It could be a timing chain issue. Hopefully not. You could make sure it is turning by grabbing the rotor while someone is cranking. If the rotor still turns your chain is intact, but not necessarily in time.
Do you feel compression when you crank the engine. If there is a problem there the engine will turn very fast while cranking because of low pressure.
Your issue is definitely close to being figured out since you have determined spark is going to but not out of the distributor.

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Chris
84 280sl
82 300d euro