The Imperial Valet is probably the grandest of the old "Built like a battleship" controllers. That being said, once it stops working, it will probably cost more to put back into service than it will cost to replace it with an entry-level solid-state controller.
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If it no longer starts automatically, it may also not be keeping the correct time of day. That points at the need for a new motor (one of three motors used in the controller) and those are expensive motors by now. Imperial is gone, and no one is stocking repair parts for them at the moment.
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Only if your zone valves are Imperial brass valves, do you need the power output of the Imperial Valet (over twice what standard solid-state controllers can supply)