[Keith Seymore]

Originally Posted by 68 DANA

Keith......you are correct......thanks for the post

Mercury batteries (two 1.35 volt cells in series = 2 .7 v) were used in the earlier Sun tachs. The mercury style batteries provided excellent constant voltage regulation over their operating life span. As a result the mercury batteries will maintain 1.35v potential until approx. 5% of their life, where the voltage will quickly decline. AA Alkaline battery technology doesn’t have the same constant voltage regulation curve. The batteries have an initial open circuit voltage of 1.5v and initially drop approx. 6.6% - 13.3% (1.4v-1.3v) then continue to decrease over their life span.

Decreasing an alkaline battery from the initial 1.5v to 1.35v before using them in the Sun tach will work however due to their poor constant voltage regulation the Sun tach’s needle deflection, representing the RPM, will not be as accurate as if it had been used with an original mercury type battery.

If accuracy is not an issue then problem solved. If accuracy is an issue then it can be corrected by the use of a simple voltage regulator circuit; 9 volt battery, resistor and a 2.7v Zener diode.

The mercury battery was Sun part number 1766-7 and was made by Mallory, commercial part number RM12R and the Eveready part number was N0. E12N.

I am restoring to period correctness and therefore prefer not to modify but use the original technology of the day. I reached out to the company that Lee mentioned above. Hopefully they have access to the Duracell RM12R battery.

Thank you for the additional detail. I was not aware of the accuracy issue; the needle moved appropriately and I never thought to check beyond that.

Fortunately the tach does not have to be very accurate for driving on and off the show field...

Let us know what you find out.

K