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From: Zen Cycle <funkmaster@hotmail.com>
Newsgroups: rec.bicycles.tech
Subject: Re: For those who believe in electricity
Date: Mon, 7 Apr 2025 11:16:02 -0400
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On 4/6/2025 10:22 PM, Frank Krygowski wrote:
> I'm not an electronics guy. But speaking of problems, I think I 
> mentioned a few days ago that on a recent night ride, my headlight (B&M 
> Eyc N plus),  powered by my good old sidewall generator, was randomly 
> and sporadically blinking off. I put the bike on the workstand, checked 
> out the wiring (it’s original and pristine), and spent some time 
> spinning the front wheel. Sometimes the headlight worked, sometimes not, 
> and it changed state at random without having been touched. I was 
> suspicious of the tiny on/off switch hidden behind a rubber plug, in 
> part because I felt no satisfying click.
> 
> I substituted a different (lower quality) LED dynamo light, which seemed 
> to work perfectly and probably absolved the generator. I eventually 
> talked by phone to Peter White, who sold me the light far longer ago 
> than I remembered - ten years! I asked if he had clues on diagnosing the 
> problem. He did not, but suggested that the identical light without the 
> possible failure point of an  on/off switch is just $35. (A sidewall 
> dyno doesn't need a switch.) Still, I opted to open the light to see if 
> I could spot anything because I have a compulsion about fixing things - 
> but a headlight has to be really, really reliable.
> 
> Here are photos of the electronic guts, with a vernier caliper for size 
> (and to educate Tom!).
> 
> Bottom view. The downward facing LED is at the top of the photo, the 
> suspicious switch is the tiny white box at the photo’s bottom. https:// 
> www.flickr.com/photos/16972296@N08/54435279894/in/dateposted-public/
> 
> Top of the circuit board. The large copper pad is part of the heat sink 
> system, contacting a metal casting that forms the top of the headlamp 
> body. I’ll probably add some heat conducting goop when I reassemble:
> https://www.flickr.com/photos/16972296@N08/54435279589/in/dateposted- 
> public/
> 
> Although a bike generator or dynamo puts out AC, I had hoped to diagnose 
> it using DC, figuring a 6 volt battery would essentially supply half of 
> the sine wave so half of the input circuit. That naturally works with 
> incandescents, and it worked with one Avenir LED dyno headlight that I 
> repaired. Not so in this case. So I ran another generator using a roller 
> chucked in my drill press, attached a digital voltmeter in parallel, and 
> poked around best I could. I saw nothing loose, no scorched components, 
> nothing obviously wrong.
> 
> With no power attached, I tried checking for continuity across the 
> switch. It always showed close to zero ohms, never infinity - but the 
> resistance readings with pinpoint probes were jumpy, and since the 
> switch wasn’t isolated from the circuit, that may not mean much.
> 
> I used contact cleaner to blast out the switch and repeatedly clicked 
> it, including while power was applied. Except for a few occasions, the 
> light did not respond to the switch, so I still suspect the switch. When 
> the light was off, I had about 12 VDC. When on, about 4 VDC.
> 
> It’s still not fixed, but it's getting late here. I would try to simply 
> short out the switch, but it’s very difficult to identify and reach its 
> microscopic solder pads and I’m no good at micro soldering.  I think my 
> best bet will be to mechanically remove the switch, crushing and cutting 
> it as necessary, hoping that it will yield access to solder points that 
> I can bridge with a short. If that cures it, fine. And I’ll probably buy 
> that other light anyway.
> 
> BTW, Peter White is not pleased about the tariff situation. He says he 
> won’t raise prices on current stock, but with the next shipment he’ll 
> probably have to.
> 

One possibility is open/"cold" solder connection somewhere. These 
assemblies are built with what is known as a 'reflow' soldering process 
- essentially, the PCB has a solder paste applied through a process 
similar to silk-screen, then the parts a put on the board using a "pick 
and place" machine, then it's run through a hot oven which melts the 
solder paste and hopefully wets the solder to both the component and the 
pad under it.

Sometimes, it doesn't quite flow right and ends up with a "cold" 
connection. The connection may work for some time, over time will 
oxidize such that it opens the connection. Normal cleaning techniques 
seldom work to fix this, it usually takes reheating the offending 
connection to reflow the solder, and some type of flux is highly 
recommended.

One way to diagnose this is to apply power to the circuit and press on 
the components, though this sometimes doesn't work either. If it does, a 
light touch with a soldering iron and on the component connections 
_usually_ works.

Someone with good understanding of soldering techniques might be able to 
see a cold solder joint under a microscope (10X usually suffices).

I recently bought a new blinky light for my commuter and the switch was 
intermittent out of the box. A cold solder joint on the switch contacts 
was the culprit.

-- 
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