As you may know, the MOSFETs used by Counterpoint in the output stage are fragile and are no longer available. The only way to replace them is to use another manufacturer. We are using Exicon MOSFETS in other projects and found them excellent performers. They are designed specifically for high power audio applications and are lateral types, thus they are far more rugged than the original MOSFETS, making them the perfect choice.
The first step was to design a new bias network to match the Exicon's specifications. Once we had the output stage working with the new Exicons, we took a hard look at the TO-3 package of the MOSFETs the heatsink was designed to hold (See inset below). We decided to use a TO-247 package for much better electrical connections. This decision led us to use a new printed circuit board on each heatsink, which gave us a tidy place for the gate resistors and zeners that were originally soldered on the TO-3 connectors.
Since we now had a new circuit board, we thought it was the perfect spot to place the new bias network, rather than try to patch it on the original board. Also, the voltage regulator circuit for the bias network has the reputation of overheating and failure. (See inset below) We decided a new LED regulator mounted on our new circuit board was just the thing to ensure reliability for our new output stage.
One of the consequences of a blown output stage is that the muting relay can be damaged. We wanted our restoration to address all potential failures, so we put the new relay on our new board as well. All that was left of the output stage on the original board was the DC offset resistor and two film capacitors, both of which were over 20 years old and a little tired. We updated the resistor and capacitors, making our board a complete replacement for the output stage.
After all of our work, it didn't make sense to use the same production quality parts that Counterpoint installed so the parts we use are as follows:
The power supply bridge rectifier is replaced with four discreet FRED diodes. These fast, soft recovery diodes supply smooth and silent power to the MOSFETs.
PRP PR9372 Audio Application leaded Nichrome thin film resistors.
Swiss made Axon True Cap Metalized Polypropylene Capacitors.
Silver-plated beryllium copper fuse clips.
All on a silver-plated, 2oz copper, double sided, through hole plated printed circuit board.
Our MOSFET board will greatly improve the sound of your amplifier and restore its reliability for years to come.
CUSTOMERS INTERESTED IN HAVING US PERFORM THE RESTORATION
CLICK HERE for shipping and warranty information
CUSTOMERS INTERESTED IN DIY KITS CLICK HERE to download instructions and necessary tool list. The kit includes 2 boards that are fully assembled and bolt into existing holes in the heatsink. Terminated connection wiring is attached to the boards for easy installation.