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Zundapp KS601 - Tail Light Refurbishment

   With all the work that had to be done on the KS601, why ever would I choose the tail light as a starting point?

I didn't know the etiquette of a road run at a rally, but I knew that a lot of the folks there, while truly loving their machines, were not highly polished riders. With the bunch I ride with, no brake light would be a point of concern, but not a show stopper. With less experienced people in a large group, and me being unfamiliar with the Zundapp to boot, I felt no brake light really meant no ride, even in the unlikely event their rules didn't stipulate that. So that's why it was first.

Here you see the tail light almost as found. The only thing you missed is how the socket was artfully strapped to the mount by electrical tape.

Zundapp KS601's did not have a brake light from the factory, only a single filament tail light with a single contact socket to match. So it appears some previous owner decided to remedy this shortcoming and removed the original socket, replacing it with this one which is correct for a dual filament bulb. That, apparently, is where that project came to a rest, after the socket was taped to the mount.

Another issue, in my view, is no reflector. I don't see any provisions for one, so I am assuming there never was one. Six volt lighting is marginal at best, and for a six volt brake light to be effective it really seemed important to me that as much of the meager light as possible be directed toward the idiot behind me, not bouncing around inside the tail light housing.

The Zündapp KS601 tail light as found.
The Zundapp KS601 tail light as found. Note no reflector.
The Process

The Zundapp KS601 tail light socket and bulb as found.    The Zundapp KS601 tail light socket and bulb as found.
The socket, borrowed from some other light assembly, had one tab missing from the insulator. The contact had nothing to hold it in place or insulate it from the spring.

The bulb is an 1154 six volt dual filament. These have the same base as an 1157 but are typically only a bit north of 5 watts for the tail light filament and 16 watts for the brake light. In comparison, a standard 12 volt 1157 puts out around 8 watts for the tail light and a substantially higher 27 watts for the brake.

Another potential candidate, although a bit harder to come across, is a 7517 which puts out 5 watts running and 21 watts braking, quite a bit closer to the 1157.

I generally have a few circuit boards lying around waiting to get the components stripped off them for reuse. I think in the future I'm going to make sure I keep a few for projects such as this one.

This board is really nice dense material and perfect to make the replacement insulator out of. Usually circuit boards this color are cheaper material, but this one was pretty high quality stuff.

I thought it would be tricky to drill the holes for the contacts without chipping out the board. Planned on using a high speed drill, but the stuff cut and shaped so nice I just used my 2500 RPM 1/4" Milwaukee.

Cutting out the new Zundapp KS601 tail light insulator.    Shaping the Zundapp KS601 tail light insulator.
The new insulator cut out of the donor circuit board. Getting the shape just right.

The insulator all nicely shaped and fitting just right in the socket.    Insulator ready for contacts and wires.
Here's the insulator all nicely shaped and fitting just perfect in the socket. Now, I need to drill those holes.

The missing tab is evident on the original insulator.
The finished insulator with holes drilled. Note the tin plated brass rivets to be used as contacts. They have an ID drilled most of their length to pocket the wires into for soldering.

The spring has the end covered with high quality heat shrink tubing to insulate the bottom from the wires.

Contacts in place with wires soldered.    Insulator finished and ready to go in socket.
Contacts in place and wires soldered.

Insulator assembled and ready to go in the socket.

Socket welded to mount.    Socket assembled and attached to mount.
Mounting tab welded to socket. I'm afraid my TIG skills were getting a bit rusty at that point. They've since made a substantial recovery, not quite to that of the days of old, but getting there.

Here's the finished socket and mount assembly with just a puff of paint on the socket bracket to keep it from rusting. Note the filaments are oriented for max light to strike the lens.

Socket and mount fitted in tail light housing.    Roughing in the reflector.
A trial fit of socket and mount assembly to verify the bulb and filaments are in the best position to get max light out the lens.

Ok, time to make the reflector. Easier to start with a chunk of poster board to get the initial size and shape, then duplicate in a piece of aluminum flashing.

Finished reflector, polished to a nice shine.    Reflector mounted on socket.
Here's the finished reflector after a bit of effort, shaped to click onto the socket and mount and polished to a nice shiny surface.

The reflector clipped onto the socket and mount, no fasteners required. The flashing is quite springy, so it's very firmly attached with no fear of it vibrating off in use.

Finished reflector, polished to a nice shine.    Checking the fit in the KS601 tail light housing.
Another look at the reflector from the back. Fits rather nicely, if I may say so.

A view of the finished assembly tucked into the KS601's tail light housing. Maybe could have extended the reflector a tad more toward the bottom, but it looks pretty good all in all.

The Zundapp KS601 has a removable rear fender to facilitate pulling the rear wheel off. You get some impression of that in the image at the top of this page. If you squint you can see, just above the tail light mount, a brown phenolic contact block. This acts as a connector for the tail light, mating with another similar half on the larger chunk of fender that remains on the bike.

The problem is that it is a single contact. I've considered layering another above it to carry the brake light connection, or making a thinner two contact version out of phenolic. Either idea would look a bit bulky. Also, I have decided I want a dedicated ground to the tail light socket, so either of these ideas would be pretty impractical looks wise. I think I will probably just run the tail light wire through the original connector, then do as neat a job as I can of running the brake light and ground wires parallel to it, with their own connectors to allow removal of the fender. We'll see...

The Results
Well. After all that it's time to see how it works, particularly if the time used making the reflector was well spent. The four comparison pictures to the right, below, were all taken with the camera and the power supply at exactly the same settings to facilitate determining relative brightness, so forgive the over exposures. I was intentionally metering off the lens and ignoring everything else.

Brightness of the Zündapp KS601 tail light alone with no reflector.
The test set up, with my Tenma 1-15V, 30 amp power supply. Very useful for automotive and motorcycle stuff.
Brightness of tail light alone with no reflector.    Brightness of tail light alone with reflector.
Tail light alone, no reflector.

Tail light alone,
with reflector.

Brightness of brake light alone with no reflector.    Brightness of brake light alone with reflector.
Brake light alone, no reflector.

Brake light alone,
with reflector.

I have to say it was well worth it. The running light is marginally improved, but the brake light is distinctly brighter. I tried to keep the camera angle similar to the angle a driver following the bike would be viewing the tail light from. Note that it was actually a little higher on the picture of the brake light with the reflector installed, so I think it's really a bit brighter than it looks here, relative to the no-reflector image.

Conclusion - definite improvement.

The astute will notice the meter is indicating 5 volts. Got this power supply for $10 from a garage sale. The meter was pretty beat up and the fan was stuck, so I took the supply apart, lubed the fan, freed and straightened the meter's needles, and replaced the bezel. The meter is slightly non linear, but no means to linearize it other than adding some electronics, so I say good enuf. I set it to be close to nominal at 10 volts, increasingly offset negative below that, and positive above. When the dial says 13.6, the output is right there, so I've got that for a reference. Just need to put a sticker on it. It regulates right on the money, pure clean DC with 30 amps behind it if you need it, and the current indicates accurately.

Last updated 11-26-17
Email:  mechanique at wmol dot com

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