Hi guys,
Thanks so much for your help. I shall go through the suggestions carefully however I can give an update as to what I already discovered. Firstly on the S4 (with the poor erase) I had a jammed bias pot and therefore had to take out the PS and free up the pot. Whilst I had it out I decided to swap the power supply units to see what happens and bingo... the S4 erased a tape fully
. So my suspicions that it’s a faulty erase head have been eliminated. There are two capacitors on the unit that are 4000pf and 10000pf which I suspect could be leaky? I did not change these caps as they are not the type that usually get replaced. My focus is now on these caps as I had changed the other leaky caps.
I shall report back as I go along.
Thanks again guys