What is the replacement Photodiode for M645 PD prism?

Discussion in 'Medium Format Cameras and Accessories' started by anikin, Mar 10, 2014.

  1. anikin

    anikin Subscriber

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    What is the replacement Photodiode for Mamiya 645 PD prism?

    I'm trying to repair my newly acquired Mamiya 645 1000s camera with PD prism. The prism was showing rather erratic
    measurements which I traced down to one cracked photodiode in the prism. I'd like to replace the photodiode, but I don't see any markings on it. It is a small black rectangle about 6x8mm with two golden wires sticking out of the back. Does anyone know what would be a good replacement for this photodiode, and/or maybe have a spare on hand? Also, does anyone know which one of the 5 pots in the prism should be used for calibration? From the circuit, I can guess it's one of the two near the top of the prism that feeds reference voltage into the op-amp. However, I'd rather not touch them until I'm sure.

    Thank you!

    Eugene.
    P.S. Yes, I know, I can buy another prism for not a lot of money. However I hate to throw away a perfectly repairable unit, at least not until I totally destroy it in my repair attempts :smile:
     
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  2. wombat2go

    wombat2go Subscriber

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  3. anikin

    anikin Subscriber

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    Thank you for the link. However, I'm not sure what I should be looking at on this page. The photodiode I'm looking for is similar to FDS10X10, but much smaller. Mouser has a few that look similar, but they all are larger than the one I'm looking
    for.
    I wish I knew the component name for it, then at least I could start looking for a replacement with similar wavelength/sensitivity characteristics.
     
  4. anikin

    anikin Subscriber

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    Ok. Since nobody wanted to tell me, I'll reply to my own question. I could not find the exact replacement, but I found a very similar diode at alliedelec.com. It is VTB8440BH ceramic photodiode. It is just a bit taller than the ones inside of the prism. I had to shorten it a bit by sanding off the rounded ends. After that, the diodes fit perfectly. And most importantly, the meter now reads precise exposure, and I did not even need to recalibrate it! Success!

    Eugene.
     
  5. wombat2go

    wombat2go Subscriber

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    Well done.