Bulova didn’t invent the use of tuning forks to measure time – but they were the first ones to pack the technology into a wrist watch. One of the great inventions in watch making in my humble opinion, and a great piece of technology.
The watches are mainly mechanical. Instead of a balance / hairspring oscillator, the Accutron uses a tuning fork that swings. The electronic part is keeping the tuning fork swinging – all the rest is done mechanically.
I got this one off ebay and it’s not working – that’s why I paid £ 37.50 for it. But for this post, I will only take it apart and have a look at some details, and put it back together at a later time.
One huge difference to a conventional mechanical watch is that the power flow through the wheel train is inverted. Whereas in a traditional watch, the spring barrel provides the power and the balance keeps the watch from winding down quickly, it’s the tiny little wheel at the bottom (called the index wheel) that is driven by the tuning fork, which then drives the other wheels of the wheel train. So there are minimal forces at work, and the movement probably will take centuries to wear out if properly maintained.
The index wheel has 320 teeth. As the tuning fork swings at 360Hz, the index wheel makes 1 1/8 turns per second.
The coil on the left side has two coils – one is used to switch the transistor, as a small current is induced in the coil every time the fork swings. The transistor then switches the second coil within the left coil and the right coil on and off, pulling the fork ends and keeping it swinging.
The capacitor and the resistor are there to provide an initial oscillator to start the tuning fork swinging when the watch is switched on.
I even found the original Accutron218 Service Manual on the internet – you can see a detailed description of the parts and their function in there.
Time to check the coils – they tend to be the main source of electrical problems. Apparently, the transistor, capacitor and resistor rarely cause problems.
I take out the screw at the top that connects the left and right coil to separate them and I take out the battery.
I measure the resistance as shown in the drawing. The two infinite resistance measurements aren’t good. It means that the coil that pulls the tuning fork on the left side is shot. The coil on the left that is used to trigger the transistor is fine, but that’s no consolation.
This means I need a left coil assembly. If you have one lying around, please contact me (e-mail address on the “About” page).
Otherwise, it’s another Accutron less in this world.
Never fear, I manage – read up here for the follow-up …