Broken Plasma TV (Need a bit of help troubleshooting + identifying part) - Page 1
Forum section: electronics / beginners

author: Ryu.Hayabusa
author_rank: Contributor
timestamp: October 30, 2012, 08:04:46 am
content: Well, I broke my plasma thanks to me using a metal screwdriver to adjust a pot, but missed and hit a resistor + grounded heatsink instead. Once that happened I got a big spark. After that the panel had tons of "sparklies", or red/green/blue pixels that flash all over the screen, which looks a bit like static. The colors for the picture are somewhat off and have some vertical banding, which makes me think the panel is now being overdriven.The pot that I was attempting to adjust appears to no longer change anything on the screen as it used to, however the other 2 pots on the upper part of the board still function.I thought I identified the problem as being a bad MOSFET since doing the diode/resistance measurements on the one closest to the pot versus an identical one on the board near the other pots gave different results, and when the TV was on the heatsink was barely warm while the others are quite hot. So I got a close enough replacement to it, fired it up and not much changed in the picture or the heatsink temp.The resistor I hit with the screwdriver is a surface mount '3R0'. I checked the value in-circuit and got a reading of over 4 MegaOhms, which is way off from where it should be. I also disovered a zener diode close by that gives a shorted reading on the multimeter (0.000v). The pot reads the same range as the other ones which are identical, ~0.3ohm to ~1kohm (~ = approximately).I have read that testing resistors in circuit may not be reliable, though all of the other surface mount resistors read as they should except for one other that was marked '104', which was also off by a large amount.The questions I have so far are:1. Is testing resistors in-circuit reliable? What about MOSFETs? What about diodes?2. The zener diode has color bands on it, how can I identify what replacement part I need based off that.Sorry for the long and probably confusing post, it's a bit hard to condense it all. If pictures or more info is needed let me know.Thanks!

author: Jonny
author_rank: Regular Contributor
timestamp: November 05, 2012, 11:31:21 pm
content: Measuring resistors in circuit won't show you a higher value than it should be unless perhaps the resistor is open circuit, then you're just measuring other stuff. Without seeing a circuit diagram it's hard to say but if a large flow of current has blown the resistor apart internally that would explain it and if the resistor is in the drive to the FET that would explain why the HS is cold, its' not being driven. Try googling a chart for zener markings. any numbers printed on the PCB near it? Might suggest its value e.g. 3v6 is a 3.6volt zener. If that's on the drive side of the FET to then that will also kill the drive being shorted. Original FET might actually be ok unless you've measured shorts between pins when out of circuit?Good luck.

author: tec5c
author_rank: Frequent Contributor
timestamp: November 06, 2012, 06:27:23 am
content: Somewhereinsidethe TV on thebackof thepanelthe is a sticker that has the panel voltages ie. Vs, Va etcIf you inspect the power supply board more closely there will be a few test points with corresponding panel voltage labels as on the sticker (Vs, Va)I would suggest measuring these and seeing if they are accurate to what the sticker says. If they are not then also on the power supply board will be trimpots that are also labelled with the corresponding panel voltage label (Vs, Va). These pots will adjust your panel voltages.What is the model number of the TV?