I have recently acquired a very nice Collins R-390A that would not turn off. As I understand it, the 26Z5W rectifier tubes were commonly replaced with solid state diodes, and this sometimes caused the contacts in the ON-OFF microswitch to weld together due to current inrush. Sure enough, I looked at the power supply subchassis, and there were two diodes, and no tubes. I took the diodes out and plugged in two 26Z5W tubes, and promptly blew the AC line fuse. I found out that one of the tube sockets was damaged and shorting to ground. I replaced both tube sockets, and plugged the tubes back in, and success! no more blown fuses. One problem fixed.
The receiver would still not turn off, and I figured I would have to find a suitable replacement switch. But as a last ditch effort, I thought I would try to repair the switch. I took off the front panel, and getting to the switch was relatively easy. The switch was designed so that it could be carefully pryed open, and just as I expected, the contacts were welded together. Using a jewelry screwdriver, I carefully and gently worked on separating the contacts. To my surprise, they actually separated and looked undamaged. I put everything back together, and all is well.
I just thought I would share this story in case someone out there may have a similar problem. I really did not think I would be able to repair the switch. and maybe I was just lucky. Better to be lucky than good.
Ron W8ACR
