The Accutron was originally designed to operate on a 1.35V mercuric oxide cell, banned decades ago over health and environmental concerns. The modern substitute is the 1.55V silver oxide cell, available in many of the common mercury cell sizes, including those required for all calibers of Accutron movement. However, the higher voltage of the silver oxide chemistry can be a problem for some movements, increasing the tuning fork amplitude beyond tolerance, and causing the index finger to advance the index wheel by three teeth per oscillation instead of just one. This is misnomically referred to as double indexing, a problem particularly noted with the 214 movement. The preferred method of correction is to introduce a voltage regulator into the original Accutron circuit, which lowers the cell output to 1.35V. Some Accutron technicians can install a voltage regulator directly into the Accutron's on-board circuitry, but the prevailing option is to install a specially modified power cell with an integral voltage regulator.
ACCUCELL-1 power cell for Accutron 214 and 218. Although the manufacturer claims 1.35V performance, the cell pictured here tested at 1.30V on both an Accutron Test Set meter and a HP digital voltmeter.
Blue Diamond power cell for Accutron 214. Blue Diamond also claims 1.35V performance, but the cell pictured here tested at 1.45V.
Replacing the tuning fork or coil assembly may also eliminate a double indexing problem, but extended use of the higher voltage silver oxide cell may shorten the life of the drive circuit over time; the 1.55V cell is suspected in many instances of coil failure. Please refer to the table below for replacement cell types.
Tuning Fork Freq. (Hz)
HgO Cell (1.35V)
AgO Cell (1.55V)
214H, 214HN, 2141, 2142, 2143
218D, 2180, 2180F, 2180G, 2181, 2181F, 2181G, 2182, 2182F, 2182G, 2183, 2183F, 2183G, 2185, 2186, 218S, 218SC; Citizen 3701A, 3701B, 3702, 3710, 3721. 3722; Universal 51, 52, 53, 54
2191.10, 2192.10, 2193.10; Citizen 3700, 3701, 3711
2240, 2241, 2242
2300A, 2301, 2301A, 2302, 2302A, 2303, 2303A, 2310, 2312, 2313, Universal 47