I have loved this old thing I picked up at a thrift shop 20 or more years ago.
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Mine was dying, with the timer timing out earlier and earlier over time. I have fixed it by replacing the two big radial capacitors on the circuit board, one 1000uF and one 2200uF, both 16V. Apparently power ripple was causing extraneous counts.Just take it off the wall, remove 4 small screws on the back, then carefully flex the top of the back away from the front. The screws will resist popping off as the case edge catches a bit on the screw tabs, then pop off. pay attention to polarity as you replace the caps.
A similar fix would probably work for the unit with the separate timer controls too (Superstar RF).