The OP already said he can do the ‘down’ part of the dips in a controlled manner but it’s the ‘up’ part he’s having trouble with, so I don’t think controlled eccentrics will help him much.
If I was you madquarker I would concentrate on replicating the movement as closely as possible. If your gym has a dip/chin assist machine, by all means use it, but remember you are trying to work up to using no assist and eventually weighted dips. If your gym doesn’t have an assist machine, than you might ask your workout partner(or just someone in the gym who you wouldn’t mind asking) to give you a bit of help with the ‘up’ part of the movement.
Nothing wrong with the suggestions already mentioned, just my two cents.