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Aphototaker
Tinkerer
Username: Aphototaker
Post Number: 265
Registered: 12-2009
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, September 10, 2010 - 09:15 pm: | 
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I had mentioned in an earlier thread (URL: https://kyp.hauslendale.com/classics/forum/messages/2/19832.html?1284003941) that I wanted to open up the mirror cage of my Canon A-1 to clean and lubricate it (the diaphragm was not closing to its minimun value of 16 on my lens, it would top at around f/9.5 or f/11). User brcamera was kind enough to comment on the problem. So here is what I tried:
1. After looking at various diagrams of the side of the mirror box which has the diaphragm setting lever, I used a syringe to lubricate the gears. Did not work, the diaphragm setting lever was still moving and making sounds like dry gears.
2. I decided to open up the mirror cage and tackle the problem head on. This is where things got a bit interesting, as described below.
To open up the mirror cage, I removed the leatherette using naphtha (to dissolve the adhesive). After removing the leather covering, I tried the diaphragm again and lo and behold, it was working without any previous noises! Looks like some naphtha made its way to the gears and did some cleaning and temporary lubing there.
What is interesting here is that I tried my lens and noticed that this time, the diaphragm seemed to be closing a couple of stops more than what was shown in the meter (VF display). So the problem has somewhat reversed. Previously, the lens was not closing to its minimum aperture, and now it was closing two stop more than required, and quite consistently. And this was without opening the mirror box yet!
Even though the diaphragm was moving like well lubricated arm, I knew that naphtha's effect would wear out eventually and the problem would resurface. So I continued to dismantle the mirror box. Desoldered the necessary wires*, remove the screws, etc. and got the box out. Lubed all the relevant parts and put the box back in**. The problem still remains though: the aperture diaphragm closes about two stops more than what is shown by the meter. Does somebody have any idea how to debug this problem?
Thanks.
* The wires to be desoldered are different (in color as well as in number) that what is shown in the service manual that I have.
* Cleaning the mirror was a breeze with the mirror box out! It was quite satisfying to do this  |
Brcamera
Tinkerer
Username: Brcamera
Post Number: 32
Registered: 08-2010
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, September 11, 2010 - 01:08 pm: |
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Well, I would first check to see which is correct, the aperture or the meter readout. This would help narrow down the problem as to either a meter issue or a diaphragm problem. I would suspect a stop down problem but you should still check before going any further. |
Aphototaker
Tinkerer
Username: Aphototaker
Post Number: 266
Registered: 12-2009
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, September 11, 2010 - 08:57 pm: |
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Edit: meant to write earlier that cleaning the prism was a breese, not the mirror.
The same thought had occurred to me, but I am not sure how to verify the meter display. Essentially you are saying that I should verify the display is showing the f-stop that the meter has calculated? I am not really sure how to do this, but I will keep this in mind.
Meanwhile, my next step is to check the soldered connections that I did during reinstallation of hte mirror cage.
Also, if anybody can confirm, is the resistance between the orange wire (from the variable resistance on the mirror box side) and ground supposed to change from infinity to 0 as the diaphragm lever is pushed down? At least that is what is written in AE-1's service guide.
Thanks. |
Aphototaker
Tinkerer
Username: Aphototaker
Post Number: 267
Registered: 12-2009
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, September 12, 2010 - 11:36 am: |
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I checked the camera's meter and compared it with another camera's meter. The readings show that this A-1's meter is measuring and displaying the exposure just like before.
So I suppose I got to look at the aperture mechanism to see what could be wrong. |
Aphototaker
Tinkerer
Username: Aphototaker
Post Number: 268
Registered: 12-2009
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, September 12, 2010 - 08:54 pm: |
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Another little fact: in Av mode, the aperture shown in the digital display in the viewfinder is the same as the one selected on the Av dial. But when the shutter is release, the lens aperture diaphragm closes more than it should and it closes to different values usually. |
Brcamera
Tinkerer
Username: Brcamera
Post Number: 33
Registered: 08-2010
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, September 12, 2010 - 10:28 pm: |
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Two suspects:
1.) MG1 diaphragm control electromagnet on the mirror box, possible oil on the surfaces, this will cause the magnet to stick and underexpose via too small diaphragm opening.
2.) Diaphragm sensing resistor. You asked about the resistance-it should change smoothly from app. 1.7K to 1K or less in the complete range. If it does not change smoothly the resistor will need to be cleaned.
I have taken this info from my notes made many years ago and it is probably a good place to start.
Bill |
Aphototaker
Tinkerer
Username: Aphototaker
Post Number: 269
Registered: 12-2009
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, September 13, 2010 - 08:19 am: |
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Yes, I agree with your first point. I read AE-1 Program's service manual last night and there is a sentence, just one, saying that MG1 is sprayed with an oil dispersant otherwise oil on it will cause the aperture to close more than it should. The sticking due to the oil delays the arm's release from the armature. So, given your suggestion and the blurb in AE-1 Program's manual, I am going to focus on this factor next.
Regarding the second point, I also discovered that it is not true in A-1's case. The AE board on the side of the mirror cage of this camera is not like the one in AE-1 but like the one in AE-1 Program. There is a arc of metal stripes over which the diaphragm contact travels thus shorting the corresponding contact to ground when over a metal strip and opening it when not. This gives a series of high and low pulses, as the diaphragm is closing, which the camera's meter circuit counts. The count is used to decide when to stop closing the diaphragm.
I will report back how it goes.
Thank you for your insights. |
Aphototaker
Tinkerer
Username: Aphototaker
Post Number: 270
Registered: 12-2009
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, September 13, 2010 - 09:13 pm: |
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Verdict: Cleaning MG1 fixed the problem!
Bill, thanks a ton for your advise and hints ... much appreciated!
Taking out the mirror cage, cleaning and lubricating it was a very interesting and rewarding experience.
Regards. |
