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Message |
    
Tronds_63
Tinkerer
Username: Tronds_63
Post Number: 13
Registered: 12-2011
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, February 10, 2013 - 11:39 am: | 
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My Pentax Super Program stops the lens all the way down when the aperture ring is set at "A". This is independent of the amount of light in the motive. This leads to massive underexposure when used in program mode or in shutter priority mode.
In manual or in aperture priority where I control the aperture, the exposure is spot on. I have tested with two different lenses and the result is the same with both, so it's not the lens.
Anyone who knows how to fix this? |
Ismaelg
Tinkerer
Username: Ismaelg
Post Number: 104
Registered: 11-2009
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, February 10, 2013 - 12:46 pm: |
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Hi,
Do you have any other "A" lens to try? That will help isolate the issue to the body or the lens. When the lens is in the "A" mode, a pin in the mount raises and makes contact with the body. If this pin is stuck retracted, the camera operates as if the lens is set to the smallest aperture. Check the lens contact while moving in and out of the A mode and see it if pops out or not. Also, clean the contacts in the camera (as well as in the lens) to ensure a proper electrical contact.
Hope this helps!
Thanks,
Ismael |
Tronds_63
Tinkerer
Username: Tronds_63
Post Number: 14
Registered: 12-2011
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, February 10, 2013 - 05:41 pm: |
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Well, as i wrote, I have tested with two different lenses. Yes, they are both "A" lenses. Both works on another camera. Both gives the same result. The camera detects that the lens is in "A" mode. It does NOT expose as if the lens was set to the smallest aperture. If it did so, the exposure would be spot on and the exposure times propotionally longer. It exposes like the aperture is set to what is displayed in the finder, but the lens stops all the way down when I press the shutter release.
So, as I said, the problem is within the camaera, NOT the lens. |
Tronds_63
Tinkerer
Username: Tronds_63
Post Number: 15
Registered: 12-2011
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, February 11, 2013 - 03:30 pm: |
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I have used the lens on a Pentax Program Plus body today, and it works perfectly.
The Super program body still has a problem i "A" mode. I have read some hints about magnets, but didn't find enough information to get any wiser. |
Brcamera
Tinkerer
Username: Brcamera
Post Number: 241
Registered: 08-2010
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, February 11, 2013 - 07:48 pm: |
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The symptoms you described on your Super Program are most likely indicating a problem with the aperture magnet or the optical encoder. Both of these are located on the inside of the front plate so the mirror box will have to be removed. Depending on your skill level/experience this may not be a trivial repair.
Bill |
Tronds_63
Tinkerer
Username: Tronds_63
Post Number: 16
Registered: 12-2011
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, February 12, 2013 - 01:54 am: |
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It will certainly not be a trivial repair. I expect that. I have disassembled and assembled again several cameras, so there is a chance it may work again. Can you please decribe the problems connected to the magnet and optical encoder and what to do to fix them?
Trond |
Brcamera
Tinkerer
Username: Brcamera
Post Number: 243
Registered: 08-2010
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, February 12, 2013 - 10:16 am: |
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The optical encoder consists of a phototransistor and a LED. As the diaphragm of the lens closes, the optical encoder sends a signal to the CPU. A spinning disk passes and cuts off light between the phototransistor and the LED. When the count to the CPU is correct (matches the value that is needed for correct exposure) a signal is sent to the aperture magnet which releases and stops the lens diaphragm. So you can see that there are several points here that can cause problems:
Dirty encoder, wiring between encoder and CPU, wiring between aperture magnet and CPU, aperture magnet sticking or its coil is open.
When I was actively repairing these cameras (not really feasible for customers to pay me to service these anymore) I would look at the encoder signal with an O scope to check the pulses. Also, I would check resistance of the aperture magnet for an open circuit.
Bill
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Tronds_63
Tinkerer
Username: Tronds_63
Post Number: 17
Registered: 12-2011
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, February 12, 2013 - 05:34 pm: |
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Problem solved. I don't reccomend this way to do it unless you can afford to destroy the camera!
I poured some lighter fluid down the slit where the aperture contol arm is. I placed the camera vertically with the side with aperture control arm down and poured some lighter fluid around the aperture control arm. I then cycled the camera some times. At first, the arm stopped moving at all, but after maybe 40 shutter releases, the arm started moving again. I attached an A-series lend and the winder and shot at continous. Pretty soon the aperture started to close down to a resonable size depending on selected shutter speed and light.
I don't know if the magnet was sticky or the optical encoder was dirty, but this camera came from Israel with a fair amount of the Negev desert on/in it. If the internals was about as dirty as the outside, the lighter fluid washed away the dust in the optical encoder.
Again, I don't reccommend this procedure unless you can afford to destroy the camera completely. |
Br1078lum
Tinkerer
Username: Br1078lum
Post Number: 482
Registered: 11-2010
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, February 12, 2013 - 09:16 pm: |
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If it's got the Negev in it, it's probably going to grind itself to death sooner or later. Could use a good teardown and cleaning.
PF |
Tronds_63
Tinkerer
Username: Tronds_63
Post Number: 18
Registered: 12-2011
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, February 13, 2013 - 02:03 am: |
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It won't be used that much. It's just one of the about 80 cameras I have. I use Nikon D300 and Bronica ETRSi for serious work.
In this way, it will probable be used for less than 20 films in the next 10 years or so. The rest of the time it will spend its life on the shelf in the local camera museum. |
