There are two excellent documents you should purchase if you are going to get that receiver back in working condition. They will have the answer to virtually any and all technical questions you will ever have regarding the SP-600.
First the military TM-11-851, available as a down load
AVAILABLE HERE as a down load at the bottom of the page, or you can get a print copy, which I highly recommend, on Ebay. The second is "The Anthology Of Technical References For The Hammarlund SP 600 And Its Variants". These two documents will make you a virtual expert by the time you've finished reading them.
I recommend you put the radio back to "stock" condition, then proceed with incorporating those modifications you consider necessary.
HERE ARE PICS of my refurb process from a couple years ago.