I found the old movement and realised that all the parts were still there.
So I took it apart again, and this time I made a few pics. Here goes:
'Before': Antimagnetic shield plate still in place, rotary switch visible in the lower left part:
Antimagnetic shield plate removed, circuit block visible:
Circuit block removed and flipped over, rotary switch hole and contact pads in the lower right part:
Movement without circuit block, circuit block spacer visible with rotary switch rotor (two-pronged thing in the lower left part):
Better view from the side:
The rotor sits on a metal part (pivot? axis? don't know the correct word):
...which is connected to the mainplate and thus to battery plus:
Detail view of rotary switch rotor and contact pad pattern:
You can even see where the switch rotor left its imprints.
I think setting the adjustment works by connecting none, one or two pads to battery plus through the rotor.
Here is an overlay picture of the adjustment markings and the contact pattern on the circuit block:
Lets name the upper two pads (connected to the same wire) 'A', the lower left pad 'B' and the lower right pad 'C'.
The marks on the antimagnetic shield plate indicate three steps in the plus direction and two steps in the minus direction.
Default position (pre-adjusted in the factory): no pads have contact to the rotor
Plus step one: pad A has contact to the rotor
Plus step two: pad A and pad B have contact to the rotor
Plus step three: pad B has contact to the rotor
Minus step one: pad A and pad C have contact to the rotor
Minus step two: pad C has contact to the rotor
With three digital bits (line A,B and C), one can represent 8 different states.
6 of 8 combinations are used here (all open, A, B, C, AB, AC). The two unused combinations are BC and ABC:
- ABC: all three pads connected to the rotor (impossible with a two-pronged rotor)
- BC: pad B and pad C connected to the rotor
this would be possible with a slightly smaller pad size (7 positions instead of 6 on the semicircle),
but it seems Seiko thought that an adjustment range of -16s/month to +24s/month was sufficient
You can read more about the 7A28 movement in the Seiko Technical Guide, available here: