Tektronix 2215A Oscilloscope Power Switch Rebuild

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:

Tek2215A - internal top view

Tek2215A – internal top view

Removing the high voltage shield below the PCB reveals the switch has DPDT terminals, but it’s wired as DPST:

Tek2215A power switch - PCB terminals

Tek2215A power switch – PCB terminals

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:

Tek2215A power switch - bottom

Tek2215A power switch – bottom

Remove the shell, remove the toggle-action U-shaped steel pin, release the spring, and pull off the top plate:

Tek2215A power switch - internal

Tek2215A power switch – internal

Remove the plunger hardware, remove the rocker arms and their springs:

Tek2215A power switch - disassembled

Tek2215A power switch – disassembled

One contact on each rocker shows signs of distress, but the other button remains pristine (having never seen any voltage differential):

Tek2215A power switch - rockers

Tek2215A power switch – rockers

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:

Tek2215A power switch - contact tabs

Tek2215A power switch – contact tabs

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]

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