Archive for January 11th, 2013
My trusty Tek 2215A oscilloscope might be useful for a Larval Engineer engaged in late-night debugging away from the lab, but the power switch has become flaky: sometimes the ‘scope didn’t turn on at all, sometimes the switch required multiple pokes, sometimes everything worked fine. Removing the cover revealed there’s a long plastic bar connecting the power button on the front panel (to the right in the picture) to the power switch near the rear panel AC line socket, tucked under the EMI filter with the red sticker:
Removing the high voltage shield below the PCB reveals the switch has DPDT terminals, but it’s wired as DPST:
This knowledge will come in handy later…
Unsoldering the switch and wriggling the bar out of the front panel puts the switch on the bench, solder terminals upward. A plastic shell snapped around the actual switch insulates the top of the six terminals from prying fingers:
Remove the shell, remove the toggle-action U-shaped steel pin, release the spring, and pull off the top plate:
Remove the plunger hardware, remove the rocker arms and their springs:
One contact on each rocker shows signs of distress, but the other button remains pristine (having never seen any voltage differential):
Pull out the fixed contact tabs and note that they’ve been scorched a bit. The one on the right corresponds to the bottom rocker above:
I cleaned everything with a fiber wipe wetted in DeoxIT, then decided that I’d take the easy way out. The tabs have heavy silver plate on both sides, so I flipped them over and reinstalled them with the unused side facing the rockers. The rockers went back in with their unused contact buttons facing the flipped tabs, so we now have fresh, shiny new contact surfaces. Reassemble the switch, soldered it in place, button up the case, and a firm push on the button lights the ‘scope exactly the way it should.
While I had the cover off, I measured the ESR of all those electrolytic capacitors: they’re in fine shape!
The next time the switch needs repair, in another couple of decades, someone can swap in the completely unused tabs from the other end of the switch, then pick whichever contact buttons look best… [grin]