Mamiya 7II: Broken Plastic Battery Cover Caps
Originally posted on Instagram. Notes from the repair bench, kept here as a searchable archive. View the original on Instagram
I have had two Mamiya 7II models come into the shop this week with broken plastic battery covers so it looks like time to revisit this topic.
Although the battery cover itself is metal, the cap for removing the cover is plastic that has been pressed into place and once this cracks, the cover can no longer be removed as the plastic top just spins around. Photo two shows how the battery cover looked on arrival. After taking off the cover, photo three shows the plastic top with the metal base. The only way to take off the battery cover at this point is with a spanner wrench, not that convenient.
From the beginning of production in April 1999 until May 2000, the battery cover was all metal. By May 2000, it was changed to a design with a plastic top. This was because Mamiya was concerned about battery drain from the metal top to the metal strap lug. On the Mamiya 7II, the strap lugs were unpainted metal and if a persons hand contacted the strap lug and the unpainted metal battery cover, a small current could flow and cause battery drain. While this is possible, it’s not that likely. Photo four shows the location of the strap lug with the new metal cover installed.
Returning to photo one, I show a quick test setup that I made to measure battery drain. In this case, I put an electrode on my wrist with conducting gel while touching the strap lug. I held another contact to the metal battery cover with the same hand (this would actually flow more current than just making contact with my hand). Running this through a sensitive picoammeter, current flow was only 0.271 micro amps. While a sweaty hand would decrease resistance, I think you would still be below Mamiya specs of <1 micro amp drain. And, your hand would have to contact both the strap lug and battery box, not that easy of a feat.
