Vintage Synth Repair

Hello.  I just purchased a CZ-1 only to discover that it has a very annoying problem.  I was 
only able to try it out with headphones - which emitted a loud, intermittent crackling sound 
that diminished in frequency (fewer occurrences of the crackling and steadier output) the 
longer the synth was on. The crackle is accompanied by near loss in volume of both crackle 
and the intended output signal.  I thought at the time that there was something wrong with 
the headphone output or jack, but when I got home, I discovered that the exact same thing 
happens via the line out jack.  After playing around with the unit, I noticed that hitting the 
keys hard often triggers a very loud instance of the crackle followed by a period of non-
crackly, full-volumed output.  Has anyone else had this issue?  I can imagine (and I know 
very very little about electronics) that the sound I am hearing is more like that of a dying 
capacitor or faulty resistor than of a bad jack (I put some Deoxit DL-100 in the jacks to no 
avail).  If anyone can point  me in the right direction of some components to test and possibly 
a schematic to accompany, I would be very thankful.

Thanks in advance!