Canon Selphy 740 Fix

I recently bought a canon Selphy photo printer in a charity shop. It was cheap and seemed like a good source of parts if it didn't work and an interesting colour printer if it did (although the paper and ink cartridges are expensive).

It didn't work.

I took it apart and at first sight couldn't see much wrong with it. It was very clean and in very good shape. One odd thing was that I couldn't get the printer ink (or film, to be more accurate, this printer doesn't use ink, more on that later) cartridge out of the printer. Even when dismantled, I couldn't get it out by springing catches open and so on. I then noticed that there was a small electric motor which didn't have any gear on it's spindle. That can't be right. Then I noticed a rattle as I turned the printer over and then out popped a small gear. It was exactly the correct size to drive the larger gear next to the motor spindle. You can just about see the small crack in the gear that caused it to fall off the spindle in this photo:

For some idea of scale, that's a 1.5mm hole in the centre of the gear.

The first thing I do with these fixes is try to get a real part, but I've only managed that once in the past, most manufacturers won't or don't sell spare parts. That was the case here. So it was either fix the gear or make a new one. I've not had much luck in fixing these gears in the past, so I decided to make one.

I created a script that generates Gcode for this gear (it's parametric so I can use it for other gears too), and set my CNC machine off to make a new gear.

The cutter is a 0.2mm D bit that I ground the end off to make it more closely match the profile of the gear. It's not perfect but it works.

I used a surplus stylus that I got with a TFT display, as the stock for the gear. It's a bit soft and if it fails I'll look at another material, perhaps.

You can see the profile of the gear I cut here, together with the real part:

You can see that it's not a perfect match, if I need to then I can cut a better one.

Here it is in position in the printer:

The new gear fixed the printer, it can now print:

This printer uses film, not ink, as I mentioned before. There's a yellow, a cyan and a magenta block of pigment on the tape that is in the cartridge, one set per photo to be printed. The printer goes through four printing cycles but I've only found three pigments on the tape, I'm not sure what the fourth stage is. Anyway, I noticed that the used tape had a reverse image in each colour on the tape. You can see part of an image here:

These correspond to the pigment that is left in the cartridge after an image has been printed. This means that you can rewind the cartridge, put it in the printer and, if you print a totally black image, print a colour negative version of the photos that were printed by the printer. That can then be scanned or photographed and colour reversed in software.