The mains transformer was replaced and initially that appeared to fix the problem but after a short period of testing it was clear that the transformer would fail again. After further investigation and testing the bridge rectifier circuit on the PCB had to replaced.
So now i have a cassette deck to play all those old tapes and i should now be able to connect up to the ZX81 and load and save some programs. It was getting the ZX81 and the I/O board going that got me thinking about repairing the cassette in the first place. Click here for further information.