|Favorite Classics Camera & Repair Articles Canon Demi S Repairs|
|Canon Demi S Repairs
by Henry Taber
Part 1: Demi S Meter Repair
There are primarily two things that go wrong on old Demis that have been set on the shelf for years: lightmeter quits and the shutter freezes up.
Lets attack the meter first, its the easiest. Basically the whole meter setup consists of a galvanometer, selenium cell, a resistor and a couple wires. To get to these parts the front and top covers must be removed.
To remove the front cover (actually it is in two pieces: a top and a bottom section), first peel off the front leatherettes. Under them there are two screws on each side of the lens. (If only the meter is to be worked on just the top section need be removed.) Next the top cover must be removed. Open the back and insert a screwdriver handle into the rewind crank fork and unscrew the rewind crank handle. Then remove the spring, etc under the crank handle. Finally remove the four retaining screws and pull the top cover off. The shutter button will fall out, dont lose it!
Now you can see the entire meter assembly. Any one of the components may be the culprit but usually its the connections between the components that has corroded or lost continuity. Wiggle the wires first... Did it work? Darn. Gonna have to go deeper.
Lets check the galvanometer first. With a small screwdriver just flick the needle. If it moves freely and springs back its probably ok. But I like to check it with a AA battery in a Radio Shack battery holder (about 80 cents and comes with wires attached) by quickly tapping the battery leads to both sides of the galvanometer. The needle should JUMP. Dont leave the wires connected, just barely flick them. The galvanometer isnt designed for that much current. Great, the needle jumped. Its ok, lets continue...
Now check the connections. With a multimeter check for continuity through the wires. If continuity exists everywhere, try resoldering the connections. Still aint workin, huh?
Only two components left: resistor and the selenium cell.
Resistors rarely go bad, but while you have your soldering iron out and hot, remove the resistor and solder together the wires that were connected to the resistor. Still nothing? Oh well, it was worth a shot. Ive never found a resistor to be at fault and Ive repaired many meters.
Finally we are down to the selenium cell. The "usual suspect", as wed say if this was a Bogart movie. Ill be very astute: the cell will either be fine after swabbing down with acetone, anemic or totally dead.
If fine, well celebrate and go have a Dew now.
But if anemic we can try removing the resistor...again. Or try recalibrating the galvanometer. Youll find an adjustment on the top of the galvanometer. Fiddle with it until the meter reads correctly. Might take it out to the patio under sunlight, selenium cells dont care much for incandescent lights. I think they are partially colorblind.
If the cell is completely dead you have no choice but to replace it. But to be sure, shine a strong light on it and connect a sensitive voltmeter to it. If your voltmeter reads nothing, toss it and find another cell.
Three choices: rob a cell from another Demi (but dont do this unless the lens was run over by a McMichael concrete truck) Find a lightmeter that has a bad galvanometer (an old GE series meter has a cell that is nearly the right size) and grind it to the same shape as the Demis. Or the best solution, go to your friendly neighborhood Radio Shack and get a new cell. Selenium if you can but there are others that are close enough in voltage to work. Grind it to fit then recalibrate.
Reassemble and you are finished... go get that Dew!
Part 2: Stuck shutter repair
The number one problem that seems to come to old cameras that have been setting around unused for years is a frozen shutter. But luckily it can be freed up easily...usually.
On the Demi S the only tools needed are a small slot screwdriver and a locking tweezer. In addition some Ronsonol (lighter fluid), very fine graphite, Kleenex and a can of air are the only disposable products needed. That is if it is a normal "setting on the shelf freeze up".
To start getting to the shutter, first the logo ring must be removed. If you look just outboard of the white letters on the ring surrounding the front lens element youll see two small notches 180 degrees apart. These are grooves for a lens spanner. If the ring is too tight youll need one with pencil point ends. But the majority of the time I can usually hook my thumbnails in the grooves and twist it right off. A few times Ive used a small crochet hook (yeah...its my favorite spring puller too!) to break rings loose. I cover the lens with my thumb. Do NOT try using a screwdriver or other sharp tools. You are working next to a very scratch sensitive glass lens! A cheapie spanner can be bought for around $15 at most "real" camera stores.
Next the front lens element must the unscrewed. This one is often tighter than the logo ring. If you remove the three screws just outboard of the element (they are under the logo ring) then lift off the plastic insert and shutter speed ring, you can get to the lens real easily with those magic thumbnails. I always try using my hands or soft tools before I pick up metal tools that may scratch. One of the hardest things about camera repair is doing it so when finished it looks untouched. Im not usually 100% successful...but I try.
Now nothing is between us and the metal shutter blades. There are only two blades and they close like crescent shaped scissors. The blades stick together and lock up where the very light shutter spring isnt strong enough to separate them. A few taps will generally free them but I like to sprinkle on a few grains of graphite and roll a bit of tissue and hold it with the tweezers (never touch the blades with your fingers!) then massage in the graphite. Be VERY gentle!
This assumes the blades are shut. If they are stuck open the job is a tad harder. The blades must first be closed to sprinkle on the graphite. This is what the Ronsonol is used for. Hold the camera upside down and squirt the lighter fluid to where you imagine the blade hinge to be. Banging it against the heel of your palm sometimes allows the shutter to close. Try cocking and releasing the shutter a few times. Btw, NEVER force a winding lever. Forcing this lever never does any good and can only bend and damage the shutter charging mechanism which on a Demi is very easy to bend. Then do as the previous paragraph suggests. Blow dry or let dry overnight.
While we are here, check the aperture blades. Massaging with graphite and the tweezer held tissue if they seem stiff. Dont do it "just because", only if the aperture ring is stiff to turn or the blades appear to be dirty or have an oil film. Neither the aperture nor shutter blades should be lubricated with anything other than graphite.
Well, thats it. Shutter should be snapping and aperture should be...err...aperturing. Just clean the rear element and blow everything out good with the can of air. All free graphite must be blown out. Reassemble and you are set to go take beautiful photos!