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
2 months ago