The aquired RVI tachometer did not work, so a repair was necessary. The circuit board was in a fairly bad shape, but fortunately the board from the aquired RVP meter could be swapped. Apparently, Smiths used the same circuit boards for several models. The two transistors were unidentifiable (not the standard GETs), so they were exchanged with two AC 128 (NTE158 is difficult to source in Europe). The major capacitors and a few resistors were also changed and, voila, a working unit.

A note on bezels. The old, nice full-V bezels are no longer available for 3″ instruments, so care must be taken to save the old one which is not always possible. Apparently, these bezels are avaiable for 2″ and 4″ instruments so maybe there is just so much more demand for 3″. One can only hope that someone starts to produce them again, and this time slotted so assembly is easy.

Matching tachometer and speedometer

While this speedometer is basically contemporary Smiths, the dial is probably not (the calibration information is missing). The instrument showed the correct mph, but the odometer counted twice the number of miles. Encouraged by Anthony Rhodes “Repairing Jaeger & Smiths Speedometers” (Google if you want it) we decided to open the box to see if we could mend it. Here we found Anthony Rhodes’ description invaluable when we had to refit the small springs and spacers in the right sequence.

Odometer wheels disassembled

The worm gear (20 teeth) and the drive gear (48 teeth) should give a calibration of 960 revs per mile, but we measured half of that on the odometer. We soon discovered that the outermost wheel (at the arrow on the picture, referred to as the “1/10th wheel” by Rhodes) had two “cogs” insted of one. This in turn caused the “one mile wheel” to turn a digit for every half revolution of the “1/10th wheel”, effectively halfing the calibration. This was corrected by filing off one of the “cogs”. The now 960 rpm calibration is low by 6% which we find acceptable.

Coincidentally, we had another (newer Smiths) speedometer which exhibited exactly the same fault, counting double miles. In this case the cause was a different one. We noticed that the pawl advanced the driving gear by two teeth instead of one, effectively halfing the number of teeth on the gear.

Pawl and odometer driving gear

The pawl movement is accomplished by a worm gear driving an eccentric axle. It was evident that the pawl movement was too large. Hence, we found an axle with less eccentricity which solved the problem.

Axles with different eccentricities.