It is a strange one - I have an opportunity to try this on another game over the coming days. My opinion is that many ops removed the knocker, because of the noise, and the software may detect that there is no knocker - if it is not connected. Somehow, the circuit is not completed.
I assume that if you set the machine to "Factory Defaults" - it would cancel any setup option that is disabling the knocker ?
Hi Nino,
I reset the old ROM to Factory Defaults but this didn't fix the problem, and I assumed the new ROM would already be set to Factory Defaults. I will try resetting it tonight just in case, but I don't expect it will resolve this pain-in-the-arse issue 
Matthew
Maybe not, Matt - But it is a step closer. I'll have a go on an Indy I have over the next two days.
