Thanks for all the help,
I changed the Diode and it got rid of the battery warning message but there was a problem with the sound cutting out so I looked on the board and there was an inductor which had been leaking and was cracked. I changed it and the sound is now fine.
In the past I have just swapped the pcb for one that works but fixing is much better as the boards are nearly 30 years old and getting hard to find. Luckily for me I have kept the old boards.
These inductors seem to leak a lot, I have seen this on most of the power supplies on this type of keyboard but most work fine so I have not given it much thought.
Question.
Should I replace them if I see leakage or should I leave them alone?
Oh So wrong again.....
After I posted I pulled the little green thing apart to see what's inside and I thought that nothing could leak. on closer inspection I think one end of the connection to the board had broke and was causing in intermittent on off of the sound. The inductor is surrounded by capacitors 220uf 25v so I assume that the capacitors have leaked. they are still working and the leakage is rock solid so should I leave well alone or replace?