So it appears that I've forgotten how to connect up a floppy drive. In my defence, I have not used a floppy disk in years... Fortunately it seems that the cable in my PDP-11/53 should be the standard type. My likely mistake was to rummage in my bin of "reclaimed" computer parts and try to use the cable I found in there. It does not have a twist in it :-( Typical.
Seriously. That might be the cause (or one of the causes) of the "Drive Error" message I was getting. Mind you, if my RAM chips have died (as I suspect) then I might get other error messages. I should expect dodgy memory to manifest itself in all kinds of ways I guess.
So after some reading on the interweb I have been reminded that the standard floppy cable has a twist in it. It looks hacky; but was the normal technique used to distinguish drive A: from the B: drive. Of course the cable I had in my box of leftovers was flat - a non-standard cable. I expect that it won't work in my PDP.
On a standard twisted cable the connectors after the twist are usually for drive A: whereas the connectors on the untwisted section are for the B: drive..
The other thing that I remembered is that 5.25" drives normally had a "card edge" connector, rather than a couple of rows of pins like you find on 3.5" drives. I had forgotten that too. So when buying floppy cables, if you want a "standard" one you need both card edge and the 34 pin female connectors, with a twist in the cable.
Maplin's still sell these cables, so I've now got the correct one (I think). Hopefully I will be able to correctly connect my RQDX3 disk controller to my RX33 disk drive now. Watch this space...