That's what I meant. Tony was saying - and rightly so - that while you can often see what a failed mechanical part was meant to be like by studying the failed component, and can then cut a new gear (the assumption was the gear isn't a standard part) / lever / cam or whatever, you can't possibly tell what a dead chip was meant to do just by looking at it. Likewise you can't tell what the value of that charred resistor used to be... I merely pointed out that with some tight tolerance cams and levers, you have a similar problem - you can't tell by looking. "Difficult" in my line above was a euphemism for "no hope" True, every word. But if you draw the distinction there - digital vs. analogue circuitry - the comment to which I was replying deserves your reply as well. The previous poster (wasn't you, William, was it?) drew the distinction at a higher level, and said something like "a digital _machine_ either works or it doesn't. Apart from video, the results are either right or wrong. With analogue/mechanical, things can be slightly out but the machine will go on working" Video is another analogue subsystem in a digital machine, just like a disk drive, and all that that implies... BTW, video may not make the change from digital to analogue until actually in the monitor, but the same applies - that which you see on the display is (at the digital level) either right or wrong. It is only when you get to the analogue bit that it is merely fuzzy. Philip.