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.
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:
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.