The input selector switches on the Mission Cyrus amplifier deteriorate over time like any switch. Unfortunately their unique construction make them difficult to clean and impossible to replace.
I have always shied clear of any attempt to strip the switches down, knowing that I couldn’t source replacements if it all went wrong. However, a recent mains transformer fail put me in a place where I had nothing to lose. So, in I went. This is what I found
The switch was surprisingly easy to liberate from the circuit board by soldering the contact pins and one point where the case was connected to ground.
To gain access I carefully folded back the tabs wit a small pair of pliers before gingerly pulling the base from the switch.
Inside we have the contacts what you would expect to see in a slider switch, but activated by an unusual mechanism.
Rotation of the control turns a black metal gear. This rotation is turned through 90 degrees by the white nylon gear. This pinion gear acts on the plastic rack to create the linear motion necessary to move the contacts. All of this works satisfactory.
A closer look at the contacts reveals the problem I expected to see. The contact surfaces and wipers are filthy and worn. No amount of switch cleaner aerosol will fix that.
My tool of choice for these situations is a glass fibre brush. I was able to get down between the two contact sets and shift all the dirt to leave shiny metal surfaces.
Re assembly was straightforward, referring to the photographs taken previously to ensure correct placement of the wipers on the contacts and alignment with the plastic actuator inside the switch body. Carefully bending over the tabs secured the switch base.
All in all it only took me an hour or so to do both switches, once I overcame my fear. Operation is now satisfying and quiet.
Finally, here’s a gallery of high resolution pictures for your delectation.
