I know that Dale Manquen used to repair these. Here's the TrainFacts Home Page, he talks about these repairs there.
From the home page:
Introducing a new service – Component-level repair of TMCC Controllers
If any of your customer’s TMCC controllers fail, now you can offer your customer a solution! Lionel won’t fix or exchange out-of-warranty devices, and you certainly don’t want to troubleshoot down to the component level. Now you can recommend that your customer contact MANCO for repairs.
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