I've just been trying to work out an odd problem with the dynamo on my 1961 A10 (positive earth, 6 volt system with solid state regulator and small gel battery).
Initially I had no charge showing on the bike's ammeter (this may not have been surprising as I'd crossed the field and dynamo wires - note to self, green fields). However, having sorted this out, I tried flashing the dynamo field from the live (-ve) battery terminal, checked that the dynamo was rotating, and measured the resistance to earth of field and dynamo connections (both a few ohms.- so the brushes and internal connections should be sound).
Then I tried the time-honoured test of connecting field and dynamo cables together (regulator out of circuit) and checking the voltage across the joined cables and earth. Nothing (well very little). In a fit of exasperation I tried connecting the field direct to the live side of the battery - and I had a healthy 12 volts and rising from the dynamo cable. But when I try the 2 wires connected together again, still nothing.
Surely my test connecting the battery direct indicates that the dynamo is working, but why nothing from dynamo and field connected? The only thing that comes to mind is that the battery is a low Ah model, but surely this is irrelevant when out of circuit? And it does seem to excite the field coil adequately.
All suggestions welcome.