Philips 20PF4121/58 power inverter board problem

[JohnNovak]

Re: Philips 20PF4121/58 power inverter board problem

Today i check again all outputs, looks OK.

[R_J]

Re: Philips 20PF4121/58 power inverter board problem

I found a hand drawn schematic for a DAC-12m018, maybe it will help, I suspect if the voltage is only slightly higher, the problem could be around the reference ic tl431 that controls the optocoupler. it's that circuit that does the regulation. I don't know how accurate this drawing is.

[JohnNovak]

Re: Philips 20PF4121/58 power inverter board problem

Thanks.

I will check again resistors in that area.
There was electric shock here which destroy TL431, maybe i did not check good before.

I found some issue or maybe is not?
When i use scart input and i power OFF TV and power ON again there is no backlight back. I must disconnect power cable to get backlight operation.
When TV is on channel mode backlight works OK.

[JohnNovak]

Re: Philips 20PF4121/58 power inverter board problem

R_J.

I check resistor all OK.
I have no idea how to fix DC output to 14.5V.
I think TL431 have nothing with 12V output.

Can we check what is possible issue with buttons on TV?
TV have basic buttons on the top.
Power, vol -, vol +, MENU, P-, P+.
Some buttons not respond correctly.
This issue was present before and still make problem.

[R_J]

Re: Philips 20PF4121/58 power inverter board problem

The output voltage is monitored on the secondary and controls the primary ic, the only way it can do this is via the optocoupler to maintain isolation between the primary and secondary
There might be a zener diode that has changed value? It could be that they monitor the +10 volt line for the feedback and not the +12v

The fault with the buttons is likely the tac switches, they can develope some resistance across the contacts and change how the functions operate.
for example when CH- is pushed it shorts r3319 (1K1) to ground, if the menu button is pushed it shorts r3324 (2k0) to ground.
Now it the CH- button sw develops 1k of resistance, it will act like the menu is being pushed.