I've been struggling with this problem now for a week - the Ultimarc Mini-PAC that I was using for MAME in Windows works with my Tempest spinner, but it doesn't work in Linux. Andy said he doesn't know why it doesn't work - and didn't have much else to say about it - but in the RetroPie forum, I've read others having a similar problem with this configuration.
Do any of you have an original Tempest spinner running in RetroPie? If so, what encoder are you using?
It's not a RetroPi/RetroArch config issue - it's recognized by the OS as a "mouse" but no signal is detected when spun - as you would expect from a "mouse".
Tx