K75S Tachometer re-assembly issue - K-Bikes.com - Excellence in Motion
 
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post #1 of 4 (permalink) Old Apr 23rd, 2011, 12:24 am Thread Starter
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Question K75S Tachometer re-assembly issue

Pulled cluster on my 1990 K75s to re-lamp but when I went to re-assemble it the tach needle is WAY below the "0" rpm line on the face. I've moved it (gently) to the stop at 9,500 rpm but every time I release it goes back down to under 0. I really don't want to lug it to a dealer and have them say "shoulda brought it here first, idiot". Ideas, suggestions HIGHLY welcomed !!
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post #2 of 4 (permalink) Old Apr 23rd, 2011, 5:03 am
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I only know about the tacho on the K12's but there may be some similarities.

Is the tacho electronic with wires, or mechanical with a cable?

If the tacho is electronic, it may have a deliberate physical offset below the zero mark which is set correct when the unit is powered.

In both cases the tacho is a delicate 'moving coil' meter built like a large clock. If you are sure that the tacho does normally read at zero with the key off then this is what you have to do:

First check you have not bent the needle and it is touching the front glass. Take it out and make sure the pointer swings freely - use a wooden toothpick to move it.

Then you need to remember if it does sit at zero with the key off, or is biased past zero.

Locate the delicate mechanism attached to the pointer. You will see two hair springs top and bottom like in a mechanical clock. First check you didn't get a coil or two overlapped, you can carefully separate them with the toothpick but don't put in a kink. Also check with a magnifying glass that both ends of the hairsprings are actually attached.

One hairspring sends the pointer anti-clockwise and the opposite identical spring sends the pointer clockwise. Working normally, the springs are set for balance which may be zero or an offset bias which is corrected by the electronics.

On one of the springs on its outer fixing point there is usually a 'tab' which can be rotated. This sets the tension on that spring so the pointer can be zeroed. It may be marked or sealed with a dab of paint.

When you have checked or made corrections you should really check the tacho readings are accurate at different rpms, it's easier with an electronic unit than cable driven. But if you are a zero or redline monkey it probably doesn't matter!

Here endeth your lesson on fixing clocks!

Vox



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Last edited by voxmagna; Apr 23rd, 2011 at 5:10 am.
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post #3 of 4 (permalink) Old Apr 23rd, 2011, 6:42 pm Thread Starter
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Hi Vox Thanks for response !! The tach is electric and the only springs I see (w/o completely dis-assembling the unit) are 2 wound ribbon style that go on the needle shaft and are soldered to pins that come down off the board. There a plastic "gear" on the shaft, with the smaller coil mounted above it and a larger one mounted below. Both ends where attached to board pins are intact. I think I'm going to look for another tach, used ones easy to come by and I'm not confident about further dis-assembly locating the problem since I don't really know how it works.

Actually removed needle and re-attached with the pendulum swung to the "0" position so I guess it's fixed (hopefully) !! Thanks again Vox !!

Last edited by MEJETSKI2000; Apr 24th, 2011 at 6:24 am.
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post #4 of 4 (permalink) Old Apr 25th, 2011, 9:41 am
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Those ribbons will be the electrical connections to the moving coil, so the hair springs I'm thinking about would be another level in.

It makes sense they would put some kind of protective cylinder around the pivot because like a clock, it would be quite delicate.



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