Vintage Synth Repair

I just got a DX7 (first version), and it plays no sound.

I first checked the battey - and indeed it was low. I replaced it with 
a new 3v lithium, preset banks has been reset as expexted. 

I tried playing some patches from the ROM cartridge (which didn't 
suppose to be affected by the battery change), and programming some new 
patches, but no sound it heard on neither. Anyhow, I can hear the hiss 
coming from the amplification circuit... 

does anyone have any idea where should I look at ?
Is that a common problem on the DX7 ?

Thanks, can't wait to make it work ;)