Tiger Tach Rebuild
Sept 27, 2000
Update: Nov 15, 2002
Motivation
The Tiger tachometer is a source of frustration for many Tiger owners. It may work, or not, or intermittently, and when it does, it is sensitive to temperature and other weather variables, and on the whole, it is not a gauge one would want to use to monitor critical engine functions. Others have reverse-engineered the Smiths design (see Mark Olson's description) to determine replacement components and (see the STOA Tech Tip) modified the tach to work with electronic ignition amplifiers, but the basic design deficiencies still remain if the circuit is kept as it was designed about 45 years ago.

The existing design uses a simple transformer to detect the current pulse that occurs every time the coil fires. The resulting signal is converted to a constant-width, constant-voltage pulse using a two-transistor circuit. The constant-width pulses then drive the meter movement; the closer together the pulses, the higher is the average voltage, and the greater the meter movement deflection. The downside of this very simple circuit lies mostly in the component choices and the fact that there is little compensation for component aging or temperature. Since I was planning to install an MSD ignition amplifier, I needed to modify the tach anyway, and I decided I might as well update the internals to at least the '70s. Interestingly enough, that didn't really simplify the internals of the tach...


Equipment
I already had a 7000 RPM tach from a series V alpine, which is internally identical to the Tiger tach. The Smiths meter movement has a mechanical stop at each end of the scale, which prevents one from using a stock Tiger tach on any high-performance engine that will exceed the tach's 5500 RPM capability. The Alpine tach is an easy way to get a 'high-performance' instrument that still matches the rest of the Tiger dash trim, as long as it is recalibrated for the V8 ignition. My calibration instrument was an oscilloscope (if you must know, a Tek 5101 analog flood-gun storage scope). Basic electronics tools are absolutely essential, as is some experience with PCB assembly and rework.


Design
The spark for this design came from a discussion I had over dinner with Gary Winblad, at SUNI III. He suggested using a 555 timer circuit to replace the tach guts. I decided to do that, and at the same time, provide an internal calibration source for the tach so that I wouldn't have to lug the scope around, or so that you could build the circuit board, determine the calibration value for the reference, and then anyone else could bolt that circuit board into their tach and do the calibration with no need for test equipment on their part. The other parts required were a power supply, and an input circuit to condition the pulses from the ignition amplifier or coil. I haven't yet designed an input circuit for the current-detector transformer that the stock tach used. The schematic is shown below:

The power supply works from the Tiger's 12V electrical system and provides a filtered 8 volts to the rest of the tach. The resistor and Transzorb on the input are intended to protect the regulator from voltage transients that exceed the regulator's capabilities. The calibration oscillator provides a 30% duty cycle output signal at a fixed frequency that corresponds to approximately 3500 RPM. The idea is that I will measure the frequency (or period) of this oscillator using the scope, determine what RPM that corresponds to, then tweak the tach driver adjustment until the tach reads the correct RPM. The ignition input basically consists of some filtering and protection so that the 555's trigger input is protected from the ignition circuit. The tach driver uses an adjustable resistor that gives a fairly wide adjustment range of the output pulse width to allow for unit to unit variability as well as tuning for 4, 6, or 8 cylinder engines. The 100 ohm resistor in series with the meter movement, combined with the pulse width set by the resistors and capacitor, together determine the meter calibration. I wanted the pulse duty cycle to be about 50% at 6000 RPM, so I juggled the output resistor and the 3.3K resistor value until I got there.

If you compare this to the Smiths design, you'll see that things definitely didn't get simpler...


Construction
I started by disassembling the tach (follow Mark Olson's procedure!) but I found that in order to remove the PCB, I had to remove the needle and face. I put a small stack of business cards between the face and the needle center, under one side of the needle shaft, then put one card on the other side, and used a screwdriver to wedge the needle off of the shaft. This took more force than I liked, but I didn't bend the shaft. Then I removed the face plate. One thing that was interesting is that the opening for the panel light is partially obscured by the printed circuit board. This tach had a large crack in the timing capacitor (dark red cylinder on the upper left).

Another view of the tach electronics, showing the meter movement. The two spiral springs transfer the coil current from the stationary contacts at the top to the coil which is mounted on the rotating shaft.

I measured the mounting hole locations and cutouts necessary to clear the meter movement, and made up a drawing on the computer. I measured the components that I was going to fit and spent a while juggling placements and wire routing options. After I got to a reasonable answer I printed the result and then cut out the pattern on a piece of prototyping board (one-sided copperclad and tin-plated). Since I don't like drilling lots of small holes I decided to use a poor-man's surface mount method, where you basically clip the leads off of anything that you mount to the board. The traces were cut using an X-acto knife, after marking all the component lead locations.

For this prototype I used primarily leaded parts (with the leads cut off as required) but a few surface mount parts did find their way onto the board. Going to full surface mount would reduce the clutter quite a bit but some surface mount parts are a little more fragile than the leaded components and I didn't want to be chasing a cracked resistor at some inopportune time. The aerial wiring is part of the calibration connection, which I removed once I'd reassembled everything and tweaked the calibration adjustment.

I needed to make some adjustments to the mounting hole locations but things went together pretty well. Note that the meter movement is a balanced assembly and until the needle is back on the shaft, the shaft position will be dependent on how you've got things tilted. This is especially important to keep in mind when you're reassembling the needle on the shaft. I left the current sensing transformer on the backplate PCB, and just soldered its wires (just visible on the lower right) to the main PCB so they wouldn't get in the way.
 

I drilled a 1/4" hole for a grommet to lead the tach input wire out. I did not make the calibration controls externally accessible but I will probably add that later.
 
 


What Next?
Even having made these changes I expect this tach to be accurate to no more than 100 RPM or so, based on the fact that repeated connections of the calibration signal would cause the needle to stop up to 100 RPM away from the calibration point (which was 3191 RPM in my case). The big difference is that I expect that the tach will be accurate at least over the normal temperature range in which I drive my Tiger, and that this accuracy will still be there five or ten years from now. The tach doesn't have very fast response, mainly due to the inertia of the meter movement, but this could be improved somewhat (at least on the acceleration side) by characterizing the mechanical inertia and the electrical inductance precisely, and then determining a more optimal control solution, so that response speed is maximized while minimizing overshoot, but this is not necessary for any but the most highly tuned engines, and in that case you'd be better off using a real tach anyway...

Update: Nov 15, 2002


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