Calculator key indicator lamp repair

I decid­ed to see what I could do about the tem­po­rary mod­i­fi­ca­tion to the Burroughs C 3207 mul­ti­ply key indi­ca­tor, before it became a per­ma­nent modification.

Burroughs C 3207 calculator keypads
Burroughs C 3207 cal­cu­la­tor keypads

After thor­ough­ly check­ing out the mul­ti­ply key con­struc­tion. I decid­ed that the only way to access the indi­ca­tor light was going to be from below the key­cap. I ver­i­fied that there weren’t any setscrews or pins fix­ing the key­cap to the shafts, before using some par­al­lel wedges under the key­cap. The cap is attached using two flat shafts and springs, and is attached very secure­ly. After using quite a bit of force even­ly dis­trib­uted across each end of the cap, it start­ed to final­ly move.
I had thought that I had cleaned the key­pad assem­blies fair­ly well before, but was a bit sur­prised to see how much garbage still remained, which was most­ly grease, and dust.

Burroughs C 3207 cal­cu­la­tor with mul­ti­ply key­cap removed

I was expect­ing some type of stan­dard minia­ture incan­des­cent 24 volt bulb, but was sur­prised to see the odd lit­tle bulb assem­bly attached between the shafts.

Incandescent indi­ca­tor lamp used in the Burroughs C 3207 calculator
Bakelite lamp bot­tom with pot­ting used in Burroughs C 3207

The bulb of this lamp assem­bly is housed in a Bakelite mold­ed fix­ture with two met­al tabs that are used to sup­port the assem­bly and sup­ply pow­er to the bulb. The back­side is pot­ted with a sim­i­lar col­or pow­dered mix­ture to seal the con­nec­tions.
After some search­ing I was unable to find any type of bulb that matched this unit. It would prob­a­bly help if I knew what to accu­rate­ly name this type of fixture.

Bulb fix­ture ter­mi­nals after removal and cleaning

It did­n’t look like I was going to quick­ly find a replace­ment bulb, so I once again put togeth­er a semi-tem­po­rary or maybe per­ma­nent fix for the mul­ti­ply key indi­ca­tor.
I knew that the orig­i­nal LED solu­tion worked well, but caused a lot of light where I did­n’t want it. So my solu­tion was to shrink it down to fit into the same space as the pre­vi­ous bulb fixture.

Red LED replace­ment for incan­des­cent bulb fixture

I end­ed up select­ing a 10K Ohm 1/8 watt resis­tor, and used the LED’s anode lead and the resis­tor lead to attach my assem­bly to the ter­mi­nal screws. This worked well, but in hind­sight a dif­fused instead of clear LED would have been a bet­ter solu­tion.
I took near­ly as much force to press the key­cap back onto the post and springs, but when com­plet­ed, every­thing looked as it orig­i­nal­ly did.
I would still like to get an orig­i­nal bulb back into this unit if any­one knows where to find one.

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