Bandido question

[mykaitch]

Well guys (Guru), here is an odd one. I have been testing my homebuild Bandido. Everything is to the original design except I needed a 40M from Pin7-IC6b to switch side of R40 (effectively across the Auto switch). Without this nothing works. Coil is my own build. Air tests > 8". So to trials. Excellent (disc mode fine). The Q: in all metal off I go with a low tone in my ears. When I detect metal it goes silent, I mean so accurately that the silence can be used as the location signal. Hit re-tune and thats what you get but a small movement then gives a different kind of beep until back over the object. So for me it is great! So then I fitted a commercial coil. Make unknown, half the diam of mine. Ditto above but much reduced range (as expected), so my coil is ok then although the second coil was easier to ground bal than mine.
Any ideas why it is working this way ? I have actually buzzed every connection and all the op-amps are connected correctly. Suppose though I am wrong, what might cause this ? I find it very odd.

Thanks you all.

[mykaitch]

Well Mike, the problem is seemples : ground balance.
Yep, setting ground balance can cause this effect, but taking time over GB and it
will behave as it should.