Vintage Synth Repair

Hey everyone,

I'm new here and to electronics repair. I am taking some classes so I
have access to all kinds of great repair equipment and I think my next
project should be to get my Arp Odyssey (2600, the white one) fixed.

The problems are: 

none of the modulation to the filter works and the filter is really
scratchy. 

How do I find the broken paths to the filter and how would I use an
oscilloscope or multimeter to do that?

I wouldn't mind an easy answer, but I'd be just as happy for a little
learning experience as well. 

Thanks for any help,
Johnny

PS: I have the service manual as well, and I'm not sure how to use it.