Why are my mosfet fuses blowing?


I have a Classe CA-200 Power Amplifier/200 watts per channel into 8 Ohms (side heat-sink version)which is driving a pair of Thiel CS2.3s with upgraded coaxial tweeter/midrange. Sound is very good. I listen at relatively high volumes and recently (over the last year) the amplifier is getting hot within 60 to 90 minutes of listening and the mosfet fuses (2AG 1/2 PT, 1/2 amp fast blow) have been blowing. Do I need a higher powered amplifier to listen at high volume? Should I look for a used CA-200 and use one to drive each speaker (700watts into 8 Ohms)? Thanks.
bcarr38

Showing 1 response by gbart

Bearr38, this is the first time I'm seeing this thread. if both mosfet fuses are blowing, that leads me to believe your problem is with the speakers. I don't understand though why the mosfet fuses would blow under extreme load conditions but not the rail fuses for the output transistors.

BTW, Classé has a specific procedure for replacing the mosfet fuses that includes discharging the main power supply caps with a 100W incandescent bulb before doing anything in the amp, including replacing the mosfet fuses. Failure to do this risks damage to the mosfets. You also need to ensure that replacement fuses fit tightly in the fuseholder, crimping it a little if necessary. Loose mosfet fuses can translate into distortion on top of the audio signal.

I have service manuals for the CA-200 and CA-201. The older CA-200 manual is very basic, and does not have troubleshooting steps. The service manual for the almost identical CA-201 does have troubleshooting steps which you should find helpful. The steps are basically the same for all Classe legacy models. PM me if youw copies of the manuals.