Due to the number of requests, today I looked into adding flash capabilities to Duo - and it turns out it's a pretty simple affair. If you watched the video on separating the 105mm lens pairs, you'll see that the PC socket gets detached. Here, I reattach the PC socket and ground it to the lens to complete the flash circuit. Click through for the full write-up!
First, a brief overview. The PC socket works by triggering the flash when a circuit in the lens closes. For the Mamiya 105's, the center contact of the PC socket attaches to the contact in the lens, and the outer contact is the ground. The original Mamiya lens board is metal, so the socket is grounded to the lens by direct contact to the lens. However, Duo is all wood, so we need to add a ground contact to the lens. The lenses come with a small metal leaf-spring that contacts the conductive aperture ring on the base of the lens.
My apologies that I didn't take a lot of photos while soldering stuff together.
Here we've resoldered the PC socket to the wire from the shutter. I've also soldered a wire between the contact spring and the outer ring of the PC socket.
Here we can see the outer and inner contacts: inner contact to the lens wire, and outer contact to the ground wire.
The leaf spring gets glued to the lens board, so that it will contact the aperture ring. I glued a small piece of hard wire to the PC socket to fix it to the front standard.
Then we screw the lens back in. Piece of cake!
For anybody wondering what the spring is doing attached to the shutter cocking lever, it's just some extra encouragement to open the shutter when it's fired - my particular shutter is a little gummy.
Now to test the circuit. When the shutter is closed, the multimeter registers an open circuit. . .
. . . and when we open the shutter up to "B", the multimeter sees that the circuit is closed, and it emits a tone.
Success! Somehow, I don't own a female-female PC cord, so I had to kludge together some wiring to hook it up to my wireless flash triggers (the el cheap-o eBay type), to fire a 285HV. Works like a charm, though.