I think I've found the problem with my RL02 drive. Tracing back through the fault detector logic, the Clock Error Detector output is set. Backing up to the entrance, the 4.1 MHz system clock is coming in correctly on the two 75107 line receiver differential input pins (E61 1 and 2) but no clock is coming out of it... no clock -> no operation -> Fault light on :) Hope they aren't unobtainium parts! Meanwhile Tim Radde also has pointed out something else important I'd overlooked, namely that the pack cover has to be put upside down over the pack (when installed) in order to activate the cartridge interlock switch. (I had somehow missed this line in the instruction manual despite what I thought was a careful reading of the text) :P so it would have been a problem sooner or later anyway. -Charles