Yepp, the 5v comes from J5 pin 30 just as you said. Most likely will be the connector itsself or dry jointed pins on the sound board if you haven't already soldered them.
Most of the problems with this board are directly related to that connector. Best to repin or replace it 
Yep, had a similar problem with my Trident once, wiggle the connector and it would come good, pins need resoldering and maybe connectors need replacing too.
