I'm not sure if any of those who has experience of those mills are still around this site.
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The Yasnac control could pretty much be described as an antique, however if it still works and meets your requirements, then I wouldn't go rushing out to replace it. I would however plan for doing a retrofit at some point.
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What kind of toolchanger is it? One arm bandit aka Summit? If it is, I do have a manual, although it is for an early design, but the same basic movements apply.
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Have you tried having a search over on the practical machinist forums?
If you can ignore all the idiots who think anything more than 10 year old or non-big name is rubbish, there are some helpful guys who know about older machines.