Philips 20PF4121/58 power inverter board problem

[JohnNovak]

Re: Philips 20PF4121/58 power inverter board problem

Check the diodes in the secondary and see if any are shorted. If you disconnect the plug to the main board does the 3.3 volts stay constant?
There is a small elec. cap. in the primary, you could try replacing it. (near u100 opto)

Sorry i overlook this advice.

I disconnect also main board (small signal board).
There is option to manual power up the board to connect pin ST.BY to +3.3V
and load outputs with 12 ohm resistors.
This info is from service manual.

There is no power up also.

I also check main board with external power +3.3V and 12V.
I got green led normally, looks the main board is working.

[R_J]

Re: Philips 20PF4121/58 power inverter board problem

Is it posible to provide the voltages that you get on U101? maybe there is a clue somewhere.
As a caution and to prevent sorting pins, measure the voltage on a component connected to the pin IE: for pin 8 measure on R122

[JohnNovak]

Re: Philips 20PF4121/58 power inverter board problem

Is it safe to disconnect the board from chassis and connect to 230V?
I never do this.
I do not understand where to measure?
The voltage between hot gnd and pin8 i think.

[R_J]

Re: Philips 20PF4121/58 power inverter board problem

Yes you can disconnect the board, and work on it off the chassis. you may not get the 12 volts but you should get a good 3.3 volts
When checking voltages on the primary (or HOT) side, be carefull and always use the (-) of the main filter cap as ground.

Also check the voltage on the fet drain, it should be close to 350 volts

[JohnNovak]

Re: Philips 20PF4121/58 power inverter board problem

Main filter cap voltage = 321V
MOSFET drain voltage variable from 321V to 329V.
IC pin8 voltage at R122 variable from 309V to 314V.

[R_J]

Re: Philips 20PF4121/58 power inverter board problem

ok , thats a start, now the other voltages in U101

[JohnNovak]

Re: Philips 20PF4121/58 power inverter board problem

ok , thats a start, now the other voltages in U101

PIN1= 9.2V to 10.2V
PIN2= 0V
PIN3= 0V
PIN4= 0.80V to 1.37V
PIN5= 0V
PIN6= 0.21V to 0.50V
PIN7= 0V
PIN8= 309V to 314V

If i check basic schematic of IC TEA1532A then PIN7 have no output to
MOSFET gate.

[R_J]

Re: Philips 20PF4121/58 power inverter board problem

Thats what it looks like but the only real way to tell would be with a scope to see if there is a pulse there.
What is the resistance measured on pin 7 to pin 2 or(hot ground)?
also measure the resistance between pin 7 and the fet gate

[JohnNovak]

Re: Philips 20PF4121/58 power inverter board problem

The resistance between PIN7 and HOT GND is infinite,
measured with digital ohm meter 20M ohm range, negative probe to gnd, positive probe to PIN7.
The resistance between PIN7 and MOSFET gate is 15ohm, resistor R143=15R0 is OK.

The only resistance i found which is not clear is R145=12C, from EIA smd marking table should be 13k ohm.
If i measure this resistor show 2k ohm.

[R_J]

Re: Philips 20PF4121/58 power inverter board problem

I keep going back to these voltages

Output DC voltages are:

+10V output = not stable = 2.8V to 3.4V goes up and down
+3.3 output = 0.88V to 0.94V
+12V output = 0.28V

This tells me that the circuit is trying to work,
the 10v is 3v about 1/3
the 3.3v is 1v about 1/3
the voltage on CE108 is likely being being charged to about 1/3 which would be around 7-9v but the startup circuit is already charging it to 9v so it is being masked.
the 12v is 0.3 about zero, this I would think should be about 4v... something must be killing it

[JohnNovak]

Re: Philips 20PF4121/58 power inverter board problem

R J do you have the same power board or you have schematic diagram for this pcb?

[R_J]

Re: Philips 20PF4121/58 power inverter board problem

No I do not have either, I am using your pictures. I have repaired a lot of smps over the years

[JohnNovak]

Re: Philips 20PF4121/58 power inverter board problem

I found suspect part which is connected to st.by
line and which turn on 12V power.
The mark is RKM25 can not found any info.
What can replace this transistor?
Position Q100 on the picture.

[JohnNovak]

Re: Philips 20PF4121/58 power inverter board problem

This LCD have 3 power options.
NO LED is power off with power button on TV.
RED LED is on st.by position when you use remote to power off.
GREEN LED is power on TV.
So maybe if this voltage is missing (Q100 faulty) the power board stay in power off position.

[R_J]

Re: Philips 20PF4121/58 power inverter board problem

power off (no led) means the tv is unpluged. You need standby or nothing works.

[JohnNovak]

Re: Philips 20PF4121/58 power inverter board problem

No there is this option here.
I also never see power off TV with no led.
This model have this position when you press
power button on TV for about 3 seconds the LED is switched off.
When electric shock was occur this TV was off but 230V line
cable was connected.

[R_J]

Re: Philips 20PF4121/58 power inverter board problem

Is that switch connected to the a/c mains in? does that switch have 3 positions?

How do you expect the microprocessor to work and know that the power button is pressed if there is no voltage to operate it????

[JohnNovak]

Re: Philips 20PF4121/58 power inverter board problem

Is that switch connected to the a/c mains in? does that switch have 3 positions?

How do you expect the microprocessor to work and know that the power button is pressed if there is no voltage to operate it????

There is no any on/off switch on this TV.
The voltages are still present here.
I did not design this TV.
I have also Samsung pc monitor with no any 230V switch
and to turn off i press power button for 2 seconds and led is off.

[R_J]

Re: Philips 20PF4121/58 power inverter board problem

What do you think that power button is connected to?

On a computer monitor the power button turns the monitor for off to on and the led will come on (pick a color) IF the video signal is removed, or the computer turned off, the monitor will go into standby (usually indicated by flashing led or yellow led)

This has nothing to do with the STANDBY voltage that is needed for the micro on the main board to work, when the power button is pressed the micro sends a signal to turn on the main part of the power supply.

[JohnNovak]

Re: Philips 20PF4121/58 power inverter board problem

What do you think that power button is connected to?

To u-processor IC7011.
There is 1.8V voltage to processor, and where is this voltage on power board?