The Tach and its secrets.....

[ViperRon]

Marty I know you aims to please but I heard that bullet go zing over many heads. You better go back and kiss the next one if you want get any credit on this one.

Kiss= Keep It Simple Stupid

Most problems with Tach meters are not the Tach they are from noise generated from loose connections coming from the alternator. When the engine is turning it generates AC current which is best described as a sign wave going from a positive to a negative what happens is with a bad connection you can loose signal completely for moment and the Tach may either receive what it believes as an extra transition from positive to negative or loose too many transitions and have a lesser count. The components in the Tach actually filter and develop a DC voltage level that drives the motor or needle to a point where it displays the amount of transitions received. Any wiring a bad regulator or a alternator going bad or intermittent short could cause issues. If the tach is really bad you will see where components have overheated or capacitors may actually come loose or leak leaving a nasty smell. The Tach is a calibrated instrument and can if dropped too hard move the mechanics out of place causing the needle to stick or drag so sometimes a little lube on the springs and shaft can free it up to much becomes a mess. If you are testing with a meter you may find online a basic electronics program to assist you in identifying components and how to test basic functionality.

[3Phase]

That would be my route, just replace the caps There's nothing else in there that's going to go stale. Except possibly the solder joints can get corroded.

One of the more likely failures is that the spindle and end bearings of the galvanometer armature are worn and dirty and only making intermittent contact. It can be cleaned and adjusted but it's seriously finicky. Yes, I've done it but I had to use three 'broken' tachometers to make one good tachometer and I broke or bent five out of six hair springs. The springs are not easy to repair. Well, actually, the springs are easy enough to repair but then the galvanometer has to be recalibrated with the repaired spring.

You can see the bottom of the tachometer in Don's picture with the lower insulated bearing and the Red wire. Inside where you can't see it, the armature spindle rests in the bearing with its hair spring and that's what gets worn and dirty and allows the the armature to 'settle' into the bearing.

In another of Don's pictures you can see the upper part of the tachometer and the hair spring that connects the Black wire to another hair spring and a non-insulated upper bearing. The Black wire carries return current from the armature.

As the bearings in the tachometer wear, the upper spindle of the galvanometer armature gradually starts to lose contact with the upper bearing and the armature starts to bounce around in its bearings. Eventually, the lower bearing gets loose and dirty too and the armature starts to jump around and make intermittent contact with the Red wire. That causes the tachometer needle to jump and bounce and 'hover' in one -- wrong -- place.

Only the upper hair spring is adjustable along with the small brass nut that is the upper bearing.

The upper hair spring sets the preload against armature rotation and makes sure it returns to zero.

The upper bearing nut is used to set the preload on the armature spindle and helps ensure the armature doesn't leap out of the lower bearing and lose contact with the Red wire.


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[BA80]

I think it's just 35 year old electrical components on the circuit board. Replace them with new ones and the problems will go away for the next 35 years.

The tach getting it's signal from the alternator pulses was a really bad idea in the first place. It would have been much simpler to just run a cable drive from the cams.

But then, you have to take into account that at the time 8 track tape decks were the state of the art and beta max vcr's were too.

[3Phase]

Go ahead and replace the components and reflow the solder joints while you have the tach opened up. The circuit will probably work without being 'scoped, bridged, balanced and trimmed, or at least it won't be so far out of kilter that it can't work. It just a tachometer, not a RADAR.

I've only worked on six XS1100 tachometers and one XJ1100 tachometer so far but only one of them had any bad electrical components and that was an obviously split and leaking electrolytic capacitor. The other five all had loose and worn armatures and/or broken hair springs.

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