Thanks Edward and Michael for your replies.
Michael, I too thought it a very odd thing and I too thought the Tape Transport board was equally involved in all three speeds. I can confirm that none of the speed indicator lights go out or even flicker, but the problem is most definitely there, very plain to hear.
I'll check the tape lifter though everything mechanical seemed to be fine on a quick but careful inspection, plus I would expect that something like that would produce at any rate some similar symptoms in all three speeds.
I do indeed have the manuals and schematics: I'll sit down and have a proper look - from memory I thought there were one or two components on the speed side of things that are unique to each of the three speeds, so I'm wondering if one of those to do with 7.5 might have drifted. The speed control side of this model is - to me - fairly complex in detail, as I recall from when I was restoring the machine.
Thank you for the offer of boards: I think I have one of them as a spare too - either the Tape Transport or the capstan, but it had its own faults and was put in a box for future salvage - might have to dig it out...
Well, at least it's interesting to find a fault that is a little out of the ordinary
