Might well be Mark - I did it a couple of weeks ago to remove a business card which had slipped down the side of the modulation wheel and I sensed that I could only gingerly lift the upper part so far off the lower part. I guess I should have had a peek underneath whilst I was at it but once I'd retrieved the business card I was only too glad to screw the upper part back down and check that everything was still working...Perhaps the service manual above has more to say...