Toshiba 37XV505DB No picture

[NipZ]

Powers on but no picture with replacement mainboard

My original problem with this tv was that it would turn on but a few seconds later it would turn off again.

Originally I checked visually for bad caps and found 2 on the inverter which were bulging top and bottom, I have replaced these(2x35v 680uf).
I then checked voltages and found an incorrect voltage at 12v_lcd. Looking a bit deeper I found that 24v goes through an mp1591ds step down converter to get this 12v_lcd, but the mp1591ds step down converter is switched by a 3.3v from the mainboard.
The 3.3v from the mainboard is where I believed my problem originated. If I disconnected the mainboard from the power supply the mainboard would read anywhere from 2.2v-2.9v, but as soon as I connected the mainboard to the power supply the voltage would disappear.

I replaced the mainboard with a 2nd hand Ebay board and now I have a constant 3.3v to the power supply, and the TV acts as it should when turned on, but I have no picture or backlight.
I have 2 questions that I am in hope of getting some help with if possible please.

1) should a mainboard swap work without having to make any other changes?
2) is anyone able to trace the 3.3v and give me any indication to what the problem might be with my original mainboard?

I've included a couple of pictures, one of them indicating where the 3.3v should be and also a service manual that shows the 3.3v(Power_Lcd) on page 23.
Any help is gratefully appreciated.

[tom66]

Re: Toshiba 37XV505DB No picture

What is the model of the LCD panel in your set?

[NipZ]

Re: Toshiba 37XV505DB No picture

LCD Panel is LG Philips LC370WUN(SA)(B1)

Did you get your Toshiba sorted?
You said you were working on the same model last week?

[tom66]

Re: Toshiba 37XV505DB No picture

I am going up to Leeds in a weeks time, I left it over the summer. Same problem originally as yours. Maybe you have the wrong main board. I think Toshiba made these with LG & Samsung panels. Will have to check.

[NipZ]

Re: Toshiba 37XV505DB No picture

All the numbers match up on the boards PE0535 B, V28A000709A1

On my original mainboard its struggling with the 3.3v to supply the MP-1591 on the power supply which means the power supply wont output the 12v_lcd. looking on the datasheet for the MP-1591(attached) it says for pin 7 "Enable Input. EN is a digital input that turns the regulator on or off. Drive EN high to turn on the regulator and low to turn it off. For automatic startup, leave EN unconnected." is it possible to unsolder pin 7 and lift the leg so it is in automatic startup mode and bypass the need for 3.3v?

[tom66]

Re: Toshiba 37XV505DB No picture

You need to ask yourself why the line was not going high in the first place. Most likely because the processor was not sending the signal. And it was not sending it because? Maybe the software on the processor is detecting a fault or has a problem, so it doesn't matter if you bypass it, the software will probably not go past that step.

[NipZ]

Re: Toshiba 37XV505DB No picture

Point taken. Is it not possible to trace the voltage back before it goes through the processor?

I spent a little more time on the tv with the replacement mainboard fitted. I managed to get it into service mode and shine a torch at the screen and I could see an image so it looks like the main board is working and the issue is with the inverter not being turned on.
I started testing voltages at the inverter and this is what I get, does this point to a fault with the mainboard?

14-1.6v , comes from ERR on mainboard
13-0 , comes from DIM_3V on mainboard
12-0 , comes from BL on/off on mainboard
11 , blank
10-GND , comes from GND on mainboard
9-GND
8-GND
7-GND
6-GND
5-23.85v
4-23.85v
3-23.85v
2-23.85v
1-23.85v