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THE AM BULLETIN BOARD => Technical Forum => Topic started by: W8ACR on April 23, 2019, 11:32:46 PM

Title: Fixed my own R-390A micro switch
Post by: W8ACR on April 23, 2019, 11:32:46 PM
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. ;D


Title: Re: Fixed my own R-390A micro switch
Post by: WD8KDG on April 24, 2019, 12:16:52 PM

Somewhere in the past there has been another fix for what ails you. A resistor & cap are used to suppress the arc when those contacts open.

Might find it here:



Title: Re: Fixed my own R-390A micro switch
Post by: WBear2GCR on April 26, 2019, 11:04:43 AM
I sent you a PM, Ron, there were a few switches on epay... also almost any suitably rated microswitch could be
subbed into that spot, would need a suitable physical dimension, but there are a lot to pick from.

glad you go it fixed - I had one that was really well fried in an R-392...

Title: Re: Fixed my own R-390A micro switch
Post by: WD8KDG on April 26, 2019, 12:53:04 PM
Found it in my notes:

Added a 0.01mF 1KV ceramic cap in series with a 220 ohm resistor across the function switch contacts. No welded contacts since.

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