So, having installed not-quite-exact replacement tail/brake and rear turn signal bulbs, I took the scooter on a little 2-mile ride last night: the new bulbs (and low beam headlight) all survived! Hooray, at least so far.
The former importer (Bajaj doesn't make scooters any more) reports they haven't got any rectifier/regulator units in stock and didn't sound hopeful of getting more, so that could get interesting. Hoping a Stella (LML, another India-built scooter) or Vespa replacement will do if needed. Trying to reverse-engineer one could get interesting, despite it being a straightforward-looking four-terminal device: vehicular electrics tend to be very specifically engineered for the application and "generic electronics" approaches can run the unwary into some interesting gotchas.
T. R. MCELROY'S STREAMLINED TELEGRAPH KEYS
11 months ago