I realize this is an old thread, but my YS01 had a similar problem. I bought it from a Goodwill store. It looked like new but didn’t charge when batteries installed. Tried bending the contacts on battery cover but that didn’t help. With battery cover on but not locked, I found that if I pushed down on the lower left part of the cover looking at the strobe from the back, the capacitor charged, red light came on flash and focus light worked. When I released the pressure, the light cut out and the strobe didn’t work.
Looking inside the battery compartment,
At the bottom and to the left there was a negative contact with a positive contact below it. The positive contact had a raised plastic rim around it. I wondered if that was somehow impeding contact with battery. I found that a 3/8-inch nut driver fit over the inside edge of the ring, And by applying a little pressure twisting the nut driver left and right I was able to ‘burnish’ the ring. After putting batteries back in the strobe works. I suspect that the ridge was a little too high in preventing contact, and after burnishing it was no longer in the way. This is only a guess, but in any case the strobe now works
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