After a number of months I had finally got my HH working. It has Pascal's board and bumper boards and I've replaced coils and switches, done the earth modification, replaced plastics etc etc and even got the up kicker to work reliably.
...but sometimes when I started a game it immediately assumes the ball is in the basement. Now the flippers & coils top and bottom have stopped working. The game goes into lockdown (Pascal's board) and the game ends but the main playfield flippers still work. A message says to check switch 44.
I suspect it's something to do with the K relay but I'm (still) a novice at this and hope someone else can suggest what I should check first?