04-02-2020, 08:55 PM
If you have delved will already have read that essential reading is the Cornwall Club site "A Charging Refresher" and Dorset Club ARR/ARQ diagram by John Cornforth. It is common for the internal resistor to be open circuit, either burned open or intentional, possibly the stray wire. (I suspect the diagram is slightly wrong, the resistance connected to all the field coils in series, not after the first as drawn). Note that on the 4 pole dynamos the main brushes are somewhat unexpectedly at right angles. Care is necessary not to constrict and load the bearings end ways.