Vintage Synth Repair

Hi just wondering if anyone has some advice/tips for me.

I have what appears to be a fully working DR-55 except the drum sounds
are double-triggering. most noticeable is the kick which obviously
flams, making it sound very farty. 

I don't have a scope but i've recorded it to my laptop and can see the
double peak. 

Anyone have any idea what it could be? I can mail relevant service
manual pages if that helps. 

Thanks