IMO, using JOAT to reduce the untrained modifier is in a way too powerful, as it gives a plaer with JOAT 4 the equivalent to all skills at 0, while at the same time being useless if you already have the skill concerned.
So, an engineer 3 JOAT 2 character is exactly the same that an engineer 3 w/o JOAT, and having JOAT4 ot JOAT 6 its exactly the same.
Imagining the situation, I think the character with JOAT 2 whould be better suited for fixing the drives (I don't think JOAT whould be useful for routine tasks as powering up PP, or starting JD), most so if there are no spare parts and have to be improvised.
I see JOAT as the capality to find unorthodox solutions where orthodox ones are not aplicable, and, IMO that is better represented by the retry options.
I think most of us whould agree McGiver has at least JOAT 4. Even so, IIRC he never tried anything he's not trained on (like piloting an aircraft or engaging on surgery), he just made the best use of the resources he had, even when other didn't see those resources there.
Of course, IMO, any reapair made by using retries allowed by JOAT hould be considered temporal repairs, and more so as more retries are used (if the character has JOAT 2+), as I see more retries used as more unorthodox the solution found.
Also, IMO, retries must be restricted to tasks where this resurcefulness may be aplicable (most instant tasks are NA, as are all combat rolls), even if the rules say 'any task failed'.