M645 PD(S?) Meter Problems

[Cholentpot]

Hi,

I have a Mamiya M645 with a PD(S?) Prism. It seems to be reading a stop off. I get proper readings for ISO 100 when I have it set to 400 and for 400 @ 800 and so on and so forth. Is this a weak battery issue? Age? Is there some sort of exposure compensation that I don't know about? The prism does have some blackline separation and quite a bit of oxidization, would this change the way the meter reads?

Thanks.

I'm not sure if the prism is a PD or PDS. It has the LEDs and a white dot on the side to turn on. It has shutter speeds from 8s to 500.

 

[mweintraub]

"I'm not sure if the prism is a PD or PDS."

If the speeds go up to 1/1000s it's the PDS made for the 1000s

 

[Cholentpot]

Thanks,

I guess it's a PD then. Any clue why the reading is off? Is this common?

 

[mweintraub]

Could be due to age. I forgot what type of batteries it needs, but maybe it's a voltage thing. Not sure.

 

[Cholentpot]

6v

I've got a Duracell PX28L from my AE-1 sitting in there. The green batt check lights up ok.

I guess if it's accurate on a different setting I can live with it.

 

[MattKing]

I wonder if the current capacity for the lithium version of that battery results in a different reading then the silver oxide or alkaline versions that the meter would have been designed for?

 

[macfred]

Here is a manual from mamiyaleaf.com - page 35 up to 38 is about using the PD prism http://www.mamiyaleaf.com/assets/files/documentation/M645_1000S_v4.pdf

 

[anikin]

I had the same issue in mine. I believe it is a design defect of the prism. It has two photovoltaic sensors on sides of the viewfinder. These sensors are mechanically clamped in their position. With time one or both sensors may crack from clamping pressure. If both crack, you get no measurement, if only one cracks, you are off by one stop. That's what happened in mine. I replaced the cracked sensor and the prism started reporting exposure much closer to reality.

Good luck!

Eugene.

 

[Cholentpot]

Thanks Gene,

Can you walk me through where to find the sensors? How did you replace it? Is it worth doing or should I just live with the sensor being one stop off?

So. Many. Questions!

 

[anikin]

Hi Cholentpot,

I'm so sorry for not getting back to you sooner. I've been digging through files on my laptop and just could not find the data. However, today I turned to google and found my own posting on APUG about this issue

(there was a url link here which no longer exists)

I really hope this helps. Unfortunately I have recently sold this camera, and it's been several years since I repaired it, so I simply don't remember anything beyond the fact that I managed to fix it. The posting above mentions the type of photodiode I used. I distinctly remember having to sand the sides of it to make sure it fits into the available space. I hope this helps.

Eugene.