I have had the 20.6’s for over twenty years and I had the exact same problem.
If the amp is on and the LED is red, that means the voltage rails have power. Cold heat sinks mean the current source from the positive rail through the driver stage has been cut off and the power resistors are thereby starved and do not conduct the bias current.
If you are confident with electronics, remove the AP-5 card. On the upper left of the rear board, and snugged next to a red capacitor, you will see a small transistor Q115 with a white spacer on the center pin. If that transistor is defective -- or it has a bad solder connection -- it cuts the power through the rail because its emitter is connected to the common bases of the two silver can transistors next to it, which are the constant current source transistors, and pulls them on to turn on the current source.
In order to access the solder side of that board you have to remove the rear heat sink off the AP-5 card (and you will need new mica insulators before reinstalling).
Try first to touch up the solder joints of the transistor Q115, especially the base (center) pad. Be careful, those pads are small. Put the heatsink back on the board, and reinstall the card. If the amp works, remove the AP-5 card and replace the mica insulators with new ones and goop. Or you can order a replacement transistor (2N5401) from Mouser and replace the original since you have the board open anyway.
Good luck.
If the amp is on and the LED is red, that means the voltage rails have power. Cold heat sinks mean the current source from the positive rail through the driver stage has been cut off and the power resistors are thereby starved and do not conduct the bias current.
If you are confident with electronics, remove the AP-5 card. On the upper left of the rear board, and snugged next to a red capacitor, you will see a small transistor Q115 with a white spacer on the center pin. If that transistor is defective -- or it has a bad solder connection -- it cuts the power through the rail because its emitter is connected to the common bases of the two silver can transistors next to it, which are the constant current source transistors, and pulls them on to turn on the current source.
In order to access the solder side of that board you have to remove the rear heat sink off the AP-5 card (and you will need new mica insulators before reinstalling).
Try first to touch up the solder joints of the transistor Q115, especially the base (center) pad. Be careful, those pads are small. Put the heatsink back on the board, and reinstall the card. If the amp works, remove the AP-5 card and replace the mica insulators with new ones and goop. Or you can order a replacement transistor (2N5401) from Mouser and replace the original since you have the board open anyway.
Good luck.