Setting Shutter Speed on a Mamiya 645

On an internet auction site I won a Mamiya 645 1000s, not for the camera but for the grip that was on the same auction. The camera itself was reported malfunctioning so the price was set low and soon I was the happy owner of a dodgy camera and a “Deluxe Left Hand Trigger Grip”.

The grip proved handy immediately for my other, working, Mamiya 645 but the 1000s was put in the cupboard awaiting the check-up. To make a long story short the last step of the overhaul was setting the shutter speed as they were off about 20 %.

Now that is perhaps not so terribly much and I can of course counter the offset timing by the PD prism ASA/ISO settings. The time error is approximately the same for both the body itself and the body with PD prism attached. Wonder why? Common aging of internal capacitors maybe?

So how do you adjust this? The service manual found on the web, is unclear at this point. It seems that there are two settings, one is to be done mechanically at 1/1000s and one should be done with a potentiometer on 1/60th. I have disassembled the PD prism and found several ”interesting” potentiometers on the circuit board, but touched nothing at the time. Behind the left side panel of the body itself I found an additional circuit board with potentiometer for adjusting the shutter time at 1/60. Does this potentimeter control all timimg? It turns out does!

With a little manipulating, it is doable to remove the Mamiya’s left side panel to access the potentiometer. According to the service manual, setting the camera at 1/60 is all that is needed and apparently all other times follow automatically!

To open the camera you have to break away/detach on its ”plastic skin” in a few places to access some screws. It was pretty messy and I have to get some soft glue to put back the lifted corners. In general, however, it appears that there is a soft, sticky surface that has been used so it is perfectly possible to stick it back in place again. But not in all places, unfortunately.

Note: Afterwards I found out that I could reach the potentiometer with a much less invasive procedure. Something to remember if there will be a next time! It will then be much easier. The part of the left panel with a raised chrome rim is in a separate piece that can be removed simply by loosening six screws! Then you do not remove the shutter speed dial and this helps considerably!

First, I put the time in 1s and adjusted the potentiometer so that the shutter speed was 1s, but then many other shutter times skewed too much. Setting the time at 1/60 instead did the trick. Some are a few milliseconds off, specially at the slower speeds, but that is of no consequence in practice.

The Mamiya with the left panel removed reveals the potentiometer for adjusting the shutter time. This should be done at 1/60 s. All other times will follow.

This is the potentiometer cutout from the picture above. By moving the slider on the black resistive compound the resistance between the slider and endpoints can be adjusted:

Screen Shot 2014-04-29 at 11.20.18

On the back of the left side plate you see six little black tabs with screws in them. Remove these screws and you will find the potentiometer much more easily! (Do as I say, not as I do:)

The light sensor probe used consists of a phototransistor (BPX43) with a 1k resistor in the collector up to plus 5 volts. The output was taken via a 470n capacitor to a DC-coupled oscilloscope. The probe is simply inserted with the 120-cassette removed and back door closed. The probe works fine on most times but is not quite optimal for 1/500 and 1/1000, where the waveform is not as distinct as at the slower speeds. For 1/60s (where accuracy was important), it was however sufficient.

Testing the shutter speed at 1/30 s. It measures 29.41 ms which is definately good enough. And probably beyond the accuracy of the oscilloscope anyway!

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9 Responses to Setting Shutter Speed on a Mamiya 645

  1. Ben says:

    Hi there,

    Thanks for this great write up, I have a Mamiya 644 1000s and my shutter speed is about 20-30% delayed. I am going to attempt what you’ve done above but if you don’t mind, I’d have a question.

    Would it be possible to mark with some pain tool the location on the image where the screw is at that needs adjusting?

    Thanks a lot,
    Ben

  2. jabcam says:

    I modified the text above. Hope this makes it clearer.
    Good luck!

  3. Ashley says:

    So, can I ask? You set the shutter speed on the camera to 1/60 and then adjusted the potentiometer until you got a satisfactory result? And then that means that the other speeds should adjust as well? I have one of these testers and the app https://itunes.apple.com/gb/app/shutter-speed/id560154244?mt=8 which I seem to make sense of, but I wanted to check that I was indeed follow your notes correctly! Thanks for a great article! Ashley

    • jabcam says:

      Exactly! The other speeds shall follow due to mechanical linkages (or cams or something).

    • bence8810 says:

      This is exactly the app and tool I used (PhotoPlug) and once I set the 1/60th – the rest just went along with it nicely. My Mamiya has been giving me some outstanding results ever since….

      While doing the tests, I noticed that my camera was giving false speeds after a while – I assume as a result of me firing the shutter constantly for 30 or so minutes while testing. I put the camera away and 10-15 minutes later when tried again, it was spot on. Not sure why this is – I was thinking maybe some capacitor or other electrical part lost it’s charge etc. Don’t know enough about electronics. Bottom line is not to test anything other than 1/60th initially and then once adjusted, put the cover back and have faith the rest of the speeds will follow 🙂

      Ben

      • Thanks guys, this is incredibly helpful and thank you for such a speedy response! So, to confirm would you pull the ‘skin’ up from the bottom front of the camera (beneath the lens release)? I have a habit of making things more difficult than they need to be!

  4. Andreas says:

    Hi,

    not sure if anyone would have the answer I have a Mamiya 644 1000s, speeds all seem as they should except for bulb, the shutter fires but does not stay open, battery is fresh, any thoughts

  5. Ashley says:

    Just thought I’d let you know, I succeeded in adjusting the speed on my 645J, but there isn’t the same panel under the vinyl. You’ll have to undo 4 screws under each of the corners as well as the screws under the shutter speed dial vinyl. But it all seemed to work to plan! Thanks again for your help!

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