Ours is probably some 20 years old initial MIT Scheme implementation added with homegrown features like "Object Oriented Scheme" etc., but a debugger was never implemented for the same. Talks have been going on to move to an implementation that has a debugger, but at such a scale changing that would be difficult.
By MIT scheme, do you mean MIT/GNU scheme? A few schemes implement their debuggers in terms of call/cc (or delimited counterparts) and exceptional conditions like errors or breaks hand control to a new 'depth' of REPL where anything in the environment can be inspected (or redefined) in-context before possibly trying to continue or doing any unwinding. If your scheme has no debugging apparatus, it shouldn't be too hard to implement something a little more useful than sprinkling in print statements. Even without wading into continuations, implementing a macro that does tracing should be possible.
Ouch... That seems painful and really like someone made a wrong decision 20 years ago. Back then scheme was a pretty inane choice compared to CL. That is not as much the case today, regardless of what the CLers say :)
