-21 volts on TP5? TP5 is the ground return for the main filter capacitor to the transformer board (J3 - 19) so should be zero volts. It maybe as simple as cracked solder joints on the header pins on your sound board or transformer board, or broken wire/connection between your sound board and J3 pin 19 on the transformer board, so id say check that first.
hope this helps...
Cheers...Mitch
