Grahame N's Web Pages
EVER HEARD OF A GBL517.5?
by the late John Cunningham - December 1991
I have collected films since 1935,and the hobby still fascinates me. It's a natural diversion, I suppose, to become interested in obsolete gauges. 17.5mm has always interested me, as well as 9.5mm ... still, all cine gauges will soon be obsolete, won't they? Over the past few years I have managed to acquire a collection of about 70 reels of 17.5mm. These came from various sources and included four projectors. Three of these were actually Pathé "Home Talkies", and after a little repair and adjustment ran quite well mechanically. but with that "grinding" noise that all of these projectors seemed to emit. As there were no lamps. all were converted to 12volt 50watt halogen Q.I. lighting. Several windings were removed from the transformer secondaries to give 12 volts. The picture was perfectly OK for home use. I was worried that if I used a 100w lamp there would be overheating of the lamphouse and, probably, the transformer. Even with 50w, the lamphouse gets hot as the flow of air from the fan seems to be poor.
The fourth projector had been converted to 16mm in a dodgy way. Back in the 1950s, I was offered one of these conversions with a few films, at a very low price. A collector friend warned me off, saying that the projectors were awful and he added, "Don't touch 17.5mm at all, the films will be a mess!" The films I obtained were not a complete mess, even after 40 years and varied from new to unprojectable. For instance. I have two Popeye cartoons which are as good as new, and a Betty Boop which has every sprocket-hole damaged. In the latter, there is strangely not a single splice in the whole film. I put this down to a "hooked" claw or, more probably, sheer incompetence on the part of the projectionist. I have been able to repair most of the films to projectable, albeit, a fragile state, by patching them in the old 9.5mm way. This works very well, but of course, is extremely tedious. However, no frames are lost and no sound cuts. If you try it, don't use modern film cement. Instead. use the cement, the formula of which I give at the end of this article. * This is good for all mono-acetate films.
Most splices had to be re-made as nearly all had been very poorly made by hand. I have come to the conclusion that there was no such thing as a 17.5mm splicer. Incidentally, a good 17.5mm splicer is easily made from a Marguet tri-film splicer. Mine works as well as it did on 16mm. After using the projectors a few times, it quickly became apparent why so much damage was caused to the films. To be fair to Pathéscope the intermittent was beautifully designed and made. There are two cams: one for the up and down movement and the other for the in and out. It runs in a sort of oil bath and the whole thing is perfectly balanced. From here on, the designer must have got tired of the whole thing and evolved something that would be as cheap as possible, but with a certain clever touch here and there. The rest of the projector is a cinematic disaster area. But then, Pathé turned out some weird ones, didn't they?
There is only one 8 picture sprocket placed just above the barrel shutter housing. The lens is within the rather large shutter so no other than the Pathé focal lengths can be used. From the feed spool, the film passes under the sprocket and immediately through the short gate, where the film is pushed down in the Super 8 manner. It then passes under the smaIl round lamphouse to a Pathé chute, and over the sound drum. There is no film wrap-around on the sound drum, so the flywheel can have only a negligable smoothing effect on the film, as it passes the scanning point. From here it passes over the sprocket and on to the takeup spool. Only two sprocket teeth are in engagement on the sprocket at any time. The sound optics are above the lamp, and the modulated light beam is reflected by a small concave mirror to the photoelectric cell, perhaps 8" to the rear. The chief trouble with the whole design is that the position of the sprocket allows only a 5 or 6 frame loop above the gate. If it is any larger, it touches the film returning from the soundhead. The sprocket is also off-centre in the Vox manner. If a poor splice goes through the gate, and all of them were poor, the loop is lost in about 1/4 second, before you can stop the projector, damage results. It quickly got to the point where I was afraid to project the films at all. It was ten years until I saw some of them.
Before long, I began to look round for a projector that could he converted to 17.5mm. The old G.B.L516 had all the design points needed, and except at the claw assembly, would be easy to convert. The only trouble was how to tackle the claw and gate assembly. It appeared so purpose-made for 16mn. Then an idea struck me, which worked. Here's how it was done. The claw assembly was dismantled and the camshaft removed. The old claw was bent straight and used as a pattern for the 17.5mm version. After making a first, rough 17.5mm claw, the 16mm section was cut off the cam box, leaving enough metal for the new claw to be attached. This was then bolted on to the cam box using 10 BA screws. Next, the cam was carefully knocked off its shaft. The cam shaft was put into the three jaw chuck of my lathe, and using shim material, the shaft was displaced from the concentric by a few thousandths of an inch in one jaw. A new spigot was machined on to the shaft. The cam was then squeezed onto the new spigot in such a way as to increase the throw of the cam. The cam and claw were assembled in the claw assembly and using a piece of film, the intermittent motion tried. After 3 or 4 attempts, each time adjusting the thickness of the shim material, a point was reached when the claw moved in and out cleanly. A final claw was made, when I had satisfied myself that the idea would work. From this claw as much metal was removed as possible in order to keep the mass of the new claw as low as possible. The claw was attached to the claw box using 10 BA screws and Araldite. This was the difficult and unknown part. After this, the rest was easy.
Several other alterations had to be made, of course. The slot in the fixed gate runner had to be lengthened, and the runner moved to take into account the wider film. Similarly, the sprung gate runner was adjusted as well as the springs. At this point, provision was made so the aperture plate could be removed, so a silent film aperture could be inserted (I have two silent 17.5mm features). The sound and silent apertures were adjusted with a file. I happened to have two new 17.5mm sprockets so was saved the trouble of making them. New sprocket retainers were made for the sprockets, the rear one giving good wrap-around. The front one was a little difficult due to lack of space. The position of the sound drum and pressure roller had to be moved away from the main palet a little. Fortunately, the sound track of 17.5mm is almost exactly the same width as 16mm, so no scanning difficulties were encountered. The spool spindles were altered. A surface silvered mirror from an old Technicolor projector was fitted in a removable mount, over the lens opening in the carrying case so that the picture conid be reversed. 17.5mm is of course, in the DIN mode (the sound track is on the inside). while the L516 is SMPE (the soundtrack outside). The lamp was converted to 24v 250w Q.I. The shutter blades had to be widened more than one would think, to stop ghosting. Of course, the larger frame was being moved further in the gate. The adjustable feet were removed from the front and the holes filled. One foot was placed at the back centre of the case so the picture from the mirror could be centred on the screen. With a photo-diode and a new transistorised amplifier, the performance was very good indeed, within the limitations of the 17.5mm sound track. Most of the tracks were scratched to a greater or lesser degree and a crazing of the emulsion through age adds to the noise. Of course, few films had noise reduction tracks at this time.
The projector used for the conversion was made in 1943 and it is remarkable how well it has stood up to what must have been millions of feet of film. Some components such as the sound drum and jockey rollers were very worn and wobbled on their shafts. Fortunately I had replacements. However, the vital parts such as the motor, main shaft and claw assembly were as good as new; surely a tribute to the makers of this old "War-Horse" of a 16mm projector. On careful examination, with a lens, the claw moves into the perforations and out again cleanly and without "pick", but there is a small amount of "sawing" action on the perforation as it moves down. Purists would say there should be no sawing action. This is true in an ideal situation, but in a conversion such as this, some compromise has to be made.
* This film cement formula comes from "Amateur Cine World" way back in the 1950's:-
Acetone .............. 80ml
Chloroform ......... 16ml
Glacial Acetic Acid 8ml
The mixture is inflammable and very volatile, so keep tightly stoppered. (When the article was written in 1991 it may have been possible to source these chemicals in a local chemists - I doubt if the big multiples even keep basic chemicals in stock today!)
This article was first published in "Flickers" magazine number 88, in December 1991.
John had converted a couple of 16mm GBL516 projectors to 17.5mm, so he offered to sell me one of these. Really ideal for showimg ancient 17.5mm film prints, as the pull-down is quite gentle and the trip mechanism for lost lower loops is very useful. I will always be grateful to John for letting me have a decent machine to use for 17.5mm. A really lovely and enthusisstic fellow, who is sadly missed. (Given half a chance, he would have made a batch of 17.5mm filmstock to try out the cinecamera!!) glnOct2016
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Last updated 07 Oct 2016
................................. ©MM2 John Cunningham
07Oct2016 - tidying and added noted.