I replaced about 10 electrolytic capacitors and a PWM controller IC (MC34067P).
Fortunately, i found an almost identical schematic diagram in the
internet, so i started troubleshooting.
To make it short, i replaced most of the electrolytic caps and mylar
caps in the Preconditioner stage and VsVa Power supply stage. And
finally the PWM Oscillator IC (MC34067P).
You have to make sure that the Preconditioner stage is running ok,
with an output of 380 to 395 volts DC. There are two fuses on the
board (in my 32FD9954), one is for the mains AC located near the mains
connector coming from RF filter (two wire connector- large wires), the
other fuse is in the middle of the board. You measure the DC voltage
on this fuse on the middle of the board and it should be about
380volts or above.
The real culprit is the PWM IC. Initially i replaced the MOSFET
(STY34NB50F) in the Preconditioner, then the caps, then finally the
IC. Then it worked!!! I put back the old MOSFET, it's not damaged, so
now i have a spare one. Just replace the electrolytics near the PWM IC
(2.2u, 4.7u, 100u, 1000u)
I think the design problem is that the IC is nearly located to the
heatsink which generates lots of heat. The IC deteriorated in time and
the electrolytes in the capacitor evaporated due to the excessive heat
inside the case. I'm thinking of incorporating 120cm computer fans at
the back to suck down the heat.
Happy troubleshooting. I have the necessary diagrams if you need it.
Oct 31, 2006 |
Philips 17PF8946A 17 in. EDTV-Ready LCD...