TDC made two models of projector, the 116 and the 716. The 716 was designed for 750 watt lamps maximum, whereas the 116 was designed for 500 watt lamps.
The 716 has a slightly better slide carrier than the 116, in that it is a sturdy stainless steel carrier with a built-in shutter for obscuring both slide windows during the slide change. However, the openings in the carrier were designed for 5 perforation Realist format slides only; if you want to project larger slides, the openings can be CAREFULLY filed to a larger size so as to accommodate so-called 7 perforation European format slides, but any larger opening would interfere with the proper operation of the shutter assembly.
The 116 can be rewired to accommodate 750 watt lamps by simply using a heavier gauge wire for the internal connections -- not a difficult chore for someone handy with a soldering iron. A test of the heat generated at the power plug would be a good idea, however. If the plug gets too hot, it would be advisable to change the socket on the projector to a heavier capacity, as well as the plug on the cord, or to wire the power cord permanently into the projector (probably a better solution). Of course, the blower fan must be in good operating condition!
The 116 has an aluminum carrier, which can be more easily damaged than the stainless steel carrier of the later model 716. In addition, a spring loaded shutter mechanism is mounted in the projector and is cammed into and out of position by a protrusion on the moving part of the aluminum slide carrier; this triangular shaped protrusion makes installation of the carrier a little tricky during set up and breakdown, and users unfamiliar with the projector can cause the carrier assembly to jam or even get bent or damaged if they are not careful. The triangular protrusion can get a groove worn into it by constant contact with the shutter mechanism, but this wear does not usually degrade operation of the shutter unless the groove gets very deep. On some models I have seen, the triangular protrusion is reinforced with a split aluminum tube slipped over the edge of the triangle that contacts the shutter mechanism.
However, the model 116 carrier has one tremendous advantage -- it has openings large enough to accommodate up to full frame 35 mm stereo pairs mounted in a single slide of the standard 1 5/8" height. The carrier also has a second channel for projecting slides 2 inches high, which accommodates standard 2 x 2 monoscopic single slides, but, unfortunately, will not support twin separate 2 x 2 slides for stereo (the support spring at the top of the channel is only just long enough to align one 2 x 2 slide -- a second slide in the channel will tip forward or back at the top and cannot be focused). This second channel WILL accommodate twin 2 x 2 slides that have been fastened together with an appropriate spacer to form a single slide, however. It will also accommodate the rare mounts that were made for full frame stereo pairs and that were 2" high.
Clearly, if full frame 35 mm stereo pairs are important to you, the model 116, despite its older design, might be the better choice for you. Be sure that there IS an aluminum carrier in good condition(!) with the projector, and that the built-in shutter is still in or with the projector; some model 116's were outfitted with a removable adapter to accept the better made and quieter stainless steel carrier, and a former owner may have discarded the older aluminum carrier and shutter mechanism.
If you decide on a TDC, whether a model 116 or 716, BE SURE THE LENSES ARE PROPERLY ALIGNED IN THE HOOD! If they aren't, the repair can be expensive, requiring somebody to machine new threaded lens holders. There were a few projectors made without proper quality control of the lens holders, and the result was a projector that could not be focused over the entire picture area of one or both images. Check by unscrewing one of the lenses, if necessary, until both lenses are even with each other. Then. looking down on them from above, lay a horizontal straight edge across the front of both lens barrels. If the straight edge touches both sides of both barrels, then the lens holders pass the first test. Now from the side of the projector, sight along the top of one barrel and see if it is parallel to the top of the other barrel; if so, the lens holders pass the second test and are probably OK if the tests still check out after you change the horizontal and vertical adjustment positions on the projector a few times. If there is a discrepancy in either test, I would recommend rejecting the projector unless you know somebody who can machine one or two properly aligned lens holders inexpensively.
Compco (no longer in business, either) made two models, the "Triad" and the "500". In this case, the older "Triad" was the better projector of the two, although the "500" (which I own) can be modified (at considerable expense and trouble) to behave acceptably. Both projectors were designed for 500 watt operation, and I have not tried to rewire either for higher wattage because the cooling fans, while commendably quieter than the TDC fans, probably do not have the cooling capacity to handle 750 watt lamps.
Compco slide carriers are sturdy but simple in design, and have the disadvantage that, during a show, you must remove a previously projected slide and replace it with the next one first on one side of the projector, and then, after the slide carrier is pushed through to project the slide you just loaded, you must remove the old slide and replace it on the other side of the projector. Unlike the TDC, you can go slightly buggy ping-ponging back and forth from one side of the projector to the other during a rapidly paced show, but you get used to it after a while.
It has been a long time since I operated a Triad (our Pasadena Stereo Club Triad, with special Simpson F1.4 - I think - lenses, was stolen about fifteen years ago). But my recollection was that the carrier is a simple frame that would accommodate slides mounted in any 1 5/8" x 4" mount, which would allow it to project slides mounted in any stereo wide format masks. Be sure to check this if you are looking at a Triad, though! But there is no built-in shutter, and the slide change movement is annoyingly visible on screen. The projector was as solidly made as the TDC and had two advantages:
1. The focusing knob is at the back of the projector and is easily accessible to the operator, unlike the TDC. And horizontal/vertical adjustment, to align the two images on screen with respect to each other, is done by a single knob on the side of the projector, which is also more easily accessible than the TDC knobs.
2. The polarizers are located between the slide carrier and the lenses, unlike the TDC, where the polarizers are located between the lamps and the slide carrier for additional heat absorption. Although the Compco arrangement requires a better grade of polarizer that is optically flat so as not to degrade projected image quality, the location after the film in the light path insures that maximum polarization is achieved. In the TDC arrangement, some films can actually degrade the polarization and cause excessive "ghosting" (unwanted "cross talk" between the left and right images when viewed through the Polaroid glasses). But in the Compco projectors, polarization takes place only AFTER the light has passed through the film, so that any degradation that may be caused by the film is eliminated. However, because of the absence of polarizers behind the film. the slides can heat up a bit if left in the projector too long. Since most good slide shows never leave a slide on screen for longer than 15 or 20 seconds at most, this should not be a problem.
The Compco 500 was an attempt to make a more compact, lighter, more modern design, but the engineering was unsatisfactory. The slide carrier is similar to the Triad's but with stainless steel aperture openings that would have to be filed or machined out to accommodate wide format slides (because there is no shutter, this can be done much more easily than enlarging the openings on a TDC carrier).
The 500 projector includes built in switches, triggered by the slide carrier movement, that switch the lamps off during a slide change and switch them on again when the change is complete. This is a much less satisfactory arrangement than the mechanical shutters of the TDC, because the part of the slide movement during a change would be visible until the moving carrier turned the lamps off. In addition, the switches were so stiff on the unit I bought that the carrier would not operate unless the projector were bolted down to a heavy table, and even then the carrier operation would be unacceptably difficult. Rather than trying to find less stiff switches, I simply took them out, designed an adapter for a TDC stainless steel carrier, and used an extra TDC carrier I was able to buy at the time. I enlarged the openings in the TDC carrier so that my modified 500 will now show wide format stereo slides up to European (7 perforation) format. This would be a difficult change to make today, because separate TDC stainless steel carriers for sale are rare.
Location of the polarizers is similar to the Triad, an advantage, in my opinion. Also, the ease of vertical and horizontal on-screen stereo adjustments is similar to the Triad's, also an advantage.
The big disadvantage of the 500 is the poor design of the focusing mechanism. For a number of reasons, there is slack between the focusing knob (conveniently located on the side of the projector) and the hood that moves the lenses, which makes it nearly impossible to do a quick, positive change in focus. Also, the tracks intended to keep the hood aligned are poorly designed, allowing the hood to skew slightly to one side or the other during focusing, which can throw one lens slightly out of focus with respect to the other. I was able to correct some of the slack but not all of it, and the poor tracking was helped some by careful lubrication of the tracks, but it is still not as precise a focusing mechanism as it should be.
One other thing I did with my 500 was to paint the inside of the hood and the surface it rides on a flat black, which noticeably improves the contrast of the projected images. And, of course, I replaced the polarizers with new ones.
If you are interested ONLY in twin 35 mm 2 x 2 stereo, you might consider a twin Kodak Carousel Ektagraphic rig. This is quieter, uses less power, creates less heat, is a more modern technology, permits automatic operation, can work with cheaper slide mounts, and can be programmed to work with slide changing signals on an audio taped sound track. Susan and David of Reel 3-D Enterprises, among others, are a good source of information about this method of projection, and they can also supply most of the accessories you would need to add to a pair of Ektagraphics for convenient stereo projection of twin 2 x 2's.
Oliver's follow up message regarding Realist projector
September 14, 1999
I had the use of a Realist projector for a short while. It was sturdy (except for the case bottom, which was made of easily broken bakelite), had sharp optics, and was bright (twin 1000 Watt lamps -- a problem for 15 Amp circuit breakers; 20 Amp circuit is usually required). But the cooling system was very noisy and the lens focal lengths were only 3", which usually put the projector too close to the screen, where it blocked the view from those valuable center seats in a large audience.
Furthermore, the tricky rotary slide carrier was a problem in that it did not position slides with consistent vertical alignment. You put the slide in the carrier in viewer position (thumb spot at the lower left), and rotated the carrier 180 degrees to put the slide in projection position. Because the mechanism to stop the rotation was not precise, the slide could be rotated too far or not quite far enough, thereby resulting in vertical misalignment.
Vertical and horizontal alignment knobs were in an idiotic position in the front of the projector and very close to the lenses, making it extremely difficult for the operator to make adjustments without getting his hand in front of the lenses!
It may have looked cool, but I would recommend staying away from it unless you have a lot of money to spend and want one as a second projector for occasional projection in close quarters or for a collection. It was a beautiful kluge!