Casio PT-1 Help

[stripyhorse]

As this is my first post, I'd just like to say thanks to all on here for the help over time - I've learned a lot from this website  .

My problem is that I recently bought a Casio PT-1 listed as faulty thinking that it would be an easy fix, because the listing said it was only working intermittently because the batteries had been left in too long. I thought this would mean it was just leakage, but I've cleaned the terminals and all I can get is hiss. This is the same regardless of whether I use the DC input or the battery compartment, and the audio output is the same. I have checked diodes/fuses. The power light and hiss only start when the power switch is set to 'play' and I press the 'reset' button, whereas I think it should begin working as soon as the 'play' option is selected.

Any ideas? If not it can always go back on the bay for spares... Thanks.