I mentioned in post #8 that I'd try to track down a small 100uA or 50uA edgewise meter to replace the 200uA meter that I used, but that quest came to nothing.
Not only that, neither could I find any small 'SEW' type meters (about 40mm square) that used to abound. The smallest panel meters now seem to be 62mm wide x 55mm tall. Furthermore, the tuning cap I'd used shorted out its vanes when fully closed and I couldn't discover where the short was, so I needed to fit a replacement tuning cap, which meant a new front panel calibrated to suit that cap was called for. I found a suitable tuning cap in my spares box, which ranged from 35 - 550 pF in my spare box.
As a wider and taller front panel was needed to accommodate the larger meter, I scrapped the original test unit and built another one (and yet another comb-jointed box!). I sourced a 50uA from e-bay (£4.23 'buy it now' post free) - and while I was at it, I thought it would be useful to add a 'scope socket on the front panel so that the null can be observed on the scope as well as the meter. Not in any way necessary, but for the sake of a slight alteration to the PCB layout and an extra socket on the front panel, it was no great effort.
The more sensitive meter means that the unit will work well on a signal generator with low output, and with the meter at full scale deflection the null is very pronounced and unambiguous.
I appreciate that it's not the sort of instrument than many home-brewers would have at the top of their list of projects to build, so I realise that it will be of limited interest, but nevertheless, I've attached the updated circuit, PCB artwork and component overlay, and some pics of the 'MK2' version.
A point worth mentioning should anyone did want to built such a unit, is that the tuning cap must be 'floating'. (IE, it must not be grounded). Initially, I'd put it on a metal bracket attached to the front panel, which is made from scrap double sided PCB, then when it wouldn't work, the penny dropped that I'd ground the tuning cap. I therefore fitted it on insulated stand off pillars and all was well.
The front panel was printed on coloured card, encapsulated, then a clear protective panel of 2mm acrylic was cut, drilled and fitted in front of the printed panel.
A lot of fun (for me, anyway) for a negligible outlay. All my projects are strictly 'MFJ' - 'made from junk'!
Hope that's of interest.