[SteveCG]

When you have got the mechanical aspects sorted out to your satisfaction then don't forget that the paper capacitors in the tape recorder almost certainly need replacing as they will be electrically leaky. Check especially those that are connected to the record/replay head as (if my memory serves me correctly) they can result in DC passing through the head. And this will slowly erase any of your recordings when you replay them. I would suggest you search for a circuit diagram of the 4A on the web to check.