[time-nuts] VCXO in a watch timing machine

Bob Camp lists at rtty.us
Tue Feb 5 07:26:36 EST 2013


I suspect the 18 MHz is simply the clock to a CPU chip. Cheap CPU + code is more cost effective than a bunch of random logic. If the gizmo works that, there's a "microphone"  picking up the ticking and the CPU does DSP to figure out what's going on.


On Feb 5, 2013, at 6:18 AM, john <john at ic0n.org.uk> wrote:

> Hi
> Some very good questions - thanks for the responses. No schematic (or documentation of any description - Ebay purchase), but I've done some dismantling and had a poke around with a multimeter. This is what I find:
> The power supply provides +5.2V and +/-6V. Strangely, the silkscreen on the board power connector says +5V, +8V, -8V and -24V. The power supply has no components or wires for this latter voltage, so that's a bit of a mystery. The -24V rail disappears off into some components, so maybe it's an 'option' on another model? Anyway, let's not get sidetracked.
> The 1k pot is sandwiched between two 3k resistors (surface mounted on the back, so not immediately obvious). I get 0V - 2.2V - 3V - 5.2V, so only 0.8V adjustment range. The lower the voltage, the lower the frequency, and vice versa, so I could just short the resistor that's connected between ground and the pot?
> Modern mechanical watches are relatively impervious to changes in temperature - balance springs and balances are made from materials which are much better in that regard than their carbon steel forbears, which required split bi-metallic balances to compensate.
> I agree that 18MHz does seem an odd number. The counter can work with watches that beat at 5, 5.5, 6, 7, 8 and 10Hz so you'd think it would relate to those in some integer way.
> Regards
> John
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