Buck Or Boost Circuit?
About 80 to 90% B+ circuit in the CRT Monitor is the boost circuit. Sometimes when we do our repairing work we could come across components that have very blur part number. This will make us have difficulty in identifying the right part. Now come back to this case, if the B+ circuit FET already shorted and you can’t see the actual part number so how do you know whether it is a P-channel FET or N-channel FET? The answer is to look at the circuit how it were connected. For a B+ boost circuit the secondary output voltage is usually about 45 Vdc and pass through a B+ coil and the line is going to the center pin of the FET and to a B+ diode (ultra fast recovery diode).
If it is a Buck circuit the secondary output voltage is usually at about 160 Vdc to 200 Vdc and going through a P-channel FET before to the B+ coil and the circuit will buck the voltage down for the primary winding of flyback transformer. So with this explanation , i hope even without the part number you will be able to know what type of FET (P or N channel) to use for replacement.