likely higher.

I have checked the drain on the mosfets (QS9, QS7) where I get 18V+ on drain of QS9 while QS7 shows 0V at all times. (again no min/max).

Checking on caps is difficult as I do not have capacitance or low ohm meter. I do have new caps for most of the ones on the PSU, so I can swap them out. I do not have the 160V 47uF for ES1 or ES3. I do have 160V 100uF in my spare caps box, but since i do not have capacitance or low ohm meter they remain untested.

Do you have specific filter caps you would recommend I should replace, or should I just take them all out?

[budm]

Re: RCA LED42C45RQ screen blinks, but will not stay on

"I have checked the drain on the mosfets (QS9, QS7) where I get 18V+ on drain of QS9 while QS7 shows 0V at all times. (again no min/max)." That means one of the LED string has open circuit, the one with 0V on the DRAIN pin (the MOSFET sinks the load). You can see how the LED strings are connected in the picture I attached in the other post.

[Pioneerx01]

Re: RCA LED42C45RQ screen blinks, but will not stay on

Right, I see the point, but when I was talking with lookimback about this, he recommended that I jump the +5V with On/Off and BL-On on the power board with 5K resistor; bypassing the main board all together. When I did that, all the LED's came on. For 5 seconds only (repeatedly), but they came on. If that Mosfet would be defective, nothing should have happened, right?

This could be my electronic ignorance speaking, but the circuit was complete and functioning for that time.

[lookimback]

Re: RCA LED42C45RQ screen blinks, but will not stay on

Makes sense. If the MOSFET is shorted between drain and source, you would still get a momentary flash of the LEDs, but then the over current protection circuit shuts it down. If I understand correctly, voltage across a short circuit is always 0v for any amount of current. What do you get for resistance between drain and source on QS7?

[budm]

Re: RCA LED42C45RQ screen blinks, but will not stay on

So ALL the LEDs did come on now after you put it back together, I thought the symptom had changed after you put the screen back.
Then check the MOSFET then, If it is shorted out I will still expect Voltage drop on the current sensing resistors + some resistance on the MOSFET since it will not be perfect 0 Ohms, it can be in milliOhms range, but your meter may be too slow for that.
BTW, you can also swap the two LED return wires which are connected to the Drain of each MOSFET.

[Pioneerx01]

Re: RCA LED42C45RQ screen blinks, but will not stay on

Hello Everyone. Just to keep you apprised of the development on this; I have decided to buy a new power supply board, as due to the limitation of my multi-meter (and my knowledge), it was my belief that there is more wrong than just two mosfets (if at all).

So, I installed the new PSU today, and guess what, same exact issue. Since this is a third time I have replaced PSU on this TV, all of which were giving me same problem, it is my current belief that the PSU is not the issue, at least not initially.

My "educated guess" (or lack there of) would be that even though none of the LED chips and modules are visibly defective and function fully, there is problem somewhere there that is causing the PSU to shut down. Since finding replacement LED strips would be difficult, expensive and time consuming to replace, I have decided to call time of death on this TV.

I do appreciate all the time and effort you have put into helping me diagnose this, and I wish this would have more fruitful result.

Thanks
-Peter