Picked up this 828 in rough shape and @dlucy dropped by and we gave it the once over. Since it has three extra switches added to the back panel we started by determining what they did. The first switch is above the preamp output/power amp input. This is what that switch controls. Takes the preamp signal and sends it either to the power amp or the preamp RCA jacks. Since this unit used a jumper bar with RCA pins, this mod makes those unnecessary since putting the signal on the Main Amp Input RCAs sends it to the amp section. The second and third switches, located between the ground and the A speaker outputs switch power. One DPST switch stops the power to the main caps, one side for - and one for +. The other switch interrupts power from the power supply board to the main board and to the protect board. The switch to the protect board interrupts PS Pin 7 to Protect Pin 11 the other side of this switch interrupts PS Pin 17 to Main Amp Pin 23 Here is the Main amp circuit: Here is the protect board: Seems the amp and protect could be turned off along with the main PS caps. The unit runs on other power so when using it as a preamp only changes could be made. Anyone know why this might have been installed on this unit? But that is not why I'm writing this today. The unit has a pop when turning off the Low Filter and a bit when turning it on. Since it is not recapped, I'm thinking that is related. Maybe someone can point to the parts that might cause this even though the switch has been cleaned a couple/few times. The low filter is S4 a,b in the middle of this scat. Thanks for any direction on this one.