Adding a thryistor (triac) to the KA-9100 Part 1
Among many models, the power switch is a Kenwood weak point. It's important to protect and one of the best ways to do that is to add a Triac inline with it. The Triac suppresses inrush current which otherwise causes arching. The arching damages the contacts which eventually destroys the conductivity of the rocker interrupting electron flow. This renders the switch inoperable and power to the amplifier is lost. When power to the amplifier is lost, it no worky. I make a point to describe this with exactness in jest having had this subject be a previous point of contention. A contemporary was being "nit-picky" about my use of semantics in describing the cause of switch failure. This time, I hope to have left no room for interpretation.
When it comes to wiring the triac I've come to use the annotation provided by Glenn McDonald. For the KA-9100 I'll be using the part number Q6040J7TP along with a safety rated X2 0.1uf film cap and a couple of 100ohm 1/2W metal film resistors. (Thanks and credit to Glenn for the diagram photo below)
The first thing to do is to identify which of the wires leading to the switch is coming directly from the 120V AC source this will be the "Hot" as noted in the diagram. In my unit the white wire is the "Hot". This white wire is connected at a terminal of the unswitched convenience outlet to the incoming brown power cord. The yellow wire connects to a red wire from the transformer(s) or "Load" in the diagram.
Next, I want to document what the switch appears like in stock form. In my unit the hot wire (white) is on the toggle side and the load (yellow) is at the middle position. There is a .01uf safety cap across the switch that will be discarded. There is a second set of contacts that is unused on the other side of the switch which will be implemented later in the install.
The wires and safety cap are snipped from the switch. All of the remaining wire and solder is cleared from the tabs of the switch in preparation for the new wiring.
The switch is then disassembled for cleaning. Notice the damage arching has caused to the upper right contact and also one side of the top rocker. This is what we are trying to avoid from happening further. The below photo is prior to any cleaning which should be done with D100L followed by an application of G100L. When reassembling I turn the damaged rocker 180 degrees toward the undamaged contact.
Once the switch is reassembled it is wired with the "hot" to the top toggle side, the gate to middle tab, and the right and left sides bridged together as shown below. Wiring both sides will divide the current, though it's not entirely necessary with the Triac in place. The Triac will dramatically lower the current that the switch sees, though in practice, bridging here is still a positive thing for the life of the switch.