Canonet QL17 G-III lightmeter

[IgalSc]

Hello
I'm reading this forum a lot, but this time I'll have to post
I have a nice Canonet QL17 G-III with lightmeter not working.
At all.
I've tried Zink-Air batteries, as well as "regular replace" batteries - the lightmeter just does not work 🙁
I'd like to try repairing it myself (if possible).
I got the repair manual from the favorite classics, but it's messy and has nothing about the lightmeter.
Would appreciate any help.

 

[Sonnar Brian]

Some things to try:

1) Clean Battery Contacts and try it. Use something abrasive like a pencil eraser;
2) remove bottom plate and inspect the solder joints to the contacts in the battery chamber;
3) make sure there is not a short. The plastic of the chamber is held in with two "nubs". If they break, the contact shorts out on the camera chassis. I used electrical tape to insulate mine after this occurred.

Beyond that- it gets complicated. I have not required anything beyond these steps for several repairs.

 

[IgalSc]

Some things to try:

....

thanks for the answer
I opened the bottom plate and cleaned the battery contacts - but no luck meanwhile.
I think my battery is discharged already - will give a try with another one, hope this will help.
BTW, I opened the front light meter cover and saw some black spot on the meter itself - can this be the source of the problem?

 

[Sonnar Brian]

I've never seen a good Cell with a Black Spot. It is a CDS cell, a photo-resistor. If the filament across the cell is broken, it's a goner. If you are good with soldering, it's possible to replace the cell with one from a parts camera. This means taking out the old cell and putting the new one in its place. Try an Ohm Meter across the cell in the camera to test it.

 

[IgalSc]

ouch... 🙁
I thought this might be a problem
Thanks, I'll try to find one working CdS (I think I have one in old broken S2)

 

[Murray Kelly]

My Canonet-28 has a 'black spot' on the lens in front of the CdS sensor.
It looks like a factory final adjustment rather like the one on my hand held meter (Unittic).
Murray

 

[Sonnar Brian]

CDS Cells are different across cameras, so a Konica might not work in a Canon.

The Canonet QL17L and GIII have an adjustment internally, accessible under the top plate for fine-tuning. I've used this to recalibrate a GIII with a weakening CDS cell for a friend.

 

[Murray Kelly]

CDS Cells are different across cameras, so a Konica might not work in a Canon.

The Canonet QL17L and GIII have an adjustment internally, accessible under the top plate for fine-tuning. I've used this to recalibrate a GIII with a weakening CDS cell for a friend.

Any tips on recalibrating? My 28 needs mercury cells and I have silver cells.
The 28 system looks way simpler than the 17 or GIII because the manual shutter speed reverts to 30th - the speeds are only different with 'auto', thus making the whole set-up more 'basic'.
I see the series resistor (?1.5K) adjuster with its paint blob, and a shunt variable resistor would pull down the microammeter but I have to work out a way to 'calibrate' the thing.
I did consider a setable light source to get the full scale with 1.35V from an external supply, then raise the volts to 1.55 and shunt the meter until it falls back to full-scale again.
This method assumes that the lower end of the scale will be left alone and presumed close to OK.
Sound anything like what you did?
TIA
Murray

 

[andynmorrison]

Solved?

Solved?

Hi I have a similar issue and I was wondering if any of you had figured out how to align the 28? Thanks!

 

[Sonnar Brian]

I was working on a Canonet last week, and noticed there was a plastic ND filter under the name ring covering the CDS cell. I suspect that Canon had different filters for the cells, to make up for differences in individual cells. So it occurred to me: put a stronger piece of plastic ND filter over the CDS cell. I've got some from my Polaroid SX-70's around, will give it a try.

Otherwise- pop the top plate, look down neat the meter area. Ypu will see a trim-pot that is accessible with a flat-head jewelers screwdriver.

 

[andynmorrison]

Thanks

Thanks

Hey! Thanks for the quick reply! I will try that out and post my results.

 

[Sonnar Brian]

Your welcome- this thread popped up on the front page, and I had missed the post before yours. I never mind a PM with specific questions. I do not use the "subscribe to thread" feature. I get plastered by Email notifications! PM system works well.

 

[dmr]

The electrical schematic that is floating around for this camera is very confusing, and an electronics geek friend helped me simplify it. Most of the confusing stuff has to do with the flash exposure circuit. The light meter is really just a simple circuit of the battery, the photo cell, and a couple variable resistances. I can post this if you like.

 

[Murray Kelly]

Hi I have a similar issue and I was wondering if any of you had figured out how to align the 28? Thanks!

As regards the 28 I went ahead and did as I indicated.
With 1.35V applied the light source was varied until there was a Full Scale (FS) deflection at 400 ASA. (600th sec)
I noted the deflection on the slowest ASA with the light at the same setting. Was between 60th and 30th. The light setting was left as it was for the rest of the work.
I then connected a variable resistor (22K) across the meter and brought the FS deflection back to 600th second with 1.55V applied. It was not correct at the bottom end of the scale. 😕
It took 300 ohms in series with the meter to bring the lowest ASA setting back to between 60 and 30th. This had almost nil effect on the FS ASA 400 setting. I suspect the adjustment next the meter is for this - it has a value of 1.5K ohms and would be ideal for just such fine tuning at the slower speeds.
I didn't touch it on mine but added the separate resistor in series alongside the shunt. Everything works fine now on a silver cell 1.55V.
To replace a non-functioning CdS cell unfortunately you'd need some sort of 'standard' light to set the 400 ASA/ FSD of the meter which is presumably what they have in the factory. I can't help there, sorry. With that they'd adjust the 400 FS deflection either with a ND filter (? exposed film?) or black blob on the CdS window and set the bottom end with the internal variable next the meter.
For more shunt/series resistor detail also see
http://www.kyphoto.com/classics/siloxidemodify.html where it is explained with diagrams how the values are derived mathematically - I did it in a more practical way, as in the first diagram of the 2 using a single variable resistor, found by trial and error, and measured the two 'arms' of the pot with a multimeter when I had both ends of the meter scale correct. Fixed resistors of the correct value were then substituted permanently. Getting small resistors is the hardest part! Surface mount would've been nice and small.
Murray

 

[dmr]

Any tips on recalibrating? My 28 needs mercury cells and I have silver cells.

I recalibrated a GIII both for the "wrong" battery and to adjust the ASA/ISO range so I could use 1600 film. I killed two birds with one stone, so to speak, by doing this.

I did a two-point (zero-span) type of calibration, similar to what's done on many clinical lab instruments. I used two laboratory-grade luminance sources (a window with a scene illuminated with afternoon sun and an eggshell blank bathroom wall with the overhead and vanity lights on dimmers) 🙂 and calibrated the low end by rotating the meter slightly and the high end by adjusting the variable resistor. Rinse-repeat until both low and high readings agree with the lab standard (the Pentax K1000).

The end result is that it's more than close enough for Kodachrome and Velvia. 🙂

 

[andynmorrison]

The electrical schematic that is floating around for this camera is very confusing, and an electronics geek friend helped me simplify it. Most of the confusing stuff has to do with the flash exposure circuit. The light meter is really just a simple circuit of the battery, the photo cell, and a couple variable resistances. I can post this if you like.

- dmr

Hi! Luckily I am an Electronics tech by trade and have found and simplified the circuit myself...mine doesn't seem to be working correctly. I was able (via trim pot adjustment) to make the needle com down from maxed out, but no deflection from different light sources. It's the same all the time. I'm going to do more measurements and I'll post my theories here!

 

[andynmorrison]

Ok, here's something I found out (not good I think): The CdS cell is only varying .1 ohms; a range of 12.3 - 12.4 ohms. I don't think that there is enough variance there to get a reading on the meter.

Does anyone know the approximate resistance range of a working CdS cell?

 

[andynmorrison]

I think I answered my own question: Google Book: Camera Maintenance & Repair

It says that the resistance range of a working CdS cell from a FTb is in the kilo-ohms. It looks like I have a dead cell. Well, it's either stealing a good cell from a broken camera or just buying another for me! 🙁

 

[andynmorrison]

Ok, so I realize that there is little point in doing this; but I am making a circuit simulation in QUCS. I hope to determine (after I have found out all of the component values) the total current range that the light meter reads. I am hoping that if I can do that I can find a cheap replacement and adjust the circuit accordingly.

Updates to follow.

 

[VíctorRF]

The electrical schematic that is floating around for this camera is very confusing, and an electronics geek friend helped me simplify it. Most of the confusing stuff has to do with the flash exposure circuit. The light meter is really just a simple circuit of the battery, the photo cell, and a couple variable resistances. I can post this if you like.

Could you post it, please?? Thaks!