RVI Tach Refurb / Mod - Ron’s 70 S2 Crossflow
My 1970 Europa uses a RVI tachometer where the primary coil current goes through the tach. Surprise, it doesn’t work after 25 years of sitting. Here is what I did.
It is completely inoperative and I needed to upgrade it to use a modern electronic ignition system that does not use points. Originally the RVI tach was connected in series with the points with the points current activating the tach.
The RVI tach schematic is shown in last pic.
This is what I ended up doing.
• Disassemble and inspect. Mine was not to bad but clearly needed some clean up and had a bad capacitor and corrosion on the internal circuit card.
• I soda blasted the exterior and painted with Rustoleum flat black. (protect the various places where there are electrical connections)
• Clean the entire circuit board with contact cleaner and a small non-metallic brush.
• Replace 2µF capacitor (it is very unlikely that any 2µF from that era is still good). I used:
https://www.digikey.com/en/products/detail/tdk-electronics-inc/B32523Q1225K000/3490025 • Remove variable resistor (calibration adjustment with yellow disc), spray with contact cleaner and rotate to clean the resistive element. Clean and gently burnish the mounting areas. Check with an ohm meter, re-install.
• Add one small drop of very light oil to top and bottom pivots, remove any excess. (I used a small wooden stick to do this.) Check for smooth easy movement.
This got me to a nicely working standard RVI tach.
Since the case was already open I added the following modification to convert it to a tach that would operate with a 5V digital pulse from an electronic ignition.
• Replace the “points winding” (the heavy 3 turn wire on the coil) with 32 turns of AWG 22 varnish coated magnet wire.
• Add the following circuit using the parts shown on the schematic.
• (next to last pic)
• Bench test using a square wave generator. RPM = Frequency(HZ) * 30. You can download software to generate this signal using the audio output of your laptop. Lot’s of options come up after a Google search.
