I have a schematic I can't send over, but I'll tell you it won't help you.
It uses a custom-ordered servo chip and a few passives. The chip is DIP8
with no heatsink.

You may have better luck hunting for datasheets of said chips (they come
with application schematics). National had the 'original' integrated
drivers, at least 15 years ago. Try an old National special purpose
catalog. They do describe the internals a little afair.

There is nothing they can do that a 12C508A can't do, except the power
driver. Normal servo chips have H bridges that can run 1A at 4V supply
with little losses (<1W).

Peter

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