Toshiba 37XV505DB missing 12v_LCD

[NipZ]

I was hoping for some help or advice with my Toshiba. The error with this TV is that the power supply turns on, but a few seconds later turns off again.
I have 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 that I dont have 12v_LCD from the power supply.

I've attached a service manual that seems to cover my power supply PE0351

[tom66]

Re: Toshiba 37XV505DB missing 12v_LCD

I am working on this same TV myself. I'm suspecting a potential main board issue, but haven't yet finished work on it as I left to go home. Unfortunately I don't have any more information.

[NipZ]

Re: Toshiba 37XV505DB missing 12v_LCD

Do you have a similar issue with the one you are working on?

I'm very limited in my experience of electronic but very keen to learn more.
It looks like Power LCD is switching mp1591ds-lf-z step down converter(on the back of the PSU) to give the 12v_LCD, but I dont know what values to expect at Power LCD as the voltage isn't consistant and disappears when turned on or connected between the mainboard and PSU

[tom66]

Re: Toshiba 37XV505DB missing 12v_LCD

Yes, same issue. Red LED, press power, goes green, few seconds later it goes back red again? I think that's what I remember.

PowerLCD should be 12V for the T-con board.

[budm]

Re: Toshiba 37XV505DB missing 12v_LCD

The out put on that 12V_LCD should be 12V, it can put out 1.2A of current. As you can see, it has 'PROTECT' sensing Diode to sense the 12V (there is also a 24v sensing circuit on the same page, page 32), if the voltage is not present, the protection circuit will kick in.
The 'PROTECT' signal is fed to Q8162 (page 31), the output 'POWER_PROTECT' is fed to connector P806B which is fed into the uController QA100 PIN81 on page 23.
That buck regulator circuit gets its 24 (22)VDC input to run the IC QE90. Are you getting steady 24vdc feeding it?

[NipZ]

Re: Toshiba 37XV505DB missing 12v_LCD

The IC QE90 is on the back of the power supply. Am I safe to unscrew the power supply from the TV to test pin 2 of QE90 for the 24VDC? does the power supply have to stay grounded to the chassis somehow? also I dont have an isolation transformer, am I still safe working on the TV? Sorry if the questions appear dumb

In the circuit diagram there is POWER_LCD connected to pin 7 of the IC QE90 used for ON/OFF but I have no voltage before or after I turn the TV on.