TV/Power supply problem - Page 1
Forum section: electronics / repair

author: whaase
author_rank: Contributor
timestamp: April 06, 2015, 09:31:59 pm
content: I have a TV I'm trying to fix. I do know a bit about electronics, but not a lotThe power supply is making a weird buzzing sort of sound and will eventually shut off until it cools down. The TV works great otherwise. Attached is a picture of what I'm looking atI replaced the capacitor that is on above the small black heat sink. But I think the sound is coming from the grey rectangular capacitor that is below the small heat sink or the coil. The capacitor is marked on top MKP 25 - 150K 450V? Is how I read it. I believe that's a 1uF, 450v. Can that be replaced with a standard radial capacitor?I will post a link to a small video of the sound, right click and save ashttp://whaase.com/walter/tv-prob.mp4Any help would be appreciatedWalter

author: cpuerror
author_rank: Regular Contributor
timestamp: April 06, 2015, 10:44:32 pm
content: I suspect those capacitors are there only for noise suppression. You likely have electrolytic capacitors somewhere else on the board that are faulty.

author: whaase
author_rank: Contributor
timestamp: April 06, 2015, 10:47:17 pm
content: Maybe I should replace them all and start there? For the few $ I suppose it would be a cheap try

author: Evil Lurker
author_rank: Regular Contributor
timestamp: April 06, 2015, 11:04:42 pm
content: Those are foil x-caps used as part of the input filtering circuit. While I have seen them go out of spec they rarely fail. Start looking at all the electrolytic capacitors. If they are from a dodgy manufacturer or show signs of failure such as bulging tops or leaking they will likely need to be replaced.

author: whaase
author_rank: Contributor
timestamp: April 07, 2015, 12:49:35 am
content: I did look them over. I couldn't notice any that looked our of the norm... but I also know that doesn't mean they are good. For the small cost to replace them all, I will try that first and see.Thanks!Walter

author: tec5c
author_rank: Frequent Contributor
timestamp: April 07, 2015, 01:52:53 am
content: What brand/model TV is it?

author: whaase
author_rank: Contributor
timestamp: April 07, 2015, 02:04:33 am
content: Its a Dynex 46" LCD from 2011.Model is DX46L262A12

author: BradC
author_rank: Super Contributor
timestamp: April 07, 2015, 03:10:17 am
content: I just did a 37" Hitachi from 2007 with similar symptoms. The fault turned out to be a 0603 MLCC ceramic cap that had become heat sensitive. It took a couple of hours to find with a can of freezer spray, a butane soldering iron and a hair dryer. All the electros are perfectly fine. so it's not always the obvious ones.

author: whaase
author_rank: Contributor
timestamp: April 07, 2015, 03:24:21 am
content: Interesting, maybe I should take that route. Do you just spray each part and see what happens until you get it?Walter

author: BradC
author_rank: Super Contributor
timestamp: April 07, 2015, 03:41:10 am
content: Quote from: whaase on April 07, 2015, 03:24:21 amI took a bit more of "scientific" approach because the PSU board in this telly is *big*. I used a binary search. Heat the whole board with a hair dryer until it is way past the point where it fails, then hit half the board with freezer. That will tell you which half of the board the fault lies on.Repeat, doing a half of each area at a time until you narrow it down to a small area. Get it nice and cold, and then just heat up each component until you get the one that causes it to fail. Mine was on the live side of the set, so I used a gas iron and just touched one pin on each SMD component until I found the one most likely to make it fault. Repeated that a few times just to be sure, and replaced the part. It was a weekend and I didn't have any SMD MLCC's. Measured the voltage in circuit to make sure I could use a standard part and just replaced it with a small leaded component.It's a lot easier with through hole components, but same principle with SMD's.I did a similar thing to repair the PSU on my Apple Thunderbolt Display. One of the SMD MLCC's had gone short and was causing the PSU to go into immediate protect mode. I suspected it was an MLCC that was faulty as all the electros are solid polymer, so I just hit each one with the CRO until I found a rail that was shorted (no pulses as the PSU tried to start up). There were only 3 on that rail, so I just removed/replaced one at a time until it sprung to life. SMD soldering tweezers made life easy there, although I have a couple of forked Hakko tips also. They are just harder to use as you need a bit of digital gymnastics to be able to grip the component whereas the SMD tweezers desolder and remove in one motion.Odds are it's probably an Electro on your telly. Mine were all top quality 105C Rubycon parts, but it didn't stop me hitting them all with freezer first up just in case. I did a PSU in a Cisco 3550 switch that had a thermally sensitive Electro in a similar fashion. Through hole Electros are easy though because you can just rest the gas iron on the aluminium cap to heat them up selectively.

author: whaase
author_rank: Contributor
timestamp: April 11, 2015, 02:59:43 pm
content: I have tested all the capacitors out of circuit. Everything tested within tolerance. I did notice while trying to narrow down the source of the sound I got a bit of a shock from the heat sink that the bridge rectifier is mounted too. I tested the rectifier and its tests good. I'm stumped lol My 24v output is all over the place on all pins and this sound continues.Walter