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Homemade Pentax 67 vacuum back - advise please

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#1 Michal1

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Posted 23 May 2014 - 06:27 PM

Hi, I finally get a cheap defective P67 body so that I can start building my own vacuum back!

Here is my plan. First, I will take off the back door form the camera and unmount the pressure plate from it. I will drill several small holes in the pressure plate copying the original Hutech vacuum back. Then I will make a small tub on the side of the pressure plate averted from film, from which the air will be exhausted. A stripe of material (metal or plastic) will be glued on the pressure plate constituting an enclosure around the perimeter of the plate. A layer of plastic glued on top of this enclosure will make up the bottom of the tub. From the tub a short tube will lead outside the camera by a hole in the camera's door. The air will be sucked out either by a reversed air pump for fish tanks as Kona describes here or by a rubber ball.

Any comments or ideas are welcome!

#2 Nebhunter

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Posted 23 May 2014 - 06:57 PM

This is definitely worth a try. I would suggest the rubber ball idea first. No need for power that way. I've found that once the vacuum holds the film, there is no leak and it should hold the film. Just be sure to release the vacuum before advancing the film, otherwise the chain on the lever will get stretched or break.

Igor

#3 TxStars

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Posted 24 May 2014 - 07:38 AM

Make sure you chamfer the holes on the film side or you might get some damage to the film.

#4 Michal1

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Posted 26 May 2014 - 04:20 AM

Nebhunter and TxStars, thanks for your comments.

I have encountered a problem. How to remove the pressure plate? According to the servicemanual, the plate is held by screws, but I can't unscew them. They don't move. Are they really ordinary screws?

#5 TxStars

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Posted 26 May 2014 - 02:44 PM

The pressure plate on my 67 is held on with spanner screws.
I have spanner drivers, but they can be made from a flat head screw driver using a file if you dont have one.
For tight screws you can use a center punch to help loosen them.

Attached Files



#6 Michal1

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Posted 30 May 2014 - 04:48 PM

The pressure plate is removed, thanks TxStars! Who is interested how the back door looks like under the pressure plate, look here and here on the detail. The lever between the pressure-making springs operates the film type indicator on the back side of the door. The other side of the pressure plate can be seen in this photo.

I didn't have any flat head screwdriver I wanted to file. I made a spanner driver from an old ballpen holder(photo) and it worked! Unfortunatelly, one of the pressure plate retainer screws was very tight. Instead of loosening the screw, the cylinder, in which the screw was mounted, started to rotate. I had to made thin pliers from cheap nail scissors I found at home (photo) and hold the cylinder between the pressure plate and screw head with it. I felt very happy when the screw finally loosened! Here is a photo of another ballpen screwdriver I made for unscrewing the door hinge.

Please can you desribe for me the way the air pump hose connects to the original Hutech vacuum back? How is the suction strength being regulated?

#7 TxStars

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Posted 30 May 2014 - 08:44 PM

What I had was a micro Air sampling pump which I ran the length of each exposure.
http://machinedesign...s-sampling-pump
You can get them on E-bay for about 20$

Not sure about a modified fish air pump, so you could use a air splitter sold at a fish store. With the splitter hook the pump to the inlet and the film back to one of the outputs then crack open a second outlet to reduce the vacuum.

On the Mamiya back I had there was a brass fitting sticking out of a hole drilled in the back nothing complicated.

#8 Michal1

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Posted 04 June 2014 - 04:59 PM

TxStars, did you grind or polish your vacuum back - the side touching the film?

#9 TxStars

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Posted 04 June 2014 - 05:45 PM

My Mamiya back was one done for Takahashi in Japan.
I did not see any grinding or polishing done just the chamfering of the holes.
The hose connector from the camera was attached to the inside back of the body so that pulling on the connector would not move the film plate.

#10 Michal1

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Posted 07 June 2014 - 02:01 PM

What about the hole sizes and their separations?

#11 TxStars

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Posted 11 June 2014 - 07:23 PM

Sorry I did not make note of that before selling the camera back.

#12 Michal1

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Posted 23 July 2014 - 03:47 AM

Dear friends, the vacuum back is done and it seems to be working! I will test it in next weeks. If it works correctly, I will write down instructions.

#13 Michal1

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Posted 09 August 2014 - 08:37 AM

I have tested my vacuum back in practice for the first time. The problem was that I didn't read the inctructions for removing the backing paper from a 120 film and I remembered it incorrectly. I removed the paper entirely instead of leaving a part of it on the film's start and end, so that I could take only 4 or 5 photos on the film.

 

At the moment I can tell you only about my test of the film tension. I removed the camera lens and opened the shutter, so that the film became visible. I touched the film with a tip of a pencil. When the suction was off, I could see how the reflection of a bulb filment on the film deformed. The film wasn't evidently touching the pressure plate properly. Then I switched on the suction and touched the film again. I was like if I touched a metal plate. The film didn't moved in the least! Only small bumplets appeared on the film with the suction on - around 1 mm wide, many acros the whole area of the frame. When the suction was off, the film appeared loose but the bumplets were not visible. Do you have any idea what they were?

 

I have two or three astrophotos on the film, but it is still being developed. I hope they will be able to develop the film with my modifications.



#14 TxStars

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Posted 12 August 2014 - 06:11 PM

My first guess is that the vacuum was too high and was trying to pull the film into the holes.

You only want enough vacuum to hold the film.



#15 Michal1

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Posted 15 August 2014 - 06:32 AM

TxStars, thank you for your support. These small bumps were not avove the holes, I have checked. Moreover, the bumps were bulging toward the lens. They looked like grains of something between the film and pressure plate. Maybe bubbles of the remaining air? Not sure if it is possible.



#16 Nebhunter

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Posted 17 August 2014 - 12:57 PM

TxStars, thank you for your support. These small bumps were not avove the holes, I have checked. Moreover, the bumps were bulging toward the lens. They looked like grains of something between the film and pressure plate. Maybe bubbles of the remaining air? Not sure if it is possible.

That's an interesting thought Michal and it could be trapped air.  The valve on my outfit is easily controlled for vacuum pressure and I engage it gently.  The Hutech vacuum has many many small holes spread evenly around the plate.  This could help prevent trapped air.  Just thinking out loud here.  I have some film loaded, and will try to do what you did to see if I have any bubbles.

 

Igor



#17 Michal1

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Posted 17 August 2014 - 02:06 PM

Igor, today I managed to do something what has been a big question for me for a long time. I managed do to the dry fire - opening the shutter without film inside. I repeated the experiment from my last post but this time with a single transparent frame cutted from a developed film. I get the same result as before: without vacuum, the film was slightly bulged toward the lens and with vacuum the film was sticked to the pressure plate but there were small waves, or how to call them, on the film. Something like small ripples on a pond. They seemed to have no relation to the holes in the pressure plate. I tried to shift them by a blunt pencil, but they didn't moved. I would expect that air bubbles would move. It looked as if the film was crumpled.

 

Igor, how many holes does the Hutech vacuum back have? I have thought that around 12. The mine has many more, around 80. I get inspired here http://www.horolezec...n=item&itemid=5 . How do you regulate the vacuum pressure - by sucking in the air from outside or by making the passage for air wider and tighter? My solution has a component that allows regulating the vacuum by sucking the air from outside, but the regulation is rough. Nevertheless, I tried to decrease the suction. The ripples get less pronounced, but the film get looser. If you try to do the same with your vacuum back, I will be interested in your results.



#18 Nebhunter

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Posted 17 August 2014 - 02:38 PM

Michal - not sure how many holes.  Will count next opportunity or if I can see thru the film as it is loaded.  I don't want to ruin the loaded roll.  Did the E200 film arrive yet?

Vacuum is regulated by restricting the air flow.  A simple valve in the air line and I turn it slowly until I do not hear the sucking sound of the air flow.  Just enough vacuum to seal off the holes.  

 

If it was air bubbles you should be able to move them around.  Gee now I'm thinking that too many holes could be an issue causing small wavelets in the film?  It's always something.

 

Igor



#19 Michal1

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Posted 18 August 2014 - 01:54 PM

I get the developed film today. Removing the paper completely from the film was a greater mistake than I thought. The film was illuminated by the light from outside, so the images get destroyed. There are only a few square centimeters of sky visible and they look well.
 
Igor, the number of holes alone cannot cause any problem. Do you know this famous photo? It was taken using the vacuum back from the link from my last post. But my holes are bigger than 0.6mm. I originally bought three 0.6mm drills, but they were enough to make only three holes. The 0.8mm drill prooved to be much more resilient. Read your PMs.


Edited by Michal1, 18 August 2014 - 01:55 PM.


#20 galaxy_jason

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Posted 18 August 2014 - 09:43 PM

We tried this years ago and found that in humid environments you end up drawing moisture into the camera. You can get uneven exposure around

the holes due to the differences in moisture. This is especially noticeable with with hypered film. The solution was to flood the camera with nitrogen. We

drilled a hole in the tripod connector and inserted an airbush hose fitting with a hose connected to a portable N2 tank. This wont flatten the film but

it will prevent curling due to moisture in long exposures which I believe is the cause of most film flatness issues. I even used this in my schmidt camera,

without it the film would expand during the exposure and pull away from the pressure plate.



#21 Nebhunter

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Posted 19 August 2014 - 07:49 PM

That definitely sounds like the issue Michal - moisture.  I've had issues with it because of the heavy dew conditions - everything is soaked except the lenses with dew heaters.  I would wrap plastic bags around the camera body.  Next was to have a bunch of desiccant pouches inside the bag to absorb moisture.  The N2 tank is the way to go, but there is cost involved.

 

Igor



#22 SMigol

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Posted 19 August 2014 - 11:31 PM

Wow!  What ambient humidity is enough to cause issues with film flatness?  I've shot Acros with 40 minute exposures and E200 with 1 hour exposures and haven't seen issues with flatness.  I'm in California where it's bone dry, so I've got that as an advantage. 

 

Is there some place inside the camera where a dessicant pack could be stashed?



#23 Nightfly

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Posted 22 August 2014 - 11:48 AM

"Is there some place inside the camera where a dessicant pack could be stashed?"

 

 

There's plenty of room in the tail end of the 400 F/4 Takumar :-)



#24 Michal1

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Posted 01 September 2014 - 11:12 AM

Thanks for your ideas regarding the moisture. I'm not going to follow them at this moment. I have finished the vacuum back now and I want to see what it can do ;).  BTW, try to breathe slightly on a piece of developed film or touch it by a cool object. Uneven temperature may deform the film considerably, too.

 

Next two rolls of film with my vacuum back are finished now! Must wait for development. 



#25 Michal1

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Posted Yesterday, 01:53 PM

Sooo, the films are developed! There are 16 frames of astrophotos taken with the vacuum back. I have seen them only with a loupe, but 15 of them don't have any noticeable traces of curl! The vacuum back is working!!! The only bad frame was that at the beginning of the film. I have probably cut the paper too far, so the film couldn't cling to the pressure plate properly.

 

I have added a few new photos:

http://www.astro.cz/...l Bilek/vacuum/

 

The construction of the vacuum back was surprisingly easy and straighforward with only a few blind alleys. I will gladly share every detail if someone is interested.


Edited by Michal1, Yesterday, 02:53 PM.







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