Technobike.2 was the one where the electrics packed in. I took the front wheel apart (that's where the motor is) and let it dry out for a couple of days, in case it was water in the motor again. But that didn't do anything useful.
After a lot of sniffing around, I decided that it was probably the controller, so I took that apart, and was immediately greeted by the acrid smell of fried electronics. So I contacted Alienocean again, and bought a new controller from them, £38. It arrived, and I looked at it for a while, gathering up the courage to do the job.
By the way, I can really recommend Alienocean as a place to buy from - even though they don't sell 24 volt bikes now, they're still supporting customers who have one; I've bought two new batteries from them now, and this controller. Customer support is extremely important when you're buying something that isn't likely to be available from other sources. I mean, I can buy brake pads anywhere, but a new controller?
Today I set to. I unplugged the old controller from the bike, and set about measuring resistances. The motor is a three phase one (that's why it doesn't need brushes), so there are three main leads to it. I measured each pair, and got about 60 ohms, which is pretty low, although a short across my ohmmeter is 40 ohms, so it's probably less than 20. That's OK, I wouldn't expect much resistance, it's just a long copper wire. But it was good to know that the resistance wasn't zero, which would mean a short circuit in the wiring, and I'd have to replace that.
So I carried on and plugged in the new controller. There's three coloured leads to the motor, yellow, green and blue, one for each phase. And there's the same three colours at the controller. but you don't plug yellow-yellow, blue-blue and green-green, or at least that's the way Alienocean wired it, and why would I try anything different?
I switched on, and tried the throttle.
Oh shit, I thought.
Then I sat and had a think, checked that the connectors were tight, and noticed that one of the connectors had two wires on one side and three on the other. That can't be right, I thought, and after a bit more thinking, I decided that the new controller had slightly different connectors, even though it was exactly the same model. So I cut the relevant connector off the old controller and the new one, soldered the new one in place, and tried it again.
Hurrah! The bike works. With the front wheel running freely, at low speed adjustment, it will do 8 mph; at high speed, 18 mph.