First of all
Canon Canonet rangefinders, which actually date back to when Canon was Bell & Howell, come in many flavors but have some common issues for the user-collector. Like I've said elsewhere I'm no expert but I've discovered a few things based on the Canonets I own, and having done a bit of research on the web. I'll only speak about the ones I'm familiar with fixing for now: Canonet QL19 (1st version, like Bell & Howell 19), Canonet GIII QL.
The Basic Issues
ALL of the 60s-70s rangefinders, not just Canonets, seem to have the following issues in common: deteriorated foam light seals (gummy, sticky, messy) and cloudy rangefinder glass. I'll talk about these first even though I describe in detail the process on my light seals replacement tips page.
Cleaning the outside is easy, just Windex/Glassex or ethanol (like a halfway decent vodka) on a Q-tip or Kimwipe (lens tissue), wiped dry with a Kimwipe. To get inside, though, requires taking off the top cap -- the metal shroud that houses the rangefinder. I'm finding this to be fairly standard construction across all makes and models, so this is pretty generic info that will apply in most cases. First remove any visible screws, sometimes there are three or four, sometimes as few as one or two. The rewind knob is almost invariably removed by opening the back, placing a bar of some sort (screwdriver, tweezers) in the fork that enters the film canister, and winding the knob counterclockwise to unscrew it. A little extra pressure is required to get it started. The wind lever is almost always held in by a top cap removed by a spanner in two holes or by gripping the outside and twisting counterclockwise. On the QL19 there is also a second retaining ring that twists off similarly. Sometimes there are additional screws, rings, washers here -- place them carefully aside, noting which way they were oriented when you removed them.
Gently lift the top cap straight up when you get all the screws and knobs removed. Often it will be attached by a wire to the hot shoe, try not to put any strain on this wire as you lay the cap aside. The rangefinder is often underneath another little cover held in by a couple of screws. Do not touch the meter needle or anything else in here, just blow out any dust and dirt, then carefully brush off any visible dust before CAREFULLY cleaning the glass and mirrors.
This should go without saying, but never put liquid directly on a lens or other glass. Always blow off dust first, then gently use your lens brush before even touching it with anything else, THEN moisten a swab or square of photogapher's wipe with solvent, and gently clean the glass, following with a dry wipe. In this case you'll use cotton swabs because of the tricky little glass placement. Don't reuse them, just wipe the wet end on and then the dry end, then get a new swab. Mirrors should be cleaned VERY gently if at all, I've had great luck with Windex on all this glass. Don't forget the inside of the glass that's fixed in the top cap. The frosted glass is perfectly ok to clean, it looks for a second like you've ruined it but it dries cloudy again. Put everything back together again.
Make no mistake, replacing light seals is a tedious, dirty, painstaking job but absolutely worth it and often absolutely necessary, again, not just on Canonets but on just about all cameras from the 60s-70s when they started using foam instead of felt, string and rubber for light seals.
Get out the Q-Tips, toothpicks, dental picks, 97% isopropryl alcohol, paper towels, and Kimwipes. Scrape, moisten, rub, carefully make it all go away, noting exactly where it all went, a section at a time so you can replace it precisely. I use strips of mousepad (neoprene rubber) carefully cut using an X-acto knife and a metal straightedge for the film door mating grooves in the camera body. If you cut the mouse pad strips a little wide and stretch them they'll fit in the grooves without any adhesive (thanks Winfried for this tip!). For the rest of the light seals I use strips of black felt salvaged from used film canisters (get them from your local friendly photo lab). For adhesives for the felt I either use Pliobond or a Duro-type all-purpose cement from Radio Shack.
For the larger sections of seals in the Canonet GIII QL17, use pieces of something called 'Foamies' which are about $.79 for an 8.5" x 11" sheet at craft and fabric stores. Get black (it comes in all sorts of fun colors). Apply it with double-stick tape. If you can find the self-stick Foamies all the better.
Another good alternative is a premade light seal kit such as the ones you can get from Jon Goodman (not the actor!), aka ebay seller 'interslice'. His kits are top-notch and include well-written instructions. Highly recommended!
Frequently you'll find that a mercury battery was left too long in the chamber and leaked. After carefully removing and disposing of the battery, and washing your hands (it's mercury, after all!), you can swab out the green residue with white vinegar and a cotton swab. Hope that the wiring is still all OK.
Canonet Special Issues
Original Canonet selenium cells dead. You can spot selenium cells easily, they look a lot like bicycle reflectors. They are light-sensitive but can eventually die and once dead, they're dead. Original Canonets have a ring of selenium cells around the lens and since they're 'always on', they're often dead. Selenium was replaced as light meter material sometime in the 60s by Cadmium Sulfide or CdS, which becomes light sensitive in the presence of electric current, eg a battery. Apparently sometimes these can die too, but much more common is the battery either simply needs replacing or a corroded battery has damaged the wiring or the circuitry. In any event, don't get your hopes up expecting any selenium cell light meters to work, and be happy if they do!
Stuck shutter. They go 'click' but nothing happens. It's the dreaded Canonet sticky blade problem. Here's what I've discovered: it looks like some of the lens elements are held in place by lacqer or shellac, which eventually becomes brittle and flakes off into the aperture blades and slow timer mechanism, if not the shutter itself. Shellac was (is?) common for holding screws in place, to keep them from vibrating loose. I see a similar thing in computers, but I think now it's more so they can tell if you've voided your warranty by dismantling your laptop. It's easily dissolved by a solvent like 97% isopropryl alcohol or lighter fluid (naptha). The real trick is getting to the blades in the first place.
One person's method is documented at Favorite Classics. I went about it another way, you can tell me if you think this is worth the trouble:
On my QL19s I went in through the front of the lens. See, there isn't enough clearance through the back to go around with a spanner, you don't have a full circle to turn. So instead I removed the retaining rings and (carefully!) all the glass elements on the way to the shutter, which is how I found out about the shellac (I'm guessing, I'm pretty sure it's not glue, that would just be stupid). The last element before the shutter blades is held in place with shellac, which if you get this far, you will see is brittle and flaking. If you're not videotaping yourself at this point, make careful notes which elements go which way, etc.
The shutter blades will not open if the button is pushed until you have dissolved the residue that holds them together. The aperture blades, located right behind the shutter blades, will not open beyond full stop (1/16) until they are freed up. NOTE that even if the aperture blades are freed up, they do not open as the ring is turned but instead open to the setting you've selected when the shutter is cocked. Look and see, that makes sense, I swear.
For anything more advanced not covered here or on the camera pages you'll probably want to get the service manual, likely available at CraigCamera or manuals2go.com and maybe a general camera repair text like Ed Romney's popular (if amateurishly published) Revised Basic Training in Camera Repair, which I have and recommend. Pricey ($40US) but pays for itself quickly if you've got the bug like me. Bear in mind that these texts are written for service technicians, not laypeople.
Patience and attention to detail are necessities. For me it's a nice calming exercise as I am rather impatient and not so much detail-oriented. Also it helps to not have toddlers running around, that should go without saying. I do my work late at night with a bad movie on and a nice bright magnifying lamp.
It's also well known that people usually ruin the first camera they try to fix. Practice on a camera you don't care too much about, like a $5 Argus. If it works when you're done, all the better, if not - no big loss! I've been lucky so far, but then my Contaflex is still sitting in a tray in many pieces...
Comments? Questions? Kudos? Flames? Go ahead and email me. I hope you find some of this useful.