I think my 1st reply got zapped..
Max, if you send me the 505 service manual scan, we will post it on
our site so others can easily grab it. we've got a bunch, but there's
still plenty we need. http://burnkit2600.com/gear/ page_id=48

Jesse, Before you've even got it open, you troubleshoot.
Are the 'shift' functions on the buttons in question working
Try to control the dead sounds from another device using midi- If they
work, then the problem is probably just in your buttons and an easy
fix & not a circuit malfunction. Alternatively, you can rule out
whether the buttons aren't working by trying to control another midi
device from the 505.

-Thom