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On 21 February 2018 at 11:44, Tom via list <list@xxxxxxxxxxxxx> wrote: > On 21/02/18 11:39, Mark Croft Redditch Linux Mint wrote: >> >> your tv probably just needs a replacement power supply can get them >> from ebay , it a bit of gamble but worth it to get the tv working >> again... you maybe lucky and find a recondition one with compete >> replacement set of uprated caps i think is what usually goes in them.. >> > > Any idea how to diagnose if this is the problem? > I've seen 3 common failures on recent PSUs, 1) Part of the feed back circuit/control circuit exploded and took out a number of transistors, pretty much unrepairable as board gets burnt and its caused by a design floor (replacement PSUs fail in same way) (component spacing/choice is bad) 2) Diode failure, one or more rectification diodes fail, you've got two sets the primary side and the secondary side, primary side is just rectifying mains to 300V DC ish and tanking it on a large cap, then switcher fets kick it through a step down transformer/inductor and its rectified again the other side, the output side is higher current and this is usually where the diodes have failed, sometimes you can just use a multimeter to check them, but when in circuit other components and paths may mask what is happening 3) Feed back failure, somewhere there is a opto isolator/opto transistor providing feedback from the low side to the high side, its usually pretty obvious as it sits right across the isolation barrier which on many PSUs is a white line and gaps in the board. This device provides an analog feedback of the output voltage that controls the switching on the primary side. You can also see cap failures where the electrolytics buldge, this starts off by allowing a lot of noise/ac ripple into the DC lines which depending on what it is can upset things. Sky boxes were good for this, would start of as distortions to picture and audio noise. If you are doing any online testing, be careful of the tank caps storing 300V DC as well as the usual risk with mains. Also if you have a scope be *really* careful of what ground actually is. Parts of the PSU may have a "ground" at -300V wrt earth. Robin -- The Mailing List for the Devon & Cornwall LUG https://mailman.dclug.org.uk/listinfo/list FAQ: http://www.dcglug.org.uk/listfaq