Philips 40PFL4908 Power Supply Problems

[neilc6]

Reported as TV not turning on and power supply squealing.

Check all transformer output diodes and not shorted.

Schematic and service manual in this thread https://www.badcaps.net/forum/showth...ight=40PFL4908

Disconnected power supply outputs and there is a very quick squeal when plugged in. AMP+13V and P-ON+21V measure half their expected output.

Bridge rectifier diodes test ok. Fuse F602 not blown. Main switching transistor Q601 not shorted. Measuring 168V on drain (expecting 292V from schematic) so the input is half as well.

Will need to check all the components going to FET of the drain and input of the transformer.

40PFL4908/F7

[budm]

Re: Philips 40PFL4908 Power Supply Problems

Reported as TV not turning on and power supply squealing.

Check all transformer output diodes and not shorted.

Schematic and service manual in this thread https://www.badcaps.net/forum/showth...ight=40PFL4908

Disconnected power supply outputs and there is a very quick squeal when plugged in. AMP+13V and P-ON+21V measure half their expected output.

Bridge rectifier diodes test ok. Fuse F602 not blown. Main switching transistor Q601 not shorted. Measuring 168V on drain (expecting 292V from schematic) so the input is half as well.

Will need to check all the components going to FET of the drain and input of the transformer.

40PFL4908/F7

The power supply board is running otherwise you will not have any output Voltage.
1) When the power supply board is not connected to the main board, the power supply will be in standby condition because it is not getting P-ON signal to turn it fully on that is why the 13V and the 21V are showing about 1/2 of the printed Voltage.
2) The Voltage on the drain of the MOSFET is switching DC at high frequency, your meter is not going to show that correctly.
At this point, you need to connect the power supply board back in place then check the RESISTANCE between AL+3.3V, AMP+13V, P-ON+21V against cold side GND to see if any one of them show low resistance.

[neilc6]

Re: Philips 40PFL4908 Power Supply Problems

P-ON+21V measured low about 8 ohms with board connected up. When I plugged it in, transformer squealed. Will trace that line in the service manual.

I have repaired this board before (blown fuse, shorted transistors and diodes) and +13V and +21V were present with just AC plugged in. The troubleshooting flowchart confirms this.

Further testing indicates squealing with the ribbon to the main board connected.

[budm]

Re: Philips 40PFL4908 Power Supply Problems

Look for C3214 (close to IC 3201) on the main board to see if it also show 8 ohms or not, see service manual page 10-14.

[neilc6]

Re: Philips 40PFL4908 Power Supply Problems

My TV is type A so the equivalent schematic is on page 10-7. C3943 by IC3901. And it measures 8 ohms across C3943.

[budm]

Re: Philips 40PFL4908 Power Supply Problems

There many filters caps connected to the P-ON+21V, so start removing them to see which one will cause the short to go away.

[neilc6]

Re: Philips 40PFL4908 Power Supply Problems

Removed both C3945 and C3943 and still reads 8 ohms. Leaves IC3901 (+12V regulator) suspect.

[budm]

Re: Philips 40PFL4908 Power Supply Problems

If you look at P-ON+21 net symbol, it also goes to other pages, I.E. DIGITAL 7 page 10 -12, there are many many caps on that P-ON+21V line.

[neilc6]

Re: Philips 40PFL4908 Power Supply Problems

On cap #5 out of 10, I found the shorted one. C3616 off IC3603. Removed it and TV turns on! Thanks budm.

[budm]

Re: Philips 40PFL4908 Power Supply Problems

Great, easy fix; lucky it is not the #10 that was shorted. C3214 is the typiacl one that shorted out, you may want to changed all those 4.7uF cap on the +21V line so you do not have to go through this again.