I bought my series II from a guy who had it (and 2 others) built and hadn't played them for at least 7 years, at least not much. The DS-5R had never been used and the stereo/mono switch wouldn't shift to mono, regardless of the switch position. Mica told me about the "exercising regimen" so I tried it 25, then 50, then 100 cycles with no success. Deciding I had nothing to lose, every day when I walked by the rack, I'd flip the switch mono to stereo and back, 100 times. Took a couple of minutes each cycle.
On the 12th cycle of this (yes, 1200 times), the switch began to work as designed and has done so ever since (13 years). Don't give up when exercising a switch!
I still preventively exercise it every 6 months or so, since now it's in a stereo rack setup with two F-1Xs and a SF-2.