AVI-8 Hawker Hunter Solar AV-4039 (add more in the comments below…)
The Seiko caliber VS42 (actually marked VS42A) is a solar-powered movement with a solar cell underneath the dial which converts light energy into electrical energy that gets stored in a capacitor (secondary battery). The dial of the watch must have at least 30% transparency for the light to reach the solar cell.
We were able to locate technical documents for the VS42A from as far back as April 24, 2015. The most recent tech sheets are from January 11, 2019. There were no noticeable differences other than the layout/design of the documents.
The example movement in this post is signed S. Epson Corp. / 2 / Japan / VS42A / Two Jewels / 712
VS42 VS VS42A:
The movement in your timepiece may actually be signed with the caliber number V157A. As with most (if not all) Seiko movements, the trailing letter indicates the evolution or change-log of the movement, starting with A as the first release. Therefore, V157 is the same as V157A. As of this post, there is not a V157B, V157C, and so on.
Seiko claims that the caliber VS42A will maintain accuracy of +/- 20 seconds per month at normal operating temperatures of about 5C to 50C (41F – 122F).
The caliber VS42A is a solar powered watch with a manganese titanium-lithium rechargeable capacitor. It is not a normal battery. The number is MT920, or more specifically: 302324Y (or Seiko part number 4020581). Order one here
The chart below shows how long different lights sources can take to fully charge the cell in the VS42A. This ranges from 47 hours to 5 hours of constant light, depending on the light source type.
When fully charged, the VS42A has an expected running time of around 6 months. When the power is low, the second hand will skip a second and move in 2 second intervals.
What is 712?
While preparing the macro photos for this post, we noticed a very tiny “712” etched into the movement. It was noticed by the naked eye. It’s still unconfirmed what this 712 represents, so please comment below if you know!
At the time of this post, replacement prices for the VS42A were found in the range of $24.99 to $39.99 USD.