Broken Philips TV power supply board - SOLVED - Page 1
Forum section: electronics / repair

author: Freeride
author_rank: Newbie
timestamp: August 04, 2023, 11:20:05 pm
content: Hi,I have a Philips TV model 32PFL7695/12 with a broken power supply board. As soon as I plug in the external power, it blows the fuse (3.15A / 250V). I have done some simple connectivity measurements and the only "strange" thing I have found is that the resistance over the component (NTC1) marked in the figure is zero Ohms - I have no clue if this is correct. I would be grateful for any advice that could help me diagnose and repair this board.Thanks in advance.

author: fzabkar
author_rank: Super Contributor
timestamp: August 05, 2023, 12:05:15 am
content: Check the bridge rectifier (adjacent to NTC1), diodes and the power semis under the heatsinks for shorts.NTC1 is a negative temperature coefficient resistor for inrush current limiting. It will have a small resistance when cold, and this then drops to zero, or close enough, when it warms up due to I2R heating.FYI, I did find a reference to your PSU in the following manual, but "all power supplies are a black box for Service". :-(https://www.brelect.fr/ve/37PFL5405H12.pdf

author: Freeride
author_rank: Newbie
timestamp: August 05, 2023, 08:52:16 pm
content: I have measured the bridge rectifier (BD1) using "diode mode" on my multimeter. The results are in the attached picture. Debugging power supply implementations is not my expertise area (I am more into ones and zeros :) so I can't judge if my numbers show an ok component, or a broken one. I'll continue with some more measurements on the other components you proposed.

author: fzabkar
author_rank: Super Contributor
timestamp: August 05, 2023, 09:03:57 pm
content: Assuming that "0" means "overrange", then your bridge looks OK.BTW, this is the internal layout:https://res.utmel.com/Images/UEditor/97c40e21-39d7-4116-8379-77dfdad93329.jpg

author: Freeride
author_rank: Newbie
timestamp: August 05, 2023, 10:51:46 pm
content: After some digging around I spotted a blown capacitor (C813) on the board, picture attached. It is marked LR 221K 1KV. When I measure the resistance over the capacitor (to test for a short) I measure ~90 kOhms. I tried to see if anything else is connect to it in parallel, but I haven't found anything yet.My question now is; can a broken capacitor like this cause the main fuse on the board to blow when I connect the external power? Or is this just a secondary affect of something else also being broken?Any advise is appreciated.

author: fzabkar
author_rank: Super Contributor
timestamp: August 05, 2023, 11:28:26 pm
content: That's a 220pF, 1kV ceramic capacitor. LR is the dielectric code. It could actually be the culprit.I suspect that there is an APFC circuit in that area, but there would need to be a controller on the other side of the PCB.

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 06, 2023, 05:01:56 am
content: Jumper the fuse and connect the AC input to a low voltage current limited bench power supply, then you'll be able to find the current path by checking what heats up. If it doesn't draw any significant current with low voltage, try going to mains with a light bulb limiting the current.

author: DavidAlfa
author_rank: Super Contributor
timestamp: August 06, 2023, 05:36:19 am
content: It seems you measured short between some rectifier legs? Are you measuring 0.x or "OL"?Measure for shorts in these componentes.If any shows short, desolder it and measure out of the circuit.The transistors will be trickier as the coil might make them look shorted. It's better to just remove them for inspection.With some skills you can heat the pin (Apply a blob of fresh solder) and bend it slightly until it comes out.I'd start by removing the rectifier. The blown capacitor might have killed the transistors too.

author: Freeride
author_rank: Newbie
timestamp: August 08, 2023, 10:44:54 pm
content: Update. I removed the broken capacitor and tested the power board in the TV - and now it works again. It must have been the broken capacitor that caused the fuse to blow. I will replace it with a new one as soon as I get hold of one.About the rectifier measurements, the zeros in the table is when my meter shows .0L, so I think that component is good.Thanks for all the help. I hope I can help someone else here in the future.