Repairing the "Unrepairable" Microscope

The school I teach at has a small number of microscopes used by our science classes. I also use them once a year for a lesson where I bring in a bunch of microcircuits made visible to let my students see what's on a chip directly. I also bring in a wafer and some dice (the microcircuit variety, not the Yahtzee variety) for the kids to see and handle.

The exhibits themselves range from a 1959 transistor with the top of the can cut off to reveal the chip and a 1965 dual op-amp similarly prepared to a 1st generation microprocessor to late 80's memory circuits. All together I have about six to eight things I can put under the microscopes that would be interesting to look at.

Since I've been with the school, one of the microscopes has had a label on it reading "Broken, Save for Parts" on it. Since there are only six microscopes, including this one, it means I have to spend time in class changing over each scope from one object to another. Fortunately I've gotten good at talking while focusing a scope and shifting the light and indicator around to the best position. But it would be awfully nice to have at least one more scope. Recently another of the scopes got damaged, reducing us to only four scopes. This is really too few, especially for our science classes.

I Can Fix That

Our science teacher found out I work on optics (I build telescopes as a hobby, in addition to my work with sensor systems professionally.) She asked if I'd be willing to take a look at our newly broken scope. I agreed, and suggested that I might take a look at the "parts only" scope as well, since neither of us knew what was wrong with it.

The office staff let us know that the other scope had been declared "unrepairable" by a scientific instrument repair service that had looked at it several years ago.

When I took a look at the "unrepairable" scope, the optics appeared to be in perfectly good condition. The adjustments and controls were likewise all in good condition. All I found was that the light built into the base did not turn on. I had a light handy, for my own scopes I prefer a light that isn't built into the base, that way I can just grab another light if one of the bulbs burns out when I'm in the middle of a job. More than half the time I'm looking at something opaque anyway, and when I do look at translucent samples I use a mirror as often as I use a lamp for a backlight.

I opened the base to the "unrepairable" scope, wondering what would prevent the light from being fixed. Perhaps a small fire from a prior fixture?

Since When Is A Bulb Replacement "Unrepairable"?

When I got inside there was nothing worse than an empty light socket. The previous light had burned out, been removed, and not been replaced. I could see why, in part. It was an odd sort of bulb. The base style is the same as that for an automotive type "1004" bulb. It's a bayonet mount with two contacts at the tip of the base rather than the usual one. The designation is BA15D. It's an unusual type of bulb, especially for 120V. They run from about $5 to $35 depending on how specialized your supplier is. But they're not unavailable, by any means.

Still, they're hard to come by for people used to picking up light bulbs at hardware and grocery stores, so I decided to replace the base with a more common screw base. A standard medium size screw base, as used on most incandescent lamps around the home, would be too large to fit in the microscope's base. A small "candelabra" base would fit handily, but I wanted to make sure it'd be easy to get lamps in the right range of brightness. Candelabra base bulbs of up to about 15W are easy to come by in a size that would fit. But above that they tend to be the larger "flame" shaped bulbs in the 25 to 40W range, which wouldn't fit properly.

So it came down to an "intermediate" size screw base. Bulbs for these are common at hardware stores and such, as "high intensity" lamp bulbs (in 40W), and as lamps for vacuum cleaners, appliances, and such. 20W to 40W are common.

You Can Get the Bulb, But Not Its Base

Now the problem was finding such a base. I checked several hardware stores and lighting stores, with no luck. They all sold bulbs to fit an intermediate base, but no actual lamp bases in that size. I checked Radio Shack, just on the off chance, and my prejudices about their current parts stocking were confirmed. I even considered using the halogen lamps with the loop and straight leads on them. While the lamps are readily available, once again the bases are not.

I went to several stores and looked at cheap lamps, looking for one I could cannibalize without paying too much for the privilege. No luck, the LED lamp rules there, and none were suitable for a microscope. Neither the light pattern nor the size would work.

Finally I tried yet another hardware store while I was in another nearby town. I was ready to give up on intermediate size and go with candelabra, and hope to find a 20 or 25W bulb to fit. They had a single bulb wired fixture with a candelabra screw base. It looked perfect. The fellow minding the shop floor mentioned a lighting store nearby that I hadn't been to, as well. So I bought the candelabra fixture, a 15W bulb that would fit it, and headed over to the other store.

There they had lamp components in parts drawers, a very promising sign! I found several different types of base, but all in candelabra or medium size. One of the workers there helped me look, but we didn't turn up an intermediate base. We did turn up an adapter to go from a candelabra base to an intermediate base, though. I got that and a 25W bulb to fit, then went home with the lot.

Once home, I test fit the new fixture in the base with duct tape. I pulled out a selection of slides, and selected one with a nice thick feather sample on it to test the light level. I started with the 15W bulb and a thick section of the feather. The 15W bulb illuminated it, but not as well as I would like for students. Student's eyes aren't trained to pick out details yet. They need things well illuminated to help them see what they're supposed to see.

Putting It Together

I put in the adapter and the 25W bulb. That worked perfectly. Bright enough, without being too bright, even with a bacterial sample on a slide. In fact, the bulb aligned with the reflector in the base better with the intermediate base adapter. So I removed the old fixture from its bracket. Prepped the bracket and plastic welded the new base's fixture into place. I cut out the old fixture's wires, desoldered one end from the light switch, soldered and spliced the new fixture in. When it was all done, it looked like it was supposed to be that way. I tested everything to check for operation and safety afterward.

Then I added some labels to describe how to change the bulb to the outside of the case, and what bulb to use.

Now the "unrepairable" microscope not only works great, but can be maintained by an ordinary person without calling the "scientific instrument repair" service.