Hi Bill
I've just checked - the oscillator does indeed go to the XTAL pins of
the Philips microcontroller. The crystal I thought was for the mc turns
out to be for the LCD display controller.
I'll have a rummage for some resistors.
Thanks
John
On Tue, 05 Feb 2013 05:20:48 -0800, WB6BNQ wrote:
Hi John,
OK ! It is good that the other resistors were there. It is also
good that a
lower EFC voltage lowers the frequency.
I am not sure what standard values are available to you in the UK. I
picked a
standard value in the US that would roughly center the pot's range
equally around
2.1 volts.
So, I would suggest paralleling a 5.1K 1% metal film resistor across
the lower 3K
resistor. Attaching from the bottom of the pot to that resistor
string's ground
point would probably be easier then trying to attach directly across
the SMT 3K
resistor.
The 5.1K added resistor puts the bottom resistance value at 1888.9
Ohms. The
whole string would nominally be 5888.89 Ohms and ups the current in
the string
from 0.743 ma to 0.883 ma. A small change and should not be a
problem as to
power dissipation in the upper 3K.
The voltage at the bottom of the pot should be near 1.67 volts and
the top of the
pot should be near 2.55 volts. That should place the range so that
the VCXo goes
below 18 MHz. If not then select the next lower standard value. Try
to obtain
1% metal film at 1/8 watt or second best carbon film at the same
ratings. Do not
use the OLD carbon composition type resistors.
As Bob points out and I also agree, if your unit looks like the type
I saw on the
web site, then the 18 MHz oscillator most likely runs the computer
system. That
does not necessarily mean it is also the clock that is clocking the
A/D that is
digitizing the sound. So, varying the 18 MHz may not change your
results. The
important clock, for accuracy, is the one that clocks the A/D inside
your unit.
Time will tell. After you add that resistor and then readjust the 18
MHz based
upon your counter, you will know if the watch is on time the next
day. If not
then it will require further study of the circuitry.
Good luck,
Bill....WB6BNQ
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