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11-16-2012, 10:33 AM   #1
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Asahi Pentax 6x7 Shutter too fast. Fix?

I have been getting consistant under exposure from my recently acquired 6x7, and thought the culprit might have been thorium in one of my lenses.

But after comparing lenses using Fuji instant film, I realized I wasn't losing light due to yellowing, (although some is present). I've discovered that my shutter is slightly too fast. I did extensive tests, and made an animation to time the shutter. The results can be seen here (using my 5D as control) more MSG!

The Pentax 6x7 shutter is slightly too fast. Now, the question is how to fix it.

Any advice?

11-16-2012, 10:48 AM   #2
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Over expose it a 1/2 stop or whatever from what the meter says. Or adjust the meter's ISO to compensate. It is quite common with film to have your own personal EI rating of a film especially with BW film.
11-16-2012, 10:51 AM   #3
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Yeah, that's what i'm going to do until I get it fixed for real. But how do I actually fix it?
11-16-2012, 11:03 AM   #4
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QuoteOriginally posted by Mgarber Quote
Yeah, that's what i'm going to do until I get it fixed for real. But how do I actually fix it?
I couldn't say apart from taking it to a repair shop. But if you are only a 1/3-1/2 stop off, not too big deal for most negative films. But positive films are more sensitive to proper exposure. Not many BW developers achieve box speed anyway, for example.

11-16-2012, 11:22 AM   #5
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The board for speed control is located on the shutter speed dial side under the mirror.
To access is going to be quite a few leatherettes and screws to be removed the lower middle shell.

Three adjusters should be on the board and control the span of speeds from 1/1000 side (VR1) and 1 sec. side (VR2) and the battery check threshold (VR3).
The adjustment will cause shifts and compression to the overall speeds... from what I'm observed.

I would say that adjustments should not be done casually, a constant test must be done. The push and pull of speeds have happened suddenly and is touchy to set.


Last edited by MysteryOnion; 11-16-2012 at 01:52 PM.
11-16-2012, 11:31 AM   #6
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I would send it to Eric Hendrickson for a CLA and speed adjustment. He does the 67 cameras and he has a great reputation here.
11-16-2012, 11:32 AM   #7
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Wow! That's specific, thanks! Well, I'll take it to the old guy who repairs film cameras and see if he thinks he can do it.

Until then, I"ll shoot TX for the latitude, although I've got 5 rolls of slide film I'm dying to use.
11-16-2012, 11:34 AM   #8
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QuoteOriginally posted by gabriel_bc Quote
I would send it to Eric Hendrickson for a CLA and speed adjustment. He does the 67 cameras and he has a great reputation here.
I appreciate the recommendation, but I live in Taiwan, so shipping could get pricey. There are experienced local guys I can talk to though.

11-16-2012, 11:54 AM   #9
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QuoteOriginally posted by Mgarber Quote
I appreciate the recommendation, but I live in Taiwan, so shipping could get pricey. There are experienced local guys I can talk to though.
Ah - didn't notice that. Yes, that would cost a bit for shipping!
11-16-2012, 07:54 PM   #10
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QuoteOriginally posted by MysteryOnion Quote
Three adjusters should be on the board and control the span of speeds from 1/1000 side (VR1) and 1 sec. side (VR2) and the battery check threshold (VR3).
The adjustment will cause shifts and compression to the overall speeds... from what I'm observed.
Are they those the flat yellow things, with what looks like a single watch hand on the face, just under the black 2 pin thing? I want to show this picture to the repair guy to make sure he's already familiar with it.
11-17-2012, 02:16 PM   #11
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QuoteOriginally posted by Mgarber Quote
...what looks like a single watch hand on the face...
Yep... But your repair guy should show your the results of the speeds after he fiddles with them.

fraction of a sec. = milliseconds

1/1000 = 1ms.
1/500 = 2ms.
1/250 = 4ms.
1/125 = 8ms.
1/60 = 16ms.
1/30 = 30ms.
1/15 = 60ms.
1/8 = 125ms.
1/4 = 250ms.
1/2 = 500ms.
1 = 1000ms.
11-27-2012, 09:00 PM   #12
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Ok, I've been to a few repair guys, and they all say the camera is too old to fix, but one of them was at least able to roughly measure the shutter speed. He said it is too fast by 1/4. Short of cannibalizing another 6x7 for internal circuitry, or constructing a laser-shutter-timer-thingy (which I have already started investigating with an engineer friend), I will work on the assumption that all speeds are 1/4 too fast.

Based on my math skills perfected in art school, the shutter dial on the camera should actually read:
1/1333
1/666
1/333
1/166
1/83
1/44
1/22
1/10.666
1/5.333
1/2.666
1/1.333 or "3/4 as my Canon would read out.
11-28-2012, 10:20 AM   #13
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QuoteOriginally posted by Mgarber Quote
constructing a laser-shutter-timer-thingy (which I have already started investigating with an engineer friend), I will work on the assumption that all speeds are 1/4 too fast.
Hey... you run across the DIY of using a audio recording program to do time testing? It is simpler and close to accurate. Check the following as an example.

Malcolm Raggett • Photography
11-28-2012, 11:27 AM   #14
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QuoteOriginally posted by MysteryOnion Quote
Hey... you run across the DIY of using a audio recording program to do time testing? It is simpler and close to accurate. Check the following as an example.

Malcolm Raggett Photography
I've made that. It works pretty good.
11-28-2012, 01:14 PM   #15
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QuoteOriginally posted by Mgarber Quote
they all say the camera is too old to fix
Pity...I have two places local to me that would be able to do the job and of course, there is always Eric. You could always look into the price to ship it to him.


Steve
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