RCA LED52B45RQ power board issue

[Keshenatech1]

Hi, I have an RCA LED52B45RQ that has a part number RS150S-4T04 power board. Other P/N's for this same power board are RE46HQ1350 and RE46HQ1351. The symptom is that I have the 5V standby voltage, but no power on the main 5V or 12V, and the red standby LED does not come on at all. After trying a brand new main board from Shopjimmy.com, which they graciously let me return for a full refund, I realized that the main board is pulling the power on signal on the connector to a high state (around 4.7V). I've also powered up the power board on the bench outside of the TV, pulled the power on pin of the connector high with a 1K resistor to the 5V standby voltage, and it still won't turn on the main 5V or 12V. So I believe the power board to be the cause of the problem, but I can't find one anywhere other than a "Non working-parts only" one on Ebay. So I'm trying to troubleshoot the board down to component level to figure out what the problem is. Toggling the "power on" signal, I traced it through some surface mount transistors and to an opto-isolator between the cold and hot side. The power on signal does turn on the opto and it turns on at the output side as well. But that's as far as I had time to trace it over the weekend. Past the opto, there are a couple more surface mount transistors. One measures normal like an NPN, and the other only has .6V drop between 2 pins but I don't know if it's a regular transistor or mosfet as there are no markings on it. (A mosfet only has one measurable junction and the other will read open)

Would anyone happen to have a schematic and/or parts list for this power board, or service manual for this TV? This same power board is also used in a model Led52b55r120q. I know some of these RCA's won't power up if the LED's are bad, but you usually get a standby light. Also, I measured all 3 sections on both plugs with my LED backlight tester and all measure equally good dropping around 100V per section, so I don't believe the LED's to be the cause of my problem but please let me know if you believe otherwise. Any documentation and/or advice on this one would be greatly appreciated. Thanks

[budm]

Re: RCA LED52B45RQ power board issue

Lets start by providing pictures of the whole backside of the TV showing all the boards and then good clear straight shot pictures of the whole top and bottom side of your power supply board.

[Keshenatech1]

Re: RCA LED52B45RQ power board issue

I took some pictures but I can't figure out how to add them because when I click on "insert image", it's asking for a URL and I can't browse to the picture on my computer. I put in the link to the picture of the power board on Shop Jimmy which I figured was better than nothing. Also, may I ask why pictures of the entire TV are necessary? And one other question, is there any way I can set it so I get notifications when I get a reply to my post or a way to subscribe to a thread so I get notifications? I'm new here and don't know how this thing works yet.
By the way, I'm pretty sure I figured out my power supply board last night. There are 2 large diodes, D18A and D18B you can see in the picture at the bottom of the board near the center just to the right of the 4T04 part number sticker. They are 200V schottky diodes, P/N MBR5200. They're used in the feedback circuit for the 12V section of the switching power supply, and one of them is dead shorted. (That is the section that is supposed to turn on when the power on signal is applied.) And from there, it bioses a mosfet between the 5V standby and main 5V. It uses the same 5V section for both, and it runs all the time. That was working, but nothing else was turning on. I found a 20 pk on Ebay for $4 and ordered them. It should fix the board if there aren't any other problems, and I'll update when they come in. I overlooked it the first time because it's across a transformer so a low resistance reading is normal, but one also read close to dead short with an ESR meter so I pulled one end of each out of circuit and sure enough one was shorted. An ESR meter is mainly used to test electrolytic caps, but it will also give a high resistance across an inductor where a regular ohm meter will read a dead short. I do this a lot to confirm my short before actually desoldering the component in question out of circuit, and it saves a lot of time and unnecessary solder work.

[budm]

Re: RCA LED52B45RQ power board issue

Those two diodes are just rectifer diodes fed by the Black and White wires (the center tap wire you cannot see), the output is filtered by two caps next to them, this DC is the input Voltage feeding the Boost converter circuit (the Toroid inductor and the associate devices on the heatsink) to provide the Voltage for constant current power supply to drive the LED strings.

[Keshenatech1]

Re: RCA LED52B45RQ power board issue

It's coming off of the same transformer that's part of the 12V switching power supply, so one of those diodes shorting out could overload the transformer and kill the 12V circuit, right?

[R_J]

Re: RCA LED52B45RQ power board issue

To add your pictures click on Go Advanced, then click on the paper clip next to the smiley face, then using Manage Attachements, browse and add your pictures, then click Upload

[budm]

Re: RCA LED52B45RQ power board issue

It's coming off of the same transformer that's part of the 12V switching power supply, so one of those diodes shorting out could overload the transformer and kill the 12V circuit, right?

That will cause the SMPS to go into shutdown/restart cycle unless it has latched shutdown protection type then it will stay in shutdown state.

[Keshenatech1]

Re: RCA LED52B45RQ power board issue

Thanks Bud, appreciate the info. The black and wires are labeled FL1 and FL2 so I guessed they stood for "feedback loop" but now I see how it connects to the circuit to make that power supply. Now one further question if you don't mind. They used schottky rectifiers in the circuit most likely due to their fast response because a switching power supply runs at a fairly high frequency. Now what I'm wondering is if it schottky's are really necessary in that circuit, or if it would be Ok to replace them with a 5A 200V general purpose fast recovery rectifier such as an NTE577? I have plenty of those on hand. Do I dare try them or do you recommend I stick with the same type of schottky originally used in the circuit? I have them ordered on Ebay but coming from overseas and haven't even been marked as shipped yet 3 days later. I'm curious to see if it works instead of waiting another couple weeks. Thanks again.

[budm]

Re: RCA LED52B45RQ power board issue

Schottky also has lower Vf so it will dissipating less heat. I would not sub those diodes.

[Hurtinkitty]

Re: RCA LED52B45RQ power board issue

Had the same issue with the same model TV. Both diodes appeared shorted in circuit; after removal it turned out one read good and one was a dead short. Replaced both with the MBR5200 Schottky as specified in this thread (after a couple quick confirmation messages from Keshenatech1 - greatly appreciated, BTW!) and TV works great again! Just wanted to say thanks to everyone on this thread!