Can be that someone was in the service manual. It's a shame too since the running advice is to never get into that since there is no document about this. I've heard folk that buy the service gripe the service menu is not documented.

As to the tapping like 1986 that points to board, panel or connection problems. Have your TV tech unplug, clean and plug in all connections. If it's a TAB BOND FAILURE, that on the panel is usually fatal.