[time-nuts] VCXO in a watch timing machine

john john at ic0n.org.uk
Tue Feb 5 06:18:02 EST 2013


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 

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.


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