Scott wrote:
How does my car clock work? It cannot be quartz or radio controlled
as it runs slightly fast. It cannot be synchronous because the supply
is DC. Is there any form of adjustment to get it to run to time?

If the clock is part of the entertainment system,
it might be harder to work with on the bench, and make
an adjustment.

Separate clock modules, fitted in a dash, are more likely
to run on the bench with a 13.5V supply for company. If
it had a ceramic trimmer, you could adjust it. When the
clock is buried in an entertainment system, getting it
open would be a problem.

Quartz time pieces can run fast or slow, and on either
side of the "correct" value. Only if the control scheme
requires a "bias", do they preferentially pull the
clock to one side. This is done on Unix computers,
so that time corrections by software, only happen in
one direction, not both directions.

Car clocks normally work too well, to not have temperature
compensation. The parabolic temperature curve for the
quartz crystal, can be corrected by attaching a network
for the purpose. As in the picture here. This can change
a 20 to 50ppm design, into a 1 to 3ppm design. The scheme
is almost as good as using an oven to keep a constant
temperature.

https://www5.epsondevice.com/en/info.../OSC-img01.gif

Paul