[RodB]

The schematic is very fuzzy but it appears the short bare wire to the 5Y3 is attached to C23, the red wires are attached to C24 and the yellow wire from the 6K6 is C16. If you look closely at the electrolytic pins where they exit the capacitor there is a symbol next to two of them. The square is C23 and the triangle is C24. Like Mike said, the negative for all three is the electrolytic case which appears to be soldered to the chassis. When installing the new caps be sure there are no connections to the old electrolytic + terminals. (I hope you're getting rid of that paper wax cap just north of the slide switch. )