NH 750 TACHOMETER CIRCUITRY

Model year 1991 through 1995 Honda Nighthawk CB-750 motorcycle tachometers tend to fail (erratic or no needle movement). A fellow rider sent me his broken tach from a 1993 model to examine and to attempt repair.

Side view of the inner case and bezel cut.

The "works" mounted on the rear of the meter movement.

The owner had opened the tachometer by using a Dremel tool on the device's bezel. Perhaps a better method would be to cut around the case about a quarter inch beneath the bezel. When the repair is completed, reassemble the case using epoxy. There is enough internal strength via four screws to hold the case physically together. The epoxy (or, perhaps, superglue) would be to waterproof this interior case.

There are two "mainsprings" in the tachometer's meter movement: one carries the signal output from the circuit board; the other carries the return (ground).

In this particular tachometer, the return (upper) mainspring had been "cold soldered" to the post and had opened completely.

In the photo you can see the positive mainspring clearly with the solder connection to its similar post. It is hard to see here, but the ground spring's arrangement is similar and very difficult to reach with a soldering iron.

Nevertheless, a successful repair was effected. From scratch this would be a two-hour job for a skilled technician. This device is worth around $120 retail. Yes, I know Honda asks — and gets — a lot more.

Inside inner case looking down.

If one elects to open this inner case by cutting around its circumference, then that cut needs to lie between the bottom of the bezel mounting lip and the top of the two vertical structural supports. This distance is one-half inch. The four tubes are light shields for the indicator lamps.

Side view of the bezel cut.

Rear view of inner case. The two outer posts hold the entire assembly in the tachometer's chrome housing. The two larger Phillips head screws mount the works to the inner case. The two smaller Phillips head screws connect the Y/G and G wires from the wiring harness.