Tachometer calibration

[Lotus 50]

My tach reads too high. Can it be calibrated? Is there a company in the US or Canada (preferably) that could do this?

[Andy8421]

The Smiths Instruments electronic tachs were widely used in British sportscars of the era, and as a result there is a lot of information on the web about them.

The electronics are extremely basic, and use electrolytic capacitors which dry out over time, leading to the calibration wandering off. The calibration is easily adjustable, but if the electronics are too far gone, you won't be able to trim it back.

Just google 'recalibrating Smiths tachs' or similar - there are some good MG owners club documents, and some very comprehensive youtube videos about how to do this. Recalibration requires no electronic skills to speak of, replacing the innards needs basic soldering iron skills, but not much else.

If you don't fancy having a go yourself, these guys will rebuild and recalibrate the tach for you:

https://www.speedycables.com/

Edit: Not the US I am afraid, perhaps others can help with a firm 'over there'

[pharriso]

Hollywood speedo have a good reputation - https://www.nhspeedometer.com/

[stugilmour]

I had mine done at Nisonger in New York, but I believe they stopped offering repair and reconditioning of the vintage Smiths stuff. North Hollywood Speedo looks like the go-to now our side of the pond. Get the entire tach upgraded would be my suggestion.

[Lotus 50]

Thanks

[Mike Ostrov]

As Andy has mentioned, many of the early RVI electronic tachometer components can fail, giving erratic readings.

If your tach readings are linear, you can attempt some re calibration. Inside the case is a potentiometer or simply a variable resister. You must remove the external case, drill a 1/4 inch hole directly over the slotted end of the potentiomenter in the case, assemble the case and now you can with a small slotted screwdriver, attempt to adjust the tach, at 3000 rpm with a known external tachometer. Use small adjustment movements.

Have a bit of fun. If the adjustments are not stable, the tach should be examined for failed components. Must confess, I send my tachs out to an electronic wizard for this work.

One indication the internals are failing is, on a very hot day, the tach will read in a linear movement, much higher than normal. ie; normal idle at 1000 rpm and tach is reading 1300 + rpm.

Good luck and please report back your success. Cheers. Mike, in the SF Bay Area with mostly Type 14 Climax Elites and some Elan stuff, too.





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[Darek Brewin]

I upgraded my Tach using a Spiyda board. I also was able to do the calibration. Took apart the tach, desoldered the RVI board and installed the RVC.

https://spiyda.com/smiths-rvi-rvc-conversion-board.html

Darek (In Edmonton)

[billwill]

One of these (or similar pulse generator) from eBay will probably let you calibrate a tachometer throughout its whole range.

https://www.ebay.co.uk/itm/364174397061?




I think Spiyda tell you a simple circuit which can calibrate a tacho at 2 points using the mains frequency 50Hz in UK as reference.

[gjz30075]

Another option:
https://momamanufacturing.com/

[Lotus 50]

I hope to get the car out of heated winter storage tomorrow. Then I can change the oil and plugs and go from there.

Thanks for the leads. I will be outsourcing the tach repair. It sometimes bounces all over the place and then it will read normally (albeit high) for a while. I need to have it bouncing when I take it for re-calibration.