Philips 20PF4121/58 power inverter board 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.

[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.

[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.

[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

Today i check again all outputs, looks OK.

[JohnNovak]

Re: Philips 20PF4121/58 power inverter board problem

Thanks for reply.

I measure DC outputs when TV is turn off.

+10V = 12.35V
+3.3V = 3.57V
+12V = 3.57V

So the only voltage which goes down is +12V.

Switch FET=Q112

[R_J]

Re: Philips 20PF4121/58 power inverter board problem

I checked the schematic for the tv and on page 29 it shows +12VSW coming from the power supply, but on ic7930 the VCC is shown as 14V4 so 15volts might be ok.
Does the +12vsw turn off when the tv is turned off? if not the fet that is used to switch it might be bad.

[JohnNovak]

Re: Philips 20PF4121/58 power inverter board problem

Hi again, R_J.

I finally receive new TL431.
I replace both Q111 and Q105.
DC outputs exactly the same, so one was faulty, one was good.
All resistors shows original value.
Still worry about high +12V output which is 15.36V.
This is more than 10% which is allowed by factory, 13.2V will be normal.

TV works normal, is it safe to run with so high +12V output?

[R_J]

Re: Philips 20PF4121/58 power inverter board problem

Thats good , its almost there, Check the reference resistors, make sure they are not damage as in changed value

[JohnNovak]

Re: Philips 20PF4121/58 power inverter board problem

R J, i have some good news.
Today i replace Q105 with TL431 i got power up with DC outputs:

+10V = 12.42V ???
+3.3V = 3.51V is OK
+12V = 15.36V should be 14.4V ??

I'm a little worried about +12V and +10V high outputs.
I will replace Q111 and Q105 with new TL431 and then will see what outputs will be.
TV works normal.

[JohnNovak]

Re: Philips 20PF4121/58 power inverter board problem

Let we take a look one detail about TL431, if you have any new outside of pcb board,
if you measure with analog ohm meter x1 from pin3 to pin1 do you have any resistance readings like normal diode and from pin1 to pin3 no resistance?

[JohnNovak]

Re: Philips 20PF4121/58 power inverter board problem

It looks the same to me like on the picture.
I make some photo but very hard to make good quality picture.

I have 10ohm readings at Q105, pin 1 and pin3 in both direction so i desolder out this part,
looks is damaged.

Pin3 goes to gnd, yes. Looks we have voltage reference here.

Q105 have pin3 to gnd, Q111 have no grounded pin.

I put pin out on the picture, should be correct?

[R_J]

Re: Philips 20PF4121/58 power inverter board problem

This pdf shows the T3GU marking for a sot23-3 on page 36
The other site I linked to also mentioned that part as being a Precision Programmable Reference
You might need to draw out the circuit for that area to see how it is used
Usually that area of the secondary near the optocoupler, can have a 3 pin reference, they may have been lazy and labeled it Q

Does one of the leads go to ground? which one? Also was the original one shorted?

Do the letters look like this picture?

[JohnNovak]

Re: Philips 20PF4121/58 power inverter board problem

Hi.
I finally found the fault and now big problem to decode this part.
I check TL431 readings with ohm meter, not even close to good T3GU.
The silkscreen show Q for transistor, for voltage reference should be U or something.
And TL431 have 431 mark.

[R_J]

Re: Philips 20PF4121/58 power inverter board problem

I can't be sure, might be a TL431BCDBZR
Found this site that mentions them also http://monitor.net.ru/forum/philips-...fo-231989.html

[JohnNovak]

Re: Philips 20PF4121/58 power inverter board problem

R J.
I need your help, i think i found the problem.

I found faulty SMD transistor with SMD mark T3GU, position Q105.
Ther are 2 transistors on this board, measuring the second good transistor Q111 with analog ohm meter
shows PNP darlington type.

Now i search replacement for this transistor.

[JohnNovak]

Re: Philips 20PF4121/58 power inverter board problem

Happy New Year to all.

[JohnNovak]

Re: Philips 20PF4121/58 power inverter board problem

I check all loads are high, no shorts or heavy loads.
+12V fuses are OK.
What can be wrong here?

[JohnNovak]

Re: Philips 20PF4121/58 power inverter board problem

I mark with red lines.

[R_J]

Re: Philips 20PF4121/58 power inverter board problem

Ok good I thought I missed something, Can you trace back the +12 volts? Is it the supply that connects to the small fuse that supplies the BL inverter section?
It does seem that the +12volt line is loading down the rest of the power supply