Vintage Synth Repair

hello, i just bought a roland mks-30 and it has a problem, maybe 
someone could help?.. theres an intermittent crackling sound from the 
outputs (headphones too) it seems to phase in and out and is worse on 
some patches than others.. otherwise the sound from the oscillators is 
crisp and clear. the crackle seems to be gated by the envelope or at 
least it crackles when it recieves midi note on. ive reseated and 
checked all connectors inside. there is no obvious damaged components.
any ideas? 
thanks, gaB