Way back in early August on a ride back to Manchester at night I was worried the see that the stand-light on my B&M Lumotec Lyt, the front lamp on my Brompton had stopped working. The back stand-light on my Brompton had been non-functional for a while too. I couldn’t really justify the expense of replacing either of these lights, especially after such a short time in service. Thankfully, I have some experience of electronics from sixth form college and had also done some work with battery-backed full-wave rectification circuits previously. The fact that the stand-light had gone on both lights, but both still worked when the bike was in motion suggested to me that the capacitor used to power the stand-light is connected in parallel with the rectified current from the dynamo and in both cases had become disconnected. The rattle was a bit of a giveaway too. I firstly too a look inside the Lumotec Lyt.
The casing of the Lyt can be gently prized apart with the edge of a screwdriver.
The business part of the Lyt, the circuit board, containing the central LED, with the £1 coin-sized capacitor next to it.
After examining the circuit board, I found the part of it to which the capacitor attached. When assembled, the capacitor sits in this position at the top right of the circuit board which sits vertically in the housing. The relatively heavy capacitor is supported at this angle by two contacts soldered in to the board in a manner which is not really ideal for applications with a lot of bumping and vibration, such as in bicycles (especially on the Brompton).
One of the contacts had broken off with the capacitor, a wire was soldered to this end,
and soldered into the circuit board at the other end.
A contact for the side of the capacitor whose original contact was still attached to the circuit board was fashioned from part of the tab from a drinks can,
and placed in contact with the capacitor using tape. The remaining end of the wire pair was soldered to the contact on the circuit board,
and the whole thing was taped (crudely) to the inside of the housing of the Lyt. Whilst hardly a professional job, the result has held just fine for over two months. The situation with the rear light, a Brompton dynamo light (made by Spanninga) was similar, although the capacitor was positioned more sensibly in this design.
The capacitor as intended to be positioned on the circuit board of the Brompton rear light was re-affixed in a similarly crude manner to the capacitor in the Lyt; using solder and a lot of tape. This repair job has also held up surprisingly well since the beginning of August.
The long-term plan is to retire both of these lights to the Yuba Mundo, which is both used less frequently than the Brompton and which, with larger wheels and more voluminous tyres will likely be less demanding on these damaged lights, and replace the front lamp with a Cyo and a new Brompton rear light which I will pre-emptively reinforce. Until I have the funds for that though, I’ll have to make do & mend.
The standlight feature on my front Lumotec (a different model to yours) also stopped working (years ago) while the main bulb continued working. I've never done anything about it, but now you have shed some light on the possible cause, I may do so. Thanks.
ReplyDeleteDavid
Vole o'Speed
Hmm.
ReplyDeleteAn article not exactly designed to make cycling appealing to those who don't cycle.
One minute you're telling everyone that cyclimg can be done by anyone without special clothes and the next you are telling them how to solder capacitors to repair a broken bike lamp.
Come on!
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ReplyDeleteInstead of what appears to be masking tape, I recommend the use of silicone sealant, as might be used around baths and windows. While it isn't meant for this purpose and there are reasons why it isn't perfect for this use, many DIY-ers may well have some silicone-based bath or window sealant under the stairs etc., but the cost of buying purpose-made electronics-grade silicone RTV sealant would be hard to justify and probably be a significant percentage of the cost of a new light.
ReplyDeleteUnlike the masking tape, which is vulnerable to abrasion and liable to loss of adhesion, silicone rubber remains flexible, is robust, weather resistant and ideal for resisting vibration levels such as found on bicycles. The long-term reliability of non-external grades of silicone rubber may be affected by exposure to UV - which rather depends upon where you store your bike.
@David,
ReplyDeleteI can imagine that the basic circuitry is similar on most of the B&M front lamps. Good luck with your exploratory surgery.
@Pete,
Variety is the spice of life! I know that most people will have no desire to attempt to repair a broken lamp (especially if it wasn't that expensive). However, if it happened to me, it'll have happened to someone else too (as Mr Arditti's experience shows). My hope is that anyone who has had the same problem (and wishes to pursue a similar resolution) can find some help here.
@amoeba,
I had considered something like that, it seems like a good idea. The main reason I used the tape is because it was what I happened to have to hand at the time. If (when) I have to revisit this repair, I'll see if I can acquire a bit of silicone sealant instead. The UV issue would be fairly minimal for the Brompton (it has a fairly cushy life really).
Reckon the lamp you have repaired with wire will last the course - it should take the stress out of the soldered joints without them holding the weight of the capacitor.
ReplyDeleteRe: the tape - there's a type used in Antenna installations & suchlike that seals but like silicone never sets known as Self-Amalgamating Tape for another alternative.