RCA WG-299D Probe Repair

One of my RCA WV-98’s from the MIT lab clean-out came with a WG-299D probe that was suffering from a very brittle plastic shell. 

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As soon as I started to use it, the main shell of the probe fragmented into shards of plastic. The probe must have been stored somewhere subject to lots of UV, perhaps near a window. 

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I removed all the broken pieces. You can see the front shell of the probe has chipped as well.

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Here’s a good view of the switch mechanism. A leaf spring contact touches a pin connected to the probe tip when the switch is in the AC/Ω position. This shorts out the 1 MΩ resistor in the probe that is in-circuit when in the DC position. The plastic switch slider rides on the center pin. When it is moved to the DC position, toward the front of the probe, the bottom of the slider raises the leaf spring contact on a ramp and disconnects it from the center pin.

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To repair the probe, I slit a length of Evergreen Scale Models 1/2” diameter polystyrene tubing lengthwise. The probe shell diameter is larger than 1/2”, so it takes a bit of persuasion to force the smaller tubing onto the probe body.

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The tubing leaves a 0.188” gap, which is wide enough for the switch slider.

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I glued the tubing down by applying a drop of super-thin cyanoacrylate glue to the edges of the tubing, wherever it met the metal shell. I then held the tubing in place until the glue set.

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I finished up by gluing in pieces of Evergreen 0.188” x 0.040” styrene strip using methyl ethyl ketone (MEK).

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Some portions were a little wider than 0.188”, so I filled the gap with gap-filling cyanoacrylate glue. One spot didn’t have any metal backing, so I cut a fragment of styrene strip and glued it in place to fill the gap.

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I then filed the filled areas with a bastard-cut single-cut mill file to smooth the assembly into a nice smooth cylinder.

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I still have to paint the replacement and mark the DC and AC/Ω legends for the switch.

© Steve Byan 2011-2019