Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems.

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sfsteamer
 
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Default JC Whitney Digital Auto Clock

I can't read the clock in my radio during the day so I stuck an LCD
clock to my dashboard. Problem was I could not
read it as night without pressing a button to light the display.
Besides, it lost about a minute a week.

I thought I found the solution in the JC Whitney LED DIGITAL QUARTZ
CLOCK SKU#ZX812798R $15.95. I cut a hole in
my dash and installed it only to find two problems:
1. I could not read it during the day - The LED display was very dim.
2. It gained about a minute a week.

This digital was the only one I could find after an extensive search
so I resolved to fix these problems.

I ordered another clock to experiment with and took it apart. The
first problem resulted from a 0.80 thick plastic
lens in front of the actual LED. It was red and dimmed the light from
the LED behind it. I cut a slot the actual
size of the led in a piece of black plastic the same thickness - a
clear piece could be used but the the PC board
the LED is mounted on would be visible. I assume this is why they used
the red color on the lens.

The second problem was a little harder to figure out - I looked up the
quartz crystal (3.93216 MHz ) which the clock
depends on for accuracy.
There is a pair of capacitors used to load the crystal and the
required value was specified at 17 pico-farad.

I found useful information on crystal timing at:
http://www.maxim-ic.com/appnotes.cfm/appnote_number/555
The first thing was that a clock running fast can be adjusted by
increasing the capactor values. The second thing
was a formula the total value for capacitors hooked up in series. If
they are the same value, the total value
is half the value of the individual capacitors.

I found the load capacitors in the clock were 15 pf which calculates
out to a load value of 7.5pf versus the 17pf
specified. I substituted two 33pf ceramic capacitors for a calculated
value of 16.5pf - much closer to the 17pf
specified. The result was the clocks (I changed both) now gain or
loose 1 second or less in a week of testing.

You may wonder why I went to this much trouble -
1. I could not find another clock that might work.
2. I had cut a hole in my dash which the clock fitted into.

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sfsteamer:
The moral of your story is:
Do NOT cut holes in your automobile dashboard until you are very
certain you have a device that meets all of your expectations.
electricitym
- - - - - - -



sfsteamer wrote:
I can't read the clock in my radio during the day so I stuck an LCD
clock to my dashboard. Problem was I could not
read it as night without pressing a button to light the display.
Besides, it lost about a minute a week.

I thought I found the solution in the JC Whitney LED DIGITAL QUARTZ
CLOCK SKU#ZX812798R $15.95. I cut a hole in
my dash and installed it only to find two problems:
1. I could not read it during the day - The LED display was very dim.
2. It gained about a minute a week.

This digital was the only one I could find after an extensive search
so I resolved to fix these problems.

I ordered another clock to experiment with and took it apart. The
first problem resulted from a 0.80 thick plastic
lens in front of the actual LED. It was red and dimmed the light

from
the LED behind it. I cut a slot the actual
size of the led in a piece of black plastic the same thickness - a
clear piece could be used but the the PC board
the LED is mounted on would be visible. I assume this is why they

used
the red color on the lens.

The second problem was a little harder to figure out - I looked up

the
quartz crystal (3.93216 MHz ) which the clock
depends on for accuracy.
There is a pair of capacitors used to load the crystal and the
required value was specified at 17 pico-farad.

I found useful information on crystal timing at:
http://www.maxim-ic.com/appnotes.cfm/appnote_number/555
The first thing was that a clock running fast can be adjusted by
increasing the capactor values. The second thing
was a formula the total value for capacitors hooked up in series. If
they are the same value, the total value
is half the value of the individual capacitors.

I found the load capacitors in the clock were 15 pf which calculates
out to a load value of 7.5pf versus the 17pf
specified. I substituted two 33pf ceramic capacitors for a

calculated
value of 16.5pf - much closer to the 17pf
specified. The result was the clocks (I changed both) now gain or
loose 1 second or less in a week of testing.

You may wonder why I went to this much trouble -
1. I could not find another clock that might work.
2. I had cut a hole in my dash which the clock fitted into.


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James Sweet
 
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The second problem was a little harder to figure out - I looked up

the
quartz crystal (3.93216 MHz ) which the clock
depends on for accuracy.
There is a pair of capacitors used to load the crystal and the
required value was specified at 17 pico-farad.

I found useful information on crystal timing at:
http://www.maxim-ic.com/appnotes.cfm/appnote_number/555
The first thing was that a clock running fast can be adjusted by
increasing the capactor values. The second thing
was a formula the total value for capacitors hooked up in series. If
they are the same value, the total value
is half the value of the individual capacitors.

I found the load capacitors in the clock were 15 pf which calculates
out to a load value of 7.5pf versus the 17pf
specified. I substituted two 33pf ceramic capacitors for a

calculated
value of 16.5pf - much closer to the 17pf
specified. The result was the clocks (I changed both) now gain or
loose 1 second or less in a week of testing.

You may wonder why I went to this much trouble -
1. I could not find another clock that might work.
2. I had cut a hole in my dash which the clock fitted into.



Many devices replace one of those loading caps with a small trimmer
capacitor, with careful tweaking you can get it to be quite accurate, as
good as a few seconds per month. The next problem you'll have is large
temperature swings in the car, but if you're feeling geeky you can overcome
that problem by fabricating a small oven for the crystal to sit in. This can
be as simple as a power resistor for heat, a thermister for feedback and a
simple circuit to control it, with the crystal, resistor and thermister
stuck together with heatsink grease and the assembly potted in a small block
of polyeurethane foam. With that arrangement you should be able to achieve
accuracy of a few seconds a year, but that's probably overkill.


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sfsteamer
 
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Good suggestions - especially about not cutting a hole first. I was
going to try a variable capacitor but, as you note, the temperature
swings will have a big effect as well. I can live with a few seconds
a week - the unmodified clock was gaining a minute a week.

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