Hi
This is just an account of my protracted tussle with a few of these power supplies.These units had the usual treatment of blanket replacement of all suspect electrolytic capacitors ,but to no avail. The problem was endlessly trying to start but failing to do so, the 24 volt supply would reach 18 volts or so and then fall away again. A logical approach was called for to get to the root of this. Where does one begin. Just to say that these power supplies will work outside of the TV with only the mains lead connected .First I made up some harness which consisted of a 3volt battery supply connected to pin 6 of PL803 with a switch,second a two lead connector with a switch connected to PL811. I decided at this stage to supply VCC1 and VCC2 with an independent 12 volt supply. Before connecting this supply I cut the connection from the standby supply to IC828 where they are normally supplied from.When the psu was connected up there was a stable 24volt supply also a PFC of 390 volts, so the fault had to lie with the standby supply. So what to do next,a lot of time spent examining the outputs 3.3 volt, 5volt ,12.2volt outputs,and the feedback loop via the optocoupler IC801 .Then I realized that that the standby supply was being shut down as if the standby switch connected to PL811 was closed. Studying the circut more closely showed that Q802 has the same effect .Q802 is in turn controlled by three other transistors,Q831 which monitors the 12.2 line viaSW1, Q833 AND Q834 which monitor the 5volt and 24 volt outputs. If the 24volt or the 5volt supplies which are potted down are missing from the bases of Q833 or Q834,it will cause Q802 to switch on and shut down the standby supply via SW2. To cut a long story short the voltage was missing from the base ofQ833. The cause being D854 the 12 volt zener diode open circut. Replacing the diode restored normality .Strange to say the other two P S U s had exactly the same fault. Ihave never read of that particular fault previously ,perhaps it is the cause of a lot of scrapped power supplies . BIGTOM
