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I have a W4 head here that is self destructing. There are no back lights and display but the buttons and controls still work fine.
When power is applied 120 oh resistor R55 incinerates. The manual suggests that the transformer/coil be checked to see if it's shorted but I am not finding a fault. The only thing I am seeing that could potentially cause that kind of current draw would be a bad display or Q50. Are there any other ideas as to what to look for?
Methinks Q50 might be your best bet. Check it to see if its always conducting and let us know what you find.
If thats okay, maybe the coil is shorted somewhere. I dont know what kind of capacitor C50 is, but you could check to see if there is a short there as well. Q50 or its driver seems most likely to me, though.
There are a limited number of components in the circuit. If you think about it for a while, you
have the FET, the transformer, the cap and the resistor. If you view the FET as a simple
switch with zero resistance and the transformer as zero resistance, that just leaves your
resistor to act as a current limiting device.
Do the math and see what you should see for current. My bet is that the diagram is wrong
for the value of the R55 resistor.
You could still have problems on the secondary side of the transformer that is loading down
the whole thing. Have you tried to remove the load by lifting R101 and seeing what happens?
i think several things have happened. You had Q50 go bad for what ever reason, it took out
R55 and you replaced it with the value on the drawing. I really thing the value of R55 is too low.
