This PCB had a crude “electronic fuse”. Tr17 and tr18 latched on when the amp was shorted out. This in turn collapsed the voltage reference for the two constant current sources therefore putting the amplifier into a standby mode. To effect a reset, the mains supply had to be removed for approximately 5 minutes. Many amplifiers were built without any fuses in the speaker line. These days use of a disconnect device is recommended should the amplifier “go DC”. A suitable crowbar device for this is described elsewhere in this site . This seems to be the first amplifier around 1980 that had an electronic latch rather than fuse or circuit breaker.