Another free TV from a friend. Fault blown fuse, no power -- same as 42" Vestel I am working on.
Found one shorted diode plus blown FET and associated driver components.
The bank of 8x3A through hole diodes were for the 24V. These diodes are all in parallel, a very bad idea. The diodes are not thermally matched, so if one gets hotter than the other, its forward voltage will drop, causing it to take more current, causing it to get hotter, causing it to take more current... This cycle is destructive to the diode. The poorly designed current limiting of the power supply means this damages the FET and other components. Only one was failed, but I elected to replace all 8.
The power supply is rather bizarre. It does not have a large 400V capacitor. Instead, it uses a transition-mode PFC controller in a flyback configuration. The main 24V output capacitors (3x3300uF) are the bulk for the PFC, plus the 12V. The measured output ripple is in excess of 1Vp-p.
This PSU is also used in the rebadge Toshiba 32BV501/502/701/702 sets. It is an unworthy pile of rubbish.
Found one shorted diode plus blown FET and associated driver components.
The bank of 8x3A through hole diodes were for the 24V. These diodes are all in parallel, a very bad idea. The diodes are not thermally matched, so if one gets hotter than the other, its forward voltage will drop, causing it to take more current, causing it to get hotter, causing it to take more current... This cycle is destructive to the diode. The poorly designed current limiting of the power supply means this damages the FET and other components. Only one was failed, but I elected to replace all 8.
The power supply is rather bizarre. It does not have a large 400V capacitor. Instead, it uses a transition-mode PFC controller in a flyback configuration. The main 24V output capacitors (3x3300uF) are the bulk for the PFC, plus the 12V. The measured output ripple is in excess of 1Vp-p.
This PSU is also used in the rebadge Toshiba 32BV501/502/701/702 sets. It is an unworthy pile of rubbish.
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