Try your multimeter on AC volts across each of the main smoothing caps?
A lot of meters (but not all) will ignore the DC component and allow you to see any AC ripple.
Also try across C1 and C2, the 15V smoothing?
It does look to be more a missing ground somewhere though, as connecting to the effects send should not do anything much, just reduce gain slightly.
With power off, try a jack plug in each socket one at a time & check continuity from the plug body/screen contact back to the 0V (white) on the two main caps?
Also check any separate metalwork such as the springline case - everything should show a dead short, or less than one or two ohms at least.