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Canon AE-1 Meter and Exposure Adjustment - Variable Resistor Functions

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koraks

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Howie1922

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Hey Everyone!

I am working on an AE-1 that is underexposing in its Auto mode by 1-1.5 stops. I would like to adjust this setting so that camera exposes correctly for my customer. I believe the adjustment involves one (or more) of the 4 variable resistors show below:

IMG_7190.jpeg


The camera craftsman article that I am referencing states that they are called:

VR1
VR2
VR3
VR Gain

Does anyone know what each of these variable resistors adjusts? The article goes into a little too much detail, and I'm looking for a simplified explanation. I haven't seen any other literature or videos that describe what these do either.

Thanks!
 
Based simply upon my own fundamental understanding of principles taught in high school Physics, I am guessing that the AE-1 employs what is known as as Wheatstone Bridge circuit in its meter, and the CdS cell of the camera is one of the four resistors employed in the Wheatstone Bridge circuit. So adjusting ONE of the three variable resistors (VR1, VR2, VR3) would be adjusted in order to achieve proper circuit balance (vs. the variable resistance due to light intensity of the CdS cell). Without seeing a circuit diagram, I cannot simply guess which one to adjust...it is best to have the guidance of a repair manual for the AE-1 telling the repair person which one to adjust in what circumstance. Is the camera powered properly with a 1.35V source during your evaluation of the meter accuracy?...deviation of Voltage CAN affect meter accuracy, dependent upon circuit design (the AE-1 might or might not be a Voltage-dependent design!)

edit: Per an AI-provided answer, "
The light meter in the Canon AE-1 is not a voltage-dependent design in the way older cameras were. While it requires a 6V battery to function, its electronic circuitry is regulated, meaning it does not rely on a precise 1.35V mercury battery for accuracy. ...
The AE-1 uses an SPC rather than older CdS (Cadmium Sulfide) cells. "
 
Last edited:
Howie1922 said:
The article goes into a little too much detail, and I'm looking for a simplified explanation.
Click to expand...

Sorry to be critical, but that sounds like a very shaky basis to start adjusting things.

1: Determine the probable cause of the wrong metering.
2: Resolve to the extent possible this cause.
3: Then determine whether re-calibration of the meter is necessary.
4: Calibrate as described in the service manual you apparently have.

I would NOT cut corners in this approach as you'd just be sticking a band-aid over the problem and you will likely end up making things worse. That's one thing if it's your own camera, but since you mention a client, this means you're in the process of messing up someone else's camera due to a lack of proper analytical approach.

wiltw said:
I am guessing that the AE-1 employs what is known as as Wheatstone Bridge circuit in its meter, and the CdS cell of the camera
Click to expand...
The AE-1 has an SPC meter cell and thus the topology you describe is not what's used here.
A further hint is in the 'Gain' label for one of the VR's.
 

Brief functional description of the light meter system is on page 12. A MOS log amp is used as a transimpedance amplifier to convert SPD current to linear voltage; this is further amplified by an opamp. Both the log and lin amps and the SPD are apparently integrated into IC4; see more detailed explanation on page 15 and further.

Adjustment procedure is discussed starting at page 40.

I'd start by verifying the voltages around IC4 to make sure these are as listed on page 61.

It would be nice if you could fine additional schematics that offer a more detailed view than the manual linked to above.

If any of the above sounds like Greek to you, IMO you should leave the metering system alone, return the camera to the customer and suggest them to offer it for repair by someone who has the know-how to troubleshoot such a metering system.
 
koraks said:

Brief functional description of the light meter system is on page 12. A MOS log amp is used as a transimpedance amplifier to convert SPD current to linear voltage; this is further amplified by an opamp. Both the log and lin amps and the SPD are apparently integrated into IC4; see more detailed explanation on page 15 and further.

Adjustment procedure is discussed starting at page 40.

I'd start by verifying the voltages around IC4 to make sure these are as listed on page 61.

It would be nice if you could fine additional schematics that offer a more detailed view than the manual linked to above.

If any of the above sounds like Greek to you, IMO you should leave the metering system alone, return the camera to the customer and suggest them to offer it for repair by someone who has the know-how to troubleshoot such a metering system.
Click to expand...

Thanks for the insight, I plan on digging deeper tonight/tomorrow.

I don’t plan on touching any adjustments until I fully understand how it all works. If I can’t get there, I do plan on giving the camera back to my client. It’s not worth messing with if I don’t understand it.
 
koraks said:
Sorry to be critical, but that sounds like a very shaky basis to start adjusting things.

1: Determine the probable cause of the wrong metering.
2: Resolve to the extent possible this cause.
3: Then determine whether re-calibration of the meter is necessary.
4: Calibrate as described in the service manual you apparently have.

I would NOT cut corners in this approach as you'd just be sticking a band-aid over the problem and you will likely end up making things worse. That's one thing if it's your own camera, but since you mention a client, this means you're in the process of messing up someone else's camera due to a lack of proper analytical approach.


The AE-1 has an SPC meter cell and thus the topology you describe is not what's used here.
A further hint is in the 'Gain' label for one of the VR's.
Click to expand...

In your experience, what other problems could cause faulty metering in the AE-1? I’d love to learn so I can troubleshoot better in the future.

The shutter speeds are good (1/500th-1/1000th have shutter capping though, I don’t use them to check the meter), and I checked that the lens I am using is giving me the proper apertures (I checked the T stops on my shutter tester).

I usually stick to mechanically controlled cameras, all the electronics in the AE-1 throw me off my game a little bit. I’m excited to learn more though!
 
wiltw said:
Based simply upon my own fundamental understanding of principles taught in high school Physics, I am guessing that the AE-1 employs what is known as as Wheatstone Bridge circuit in its meter, and the CdS cell of the camera is one of the four resistors employed in the Wheatstone Bridge circuit. So adjusting ONE of the three variable resistors (VR1, VR2, VR3) would be adjusted in order to achieve proper circuit balance (vs. the variable resistance due to light intensity of the CdS cell). Without seeing a circuit diagram, I cannot simply guess which one to adjust...it is best to have the guidance of a repair manual for the AE-1 telling the repair person which one to adjust in what circumstance. Is the camera powered properly with a 1.35V source during your evaluation of the meter accuracy?...deviation of Voltage CAN affect meter accuracy, dependent upon circuit design (the AE-1 might or might not be a Voltage-dependent design!)

edit: Per an AI-provided answer, "
The light meter in theCanon AE-1 is not a voltage-dependent design in the way older cameras were. While it requires a 6V battery to function, its electronic circuitry is regulated, meaning it does not rely on a precise 1.35V mercury battery for accuracy. ...
The AE-1 uses an SPC rather than older CdS (Cadmium Sulfide) cells. "
Click to expand...

I can try and find the circuit diagram later and post it to see if that helps.
 
IMO every AE-1 left the assembly line being adjusted by conscientious employees using purpose-made and accurate calibration equipment.
What I'm getting at is that the root cause of improper exposure is probably not a mis-adjusted potentiometer (unless a wannabe repairman was inside before). Maybe oxidation of the wiper of one of the VR*? Or a trivial cause like a broken flex?

In addition to the document linked to by @koraks I have three docs in my computer, all addressing assembly, repair, and adjustment of the AE-1:
  • A document from Camera Craftsman, 72 pages
  • An article from SPT journal, 8 pages
  • A document from NatCam, 118 pages
Since they "weigh" up to 50MB, I believe I can't attach them here. Send me a PM if you are interested.

​

 
koraks said:

Brief functional description of the light meter system is on page 12. A MOS log amp is used as a transimpedance amplifier to convert SPD current to linear voltage; this is further amplified by an opamp. Both the log and lin amps and the SPD are apparently integrated into IC4; see more detailed explanation on page 15 and further.

Adjustment procedure is discussed starting at page 40.

I'd start by verifying the voltages around IC4 to make sure these are as listed on page 61.

It would be nice if you could fine additional schematics that offer a more detailed view than the manual linked to above.

If any of the above sounds like Greek to you, IMO you should leave the metering system alone, return the camera to the customer and suggest them to offer it for repair by someone who has the know-how to troubleshoot such a metering system.
Click to expand...

It looks like this article is for the AE-1 Program, I'm working on just the plain AE-1 unfortunately. The AE-1 program does not have the same Variable resistors
 
@Howie1922 try the documents offered by @bernard_L

Howie1922 said:
what other problems could cause faulty metering in the AE-1?
Click to expand...
Instead of wildly guessing at causes, I'd really start by sourcing the right schematic and understand the circuit, do measurements based on whatever standard values you can find (similar to the table in the document I linked to) and verify each step the signal goes through and see what anomalies you note. Then work that into a hypothesis of what the root cause must be, and start addressing it.

Since SPD-based metering systems are generally very stable, it's really anyone's guess what's wrong with this one here. It's really rare to see a meter like this be off by much at all, even after a long service life.

Taking a stab at it anyway: assuming that the metering system is conceptually similar in the AE1-Program and the AE1, my first thought would be drift in the effective capacitance of the capacitor labeled 'CAD' in the pdf I linked to. This capacitor is instrumental in converting the linearly scaled voltage representing the measured light value in a time period. Drift in the value of this capacitor, or leakage currents around that capacitor (e.g. due to moisture, fouling etc.) would affect the digital conversion and could well explain a constant deviation across the exposure range. This also illustrates why it's so important to not just jump in and start adjusting pots. If this is e.g. a droplet of moisture/condensations and it ultimately dries out, the problem can correct itself (at least partly) and the meter drifts out of calibration again. If it's dirt or debris causing leakage currents, its conductance my vary with atmospheric conditions and over time as more grime collects, and the meter drifts off more. Etc. So IMO you really need to address the cause, not the symptom.
 
Last edited:
Any chance the ASA setting dial is mis-aligned/mounted in the wrong position?
 
Just in case, on page 13 of the camera-craftsman document:

1776540036332.png


I should have remembered this earlier, as I was caught myself ~1yr ago. I ended up replacing the broken tungsten wire with a polyethylene string (fishing gear), same diameter, breaking strength 28 pounds.

As chance would have it, I have now another AE-1 so far considered good to go, and on the last outing I became suspicious, checked, and found it over exposes by 1⅔ stops. Under investigation.
 
Hey Everyone!

I just wanted to give an update that I have decided to give the camera back to my client. The electronics are a little over my head and I plan on trying again on a junk AE-1 the next time I get a chance.

Also, the documents that bernard_L sent were very helpful. They described how to check everything and make the adjustments. Someday I'll figure it out, but for now I need to move onto other projects.

I am including what the resistors do here for my own reference, and for anyone who needs them in the future.

See below for what each of the variable resistors do. There are equations that you use to find out what each resistor needs to be. They are all based on the Vc voltage. All of these screenshots are from page 38-40 of the camera craftsman article, and 110-115 in the NatCam AE-1 guide that bernard_L sent me.

Finding Vc:
1776786102033.png


VR3 - maximum aperture adjustment (AVO)

1776785439436.png

1776786062166.png



VR2 - actual aperture adjustment (comparator)

1776785740381.png

1776786028591.png


VR1 - AV level adjustment

1776785639332.png

1776786006764.png


VR Gain - maintains proper differences between light levels - (Vc/16)*EV range)

1776785508812.png

1776785981561.png
 
My reply is late, but maybe it will still be helpful.

I’ve studied the AE-1 in detail. You’ll find information on adjusting its electronics on page 1 of the post, toward the end:

Canon AE-1: Adjusting the exposure metering system and performance on the camera tester

The post also lists all available technical documents.

Exploring the Canon AE-1: Disassembly, service, technology

+++

All information provided without guarantee and use at your own risk.
 
Last edited:
Andreas Thaler said:
My reply is late, but maybe it will still be helpful.

I’ve studied the AE-1 in detail. You’ll find information on adjusting its electronics on page 1 of the post, toward the end:

Canon AE-1: Adjusting the exposure metering system and performance on the camera tester

The post also lists all available technical documents.

Exploring the Canon AE-1: Disassembly, service, technology

+++

All information provided without guarantee and use at your own risk.
Click to expand...

Thanks so much for linking your post! It was very helpful. I'll be referencing it on the next AE-1 I service.
 
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