Hi Guys
I think the answer is Clear & Present - as Harrison Ford once said !
However, to answer the questions .. ..
1. The issue of the unwanted effects of the PSU came up in an earlier post, related to current limit not the earthing potential, but as a result I have been powering the circuit from a 9v battery for a while.
2. I hadn't realised how critical the tolerance of the resistors is to the function of the circuit. AK is talking about an over budget situation based on 1% tolerance resistors. The resistor components I'm using for this circuit are cheap chinese imports and I've just got one out of a packet marked 10k @ 1% and it meters at 9332 which is almost 7% .. .. .. I'm not just over budget, more up the cut in a punt !!
Time to abandon the project in favour of the alternative in post #5 .. you are quite right about the small signal transformer, but I'll look into that .. ..
As a matter of course, I would not ever consider disconnecting a mains earth from anything .. .. for two reasons ... firstly, if the manufacturers put it there I've no doubt they have good reason; also, this 'scope is a brand new gift from my two sons .. ... there would be hell to pay if I made such an alteration and did some damage to either the instrument or myself in the process .. .. .. secondly, and much more important to me .. ... 30 years ago, my business partner was killed in a substation in Warwickshire because some metal thieves had removed busbar effectively disabling the safety earth device .. no chance!
Thanks for all the information and help; AK particularly grateful for your detailed explanation, makes a lot more sense of it for me .. ..
I'll be in touch when I can't get the other one to work !!
S