A few months ago, I noticed that the light would only come on while I was moving. A month or so after that, it stopped working altogether. The front light still worked (although it had separate problems later), so I knew that the dynamo was supplying power. I used a Knoc Frog (wrapped around the seatpost) as a temporary substitute, and that was ok while I was mostly cycling in daylight. However, I've now replaced it. This was a bit of a fiddly process, so the info may be useful to someone else.
I decided to order a battery powered lamp instead of a new dynamo version. This matches what I've done with my touring bike: it should make the rear light more reliable, and also make more power available for the front light. Looking on the Brompton website, I needed part QVBATRLAM (£17.50 at SJS). This doesn't include the bracket to attach the lamp to the bike (part QVBKTRLAM-R if you have a rack, £6.75 at SJS Cycles) but I can confirm that I didn't need that: the brackets for the dynamo lamp worked fine with the battery lamp.
Here's the original lamp attached to my rear rack:
You can just about see the brackets that attach the lamp to the rack. There's also a black power cable connected to the back of the lamp, which winds around the rack and then goes towards the front wheel.
It's easier to see how the lamp is attached to the rack when the bike is folded (i.e. the rack is upside down):
The hexagonal nuts are 8mm across, so a 5/16" spanner is approximately the right size:
The main snag was that I couldn't turn the spanner very far. There was only a short arc between the mudguard and the rack, so I had to do a small turn then take the spanner off the nut, move it around and then put it back on the nut again. After I'd removed the first nut, the lamp started to slide around, so I used my spare hand to "pin" the nut in place while I unscrewed it.
Anyway, I got both nuts off, but the lamp was still attached to the power cable:
Since I won't be using this lamp again, I considered just cutting through the wires, but I wanted to find out how to do it properly. I suspected that the wires would terminate inside (like a power plug that goes into a wall socket) so I used a small Phillips screwdriver to take the front off the lamp. (I had to push quite hard to avoid stripping the threads, possibly because it's been exposed to the elements so much; this wasn't a problem with the new lamp.)
When I got it open, I saw a circuit board inside. There's also a small object which looks like a watch battery, but it was just rattling around loose inside. I assume that it's supposed to be soldered to the circuit board, so this probably explains why the lamp turned off every time I stopped. That also means that I may have been able to repair this lamp rather than replacing it, but I've never done any soldering and I'm not really confident of my abilities there.
Meanwhile, the back of the lamp was still attached to the power cable:
It turned out that all I had to do was tug the cable, then the power plug on the end slid out:
I wonder whether this cable was loose; it didn't feel like it, but that's one possible explanation for why the lamp suddenly stopped working altogether. Anyway, these may be useful troubleshooting steps for anyone else with a "broken" lamp.
As a side note, I obviously don't go through all these steps every time I lock the bike up somewhere. If I had quick release lights then I'd take them off to avoid someone stealing them, but I don't think any thieves are likely to spend this much time fiddling around with a spanner.
Coming back to the bike frame, the dynamo power cable was now redundant. I could have left it in place, since it wasn't really in the way, but it's neater to remove it. So, I started with the loose end attached to the plug:
I unwound that from the rack. After that, the cable went inside the chain stay:
I've included a close-up view so that you can see where the cable is visible behind the gear chain indicator (the rusty brown chain). Digressing again, I apologise for the amount of dirt in these photos; the bike is overdue for a proper clean.
At the other end of the chain stay, the cable comes out behind the chainring (the big cog wheel next to the pedals). I've taken photos from both sides of the bike to make this a bit clearer:
There are 3 cables there: the top 2 are for the rear brakes/gears, and the bottom cable is for dynamo power. Looking at the size of the hole where the dynamo cable emerges, I didn't think that I'd be able to tug it through, i.e. the plug on the end would be too big to fit.
Looking towards the front of the bike, the dynamo cable is tied to the other 2, then it passes between the forks and the mudguard to get to the (missing) front lamp:
I'll come back to the front lamp in a future post; suffice it to say that the fixing broke, then it dangled for a while until it finally fell off. (This was the cumulative effect of cycling along bumpy roads.) In this case, that's actually an advantage, because I didn't have to work out how to unplug the rear cable. So, I just pulled it through.
Looking at the section of cable where it passed through the forks, the black outer covering has come away:
The 2 inner cables still seem to have their plastic insulation intact, so I don't think that would cause any problems, but it's probably not a good thing either.
The next step was to cut through each of the cable ties, so that I could separate the dynamo cable from the brake/gear cables. I couldn't fit scissors inside, so I used a Stanley knife for this. Since I didn't need the dynamo cable anymore, I cut towards it. That way, if I misjudged what I was doing and cut too far then I'd only damage the obsolete cable. As it turned out, I was able to stop at the right time, but it doesn't hurt to play it safe.
This left the front half of the cable hanging loose next to the chainring, so I was able to pull it through the chain stay (towards the rear) and remove it from the bike altogether:
Comparing the old and new lamps, they look very similar from the outside:
The only differences are that the new lamp has a power button on top and the old lamp has a socket on the back for the dynamo cable.
Looking inside the new lamp, it has space for 2 AAA batteries, which came pre-installed:
It should be possible to change these batteries while the lamp is attached to the bike, just by unscrewing the front half.
So, the final step was to attach the new lamp to the bike:
I positioned the lamp so that it's slightly above the mudguard (rather than resting on it). I think it's best to do this bit while the bike is unfolded, but that did mean that the screws were sloping downwards slightly. That meant that it was slightly fiddly to get the nuts on (working against gravity) but when they were both finger tight they stayed put and I used the spanner to do them up properly.
So, mission accomplished! I'm gradually learning more about bike maintenance, as I replace one component at a time.