Dear David,
Board tapping is a good method in finding an intermittent connection sometimes. I have done this with some success in the past. You might help narrow down the area using a narrow insulated “tapper” instead of a finger, like (for example) a chopstick. Then you can nudge various components individually or board areas until you find it.?
I think you are getting close.
73,
Gregg W7GRM
