Mixed success with the Gamers Choiceᵀᴹ HP 54615B, the replacement monitor IC arrived today, it didn't cure the fault though, although after a while of trying to get the H-hold to work it suddenly locked.
Great I thought and put the cover back on only for the problem it to reappear.
Again removed the cover, tried to find out the problem and it suddenly came good again, this time it continued to work after sliding the cover back on (no screws fitted).
As least I could now use it, I gave it a check with the HP sweeper, seems to be working on both inputs up to 500MHz.
Oddly the built in game works without the cartridge installed.
David