I had never seen or heard of these before. I’m glad I got to repair one though. The first test that I did on this one was to disconnect the secondary taps from the transformer. It didn’t blow the fuse. Then I connected the tap to the power supply/amplifier board and that blew the fuse. I disconnected that one and connected the secondary tap for the preamp. That didn’t blow a fuse either. So that isolated the problem pretty quick. There was nothing obviously wrong on the board to make it easy to find the problem. No busted/burnt parts/spots. All diodes checked fine with my multimeter. A few bad looking solder joints too but they were connecting properly. After scratching my head for a while, I checked the KBPC6-02(6A) bridge rectifier. One of the internal diodes didn’t give me a reading. I was able to find a set of 6A 50v diodes locally and got to work quickly. I connected the secondary tap to the board and that didn’t blow the fuse. I put the amp together and fired it up. What can I say, it works fine now and it doesn’t blow fuses anymore.