Okay, now re-done. I was having issues with a dodgy patch-lead which I've now replaced with a known good one. Attaching direct to "INT" this time, with the switch still set to "INT" I'm seeing 10Mhz stable on the frequency counter and? what looks to be a pure sine wave of about 1.2V P-P on the 50 ohm scope input. Can't fault that IMHO.
My scope for carrying out any more tests this evening is very limited, but I'll have a lot more time tomorrow if anyone has any follow-up tests I should try. I don't think there's any point now in injecting an ext. 10Mhz in the light of the foregoing, quite honestly, since unless that signal level is too low, it should do the job I'd have thought.
Jinxie