[Pecal]

I seem to have hijacked this thread - sorry TL.
Technical stuff really gets the forum fired up and it's great to hear all the experience and knowledge.
Greg, I'll take you up on the offer to hook a CRO up to measure the signal, then I have a basis to work from - speak to you next meeting.
Thanks everyone for your input, it seems that this problem isn't as mysterious as it seems.

Peter