Getting into debugger on Apollo machines

Before I go insane, can someone tell me the magic key sequence used to get into the mnemonic debugger at startup on Apollo machines? (Specifically a 425t, but I expect it's the same sequence on all flavours) I managed it - once, after much random tapping, and thought it was a simple double return that did it. Made the mistake of rebooting so I could move the machine to a more useful position, then discovered that it *wasn't* a double return. Nothing in the manuals (install / owner's guide, anyway) saying how to do it - they only mention how to do it from a machine that's already booted into the OS. The machine's sitting there trying to load across the network when it can't find an OS on local disk, which I don't want it to do - I've put an empty disk in there *because* I want to install an OS on it from local tape. There must be some way to interrupt it in order to get into the debugger. Grrr! (I could pull the token-ring board, but I have a feeling it's trying to boot across the onboard Ethernet anyway, not token-ring, so it wouldn't help) Maybe if I leave it long enough (like, more than the 20 mins I've given it already) it'll time out... cheers Jules