Canonet QL17 Giii. Finally getting some use

John Bragg · Jun 8, 2020

Hi All. I have a fairly well looked after and lightly used example which I bought a while ago and had a partial CLA done on the shutter. I installed new seals, courtesy of Jon Goodman, and it has lain unused for a while, as life sometimes has other plans for us. I have finally gotten round to using this charming little camera and it currently is being fed some HP5+. Lesson one today was that although the meter will function with a 1.5v silver cell, it will not meter accurately, so back to sunny 16. I have a solution to that problem in the post in the shape of a converter and some hearing aid batteries. Also on the way to me is a couple of hoods from China. One straight and one flared with cutouts. I figured to get both as they were crazy cheap and I will use the most appropriate one. Any further tips and useful hints would be gratefully received.

Regards, John.

bernard_L · Jun 8, 2020

If you are reasonably handy with electrical stuff, you might consider to adapt your camera permanently to silver oxide batteries. All it takes is a Schottky diode with a suitable current rating and forward voltage. You'll want a rectifying diode with a current rating of 500mA...1A. Do not pay attention to the nominal current and forward voltage ratings, because you are going to use that diode at a much smaller current. You need to look at the datasheet (example below)

Circled in red is a typical operating point in the camera's meter circuit: a fraction of a mA, and a voltage drop of 200mV. Just what you need to convert the 1.55V of silver-oxide to the 1.35V expected from mercury. Typically the diode would be installed under the bottom plate, near the battery compartment. I can't give step-by-step instructions, as this depends on the camera and your proficiency in things electrical. And probably naysayers will point out that the voltage drop varies with current... I contend that the associated error is less than systematic metering errors as arise from, e.g. a patch of sky or sunlit rock in an otherwise shady scene.

John Bragg · Jun 8, 2020

bernard_L said:
If you are reasonably handy with electrical stuff, you might consider to adapt your camera permanently to silver oxide batteries. All it takes is a Schottky diode with a suitable current rating and forward voltage. You'll want a rectifying diode with a current rating of 500mA...1A. Do not pay attention to the nominal current and forward voltage ratings, because you are going to use that diode at a much smaller current. You need to look at the datasheet (example below)
View attachment 247911

Circled in red is a typical operating point in the camera's meter circuit: a fraction of a mA, and a voltage drop of 200mV. Just what you need to convert the 1.55V of silver-oxide to the 1.35V expected from mercury. Typically the diode would be installed under the bottom plate, near the battery compartment. I can't give step-by-step instructions, as this depends on the camera and your proficiency in things electrical. And probably naysayers will point out that the voltage drop varies with current... I contend that the associated error is less than systematic metering errors as arise from, e.g. a patch of sky or sunlit rock in an otherwise shady scene.

Thanks Bernard for your in depth reply. I am not short of hand held meters, but as luck would have it, I didnt have one on the day and I had only tested the camera with a silver cell in low light and adjusted the ei accordingly to agree with my Euromaster V. In bright light it was way off by 3ish stops over and kept locking the shutter in Auto. I am comfortable with sunny 16 and have since found a more reliable Android meter app for the odd sanity check. I will bear the diode fitting in mind though as with the Polar Bear adaptor I have on order, I can use SR44 cells which work well in my adapted OM1n and I always have some on hand.

Donald Qualls · Jun 8, 2020

bernard_L said:
And probably naysayers will point out that the voltage drop varies with current.

It does -- but what's the full scale current for that milliammeter in the metering circuit? I'm guessing if you select your diode to correct for that level, the error in lower light will be completely acceptable (as was the case with no diode).

Another way to solve this problem is with an actual regulator device (zener diode?) and lithium coil cell. I bought a converter recently for my Pentax 1/21 Spotmeter that uses a 1632 cell and a regulator, ought to run for years with the tiny current draw of that meter (especially since it draws effectively nothing while in the closed case or with lens capped). What's the original cell in that QL17? I've got one, but I'm not at home. If it's a PX640, there's a one-time solution for $30 that I can link when I'm home.

John Bragg · Jun 8, 2020

Donald Qualls said:
It does -- but what's the full scale current for that milliammeter in the metering circuit? I'm guessing if you select your diode to correct for that level, the error in lower light will be completely acceptable (as was the case with no diode).

Another way to solve this problem is with an actual regulator device (zener diode?) and lithium coil cell. I bought a converter recently for my Pentax 1/21 Spotmeter that uses a 1632 cell and a regulator, ought to run for years with the tiny current draw of that meter (especially since it draws effectively nothing while in the closed case or with lens capped). What's the original cell in that QL17? I've got one, but I'm not at home. If it's a PX640, there's a one-time solution for $30 that I can link when I'm home.

Thanks Donald. Are you talking about the CRIS converter ? The one I have ordered is a dumb version made in stainless steel but supplied with 6 zinc air wein type cells. I anticipate needing the meter more in low light situations as I like the exposure lock for contrasty light or backlit portraits. I think a lens hood will help as right now the meter seems overly influenced by the sky.
Regards, John.

Donald Qualls · Jun 8, 2020

@John Bragg No, I'm talking about a converter (specific to PX640, probably due to the dimensions of the regulator device) that takes the 3V input from a lithium coin cell and makes a regulated 1.3V out of it, at least at CdS meter current levels. If you google for "power my Pentax spotmeter" you will probably find the page I found -- but this will only work for a PX640. Same guy sent me one that replaces 2x PX625 that he doesn't talk about on his page.

The big problem with the zinc-air hearing aid cells is their short life. They also take some time to "ramp up" after you peel the label and let air into the cell -- 15-20 minutes, in my experience before you can even tell if a still-sealed cell in your camera bag is good or bad.

John Bragg · Jun 8, 2020

Donald Qualls said:
@John Bragg No, I'm talking about a converter (specific to PX640, probably due to the dimensions of the regulator device) that takes the 3V input from a lithium coin cell and makes a regulated 1.3V out of it, at least at CdS meter current levels. If you google for "power my Pentax spotmeter" you will probably find the page I found -- but this will only work for a PX640. Same guy sent me one that replaces 2x PX625 that he doesn't talk about on his page.

The big problem with the zinc-air hearing aid cells is their short life. They also take some time to "ramp up" after you peel the label and let air into the cell -- 15-20 minutes, in my experience before you can even tell if a still-sealed cell in your camera bag is good or bad.

Thanks again Donald. I use SR44 in 2x converted OM1n and I like the long life of them. Also they are cheap and two will power my Leica M6. I know the zinc air have limited lifespan, but they will be good to test the camera. If I find myself liking it, then full on conversion is the way to go. I just wish Cris converter was cheaper. My Om1ns are a doddle. Internal conversion and just slip an O ring on an SR44. Meter reliable right across the board and the guy that did it matched the two as closely as possible. Real old school service.

reddesert · Jun 8, 2020

The Canonet QL17 uses a single PX625 battery, not a PX640.

In the 1990s, I used a PX625A alkaline battery in mine when mercury batteries disappeared and needed less than one stop ISO adjustment, but over the >25 years I have had it, I think the meter properties have shifted a little.

John Bragg · Jun 8, 2020

reddesert said:
The Canonet QL17 uses a single PX625 battery, not a PX640.

In the 1990s, I used a PX625A alkaline battery in mine when mercury batteries disappeared and needed less than one stop ISO adjustment, but over the >25 years I have had it, I think the meter properties have shifted a little.

I have an alkaline in it at the moment. It is off by 1/3 of a stop. I set it at 180 instead of 200iso.

Donald Qualls · Jun 8, 2020

reddesert said:
The Canonet QL17 uses a single PX625 battery, not a PX640.

Okay, same battery callout as my Rollei 35. I recall the Rollei working okay with the alkaline cell. but it's been a long time since I used it much.

grat · Jun 10, 2020

Wein and Exell both make PX625 replacements that are a bit larger than hearing aid style batteries (or that's one big hearing aid!). Being zinc-air, they're a bit quirkier and not as long lasting as a mercury cell, but they're also a bit less toxic.

Canonet QL17 Giii. Finally getting some use

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