Seiki SE47FY19 - No backlights

[MEB]

Re: Seiki SE47FY19 - No backlights

Any help? Or schematic for MEGMEET MIP550D-TF

[MEB]

Re: Seiki SE47FY19 - No backlights

I am caught in an endless loop undoubtedly limited by my lack of knowledge, and desperately need help. I have attached photos of the affected area of the power supply board. The situation is as follows.

There are 2 separate strings of 27 leds.
Each string is powered from the power supply/driver board via a connector for each string.
I have powered up each individual led and by using a power supply have powered up each strip of 9 leds and they all light with success and equal illumination for many minutes using 23V/20mA and 26V/50mA and I don't know what else I can do. From these results I feel they are fine. Regardless of which strips are connected to make the 2 separate strings, I always get excessively high current through the top 27 leds and low current in the bottom 27 leds as I reported above. If there was a problem with the leds, I expect the current problem to follow the strings. Is this correct?

I could then see a problem with the leds. So I am stuck on thinking the problem has to be in the board. Does that make sense?

I have attempted to trace the drive circuit on the board, but it seems to me that both connectors are driven by the same circuit on the board and I can not understand how I can get high and low current through the 2 strings if the leds are not defective unless the board somehow is.

Please, someone enlighten me. This does not make sense to me, I am sure because I lack knowledge and feel free to call me an idiot, but if I can not get this straightened out in my mind, this hobby of mine is surely to take a turn for the negative. I am not sure what direction to take to solve this problem.

[dick_barton]

Re: Seiki SE47FY19 - No backlights

It could be a board problem if the led driver IC has feedback pins for over current and over voltage to monitor the voltage and current fed back from each led string.

I would identify the led driver chip and look at its data sheet so that you can compare the two feedback circuits.


I would also check out the two mosfets QL1 & QL2 which look as though they may be part of the led output stage.

[MEB]

Re: Seiki SE47FY19 - No backlights

Thank you for the information. I will gather and get back.

[MEB]

Re: Seiki SE47FY19 - No backlights

OK, What I found out:

QL1 -TF450 N-Channel Mosfet, Alpha&Omega
QL2 - HBR10150 Schottky Barrier Diode, Jilin
UL1 - OB3350CP LED Controller, On-Bright

Datasheets attached. With multimeter QL1 tested no shorts between any pins in circuit, QL2 tested directional like 2 diodes. The LED Controller chip I found, just adds more questions as I expected to see feedback and ouputs for 2 channels (LED strings) but didn't, which further confuses me as to what I am thinking. I can provide more information if directed, but I am about out of ideas on my own. That is why I am here though!

[dick_barton]

Re: Seiki SE47FY19 - No backlights

Looking at the pdf of the led driver it only has one output so it looks as though one output is driving the 2 strings of 27 leds in parallel.

If this is the case then the voltage applied to both sets of leds is identical.

Con6 looks as though it has the two Led1- pads connected together and also the two led + pads connected together.

If there's a difference in brightness between the two led strings then it definitely has to be a led problem since the electronics is common to both led strings.


Edit
I noticed that some leds strips have what may be a protection diode / zener diode fitted on them. Do any of your led strips have extra components?

[MEB]

Re: Seiki SE47FY19 - No backlights

Thank you very much for the reply. I have been gone for our Thanksgiving Holiday and not able to respond quickly. I will gather information to answer your questions later today.

[MEB]

Re: Seiki SE47FY19 - No backlights

I have attempted to attach, if you can view them, a sketch showing how I believe the strings are wired and an image showing the back section of the power board on which I attempted to trace out the circuit previously. There is some shorting of pins on Conn6 and Conn8 but the pins are actually not used on the board in my opinion.

I could not locate any additional components on the strings themselves, only connectors and LEDs as shown in the sketch. Am I not sufficiently testing the LEDs/strings cause I still feel they are all good?

If you can see on the attachment Conn6 and Conn8 connect to the circuit through a jumper (Conn6) or a no value resistor (Conn8) and the only other difference is through R29 and R33. All other routes are through common components in the single driver circuit I believe.

Thanks for your help and I will attempt to supply all the information you request.

[dick_barton]

Re: Seiki SE47FY19 - No backlights

Have you checked the resistance of all the resistors including the 000 ohms shown in the photo seiki power.bmp ?

[MEB]

Re: Seiki SE47FY19 - No backlights

000 resistor checks shorted.

There are the 2 resistors marked R470 that I think are supposed to be .47 ohms. One goes between Conn6 and drive circuit, and ones goes between Conn8 and drive circuit. The Conn6 resistor reads .9 ohms and the Conn8 reads open. This surely must be a clue! I will await your instructions, but am prepared to extract and read out of circuit. These reading were all with the board totally disconnected from the unit.

[dick_barton]

Re: Seiki SE47FY19 - No backlights

I think it's best to lift them carefully. The resistor marked 000 is just a short circuit of 0 ohms so if it is open that is a real fault. The other two marked R470 I believe are 0.470 ohm resistors. If they don't read that when lifted then change them. I suspect they limit the led current in each of the strips.

[MEB]

Re: Seiki SE47FY19 - No backlights

I removed both of the R470 marked resistors from the board and they checked same as in circuit, open and .9 ohms. I have ordered some and will get back in a few days. Will I just fire it up with the strings reconnected or is there a gentler approach? Thanks again for your help.

[dick_barton]

Re: Seiki SE47FY19 - No backlights

Try it with the strings reconnected and if it comes up without any problems which I hope it does, then turn down the backlight to save the lifetime of the led's.

[MEB]

Re: Seiki SE47FY19 - No backlights

Thank you. I will report back probably Thursday!

[dick_barton]

Re: Seiki SE47FY19 - No backlights

OK Await patiently for your return.

[MEB]

Re: Seiki SE47FY19 - No backlights

Ok, received and installed the current sense resistors, R29 and R33. Replaced strips and all 54 LEDs now light with equal intensity! I had previously opened each string's wiring to measure current and now am reading .338 and .328A in each string. They are burning brightly. Ran them just long enough to make readings. Is that too much current or am I measuring incorrectly? It seems rather high to me. Am totally excited and thank you for getting me this far.

[dick_barton]

Re: Seiki SE47FY19 - No backlights

Well done.

The rule of thumb appears to be:-
The more you can reduce the backlights (hence the current through them), the greater the increases in the lifetime of the leds.

[budm]

Re: Seiki SE47FY19 - No backlights

Great job in deed, turn down the backlights level to as low as possible, if it is really running at >300mA then I do not expect the LEDs to last very long time. BTW, for the LOW Ohms resistor to go open circuit, lots of current must have flown to it.

[MEB]

Re: Seiki SE47FY19 - No backlights

Thank you very much. The internet called those 2 resistors, current sense resistors. Are there any chance of being some "current limiting" resistors that may be responsible for the high current? I say high current, not from experience, but you told me to test at 50mA and I am quite a bit higher than that now. I will try to see if I can get into settings and lower the output, but I don't have the remote and not sure if I can do that from the front buttons or not.

[budm]

Re: Seiki SE47FY19 - No backlights

50mA is just testing level to start with.
The typical LED drive setup is that output of the boost converter power supply + is fed to the Anode of the LED string, the Cathode of the LED string is connected to the Drain pin of the PWM Controlled (To maintain constant current through the LED string) Power MOSFET, Source pin of the MOSFET is then connected to the top end of the current sensing resistor, the low end of the resistor is then connected to the ground return, the Vdrops on the resistor is used as the feed for maintain the constant current.