Help with quartz oscillator circuit

[steve1811uk]

Hi, I am trying to get a 1976 citizen quartz watch working. There has been some damage to the circuit board due to battery leakage. The oscillator capacitor had disintegrated and the trimmer capacitor crumbled when rotated. Does anyone know typical values for these components in pF? Has anyone actually fixed a quartz circuit by replacing those components? I have sketched the circuit diagram. The two parts in question are C1 and C2. Thanks, Steve.

[praezis]

C1, C2 are in the range of 15...22 pF.

Frank

[AndyHull]

Finding trimmer capacitors physically that small is a little tricky. Ali express might be the best place to try. Something like these perhaps.

https://www.aliexpress.com/item/32971499898.html?spm=a2g0o.productlist.0.0.3ec8703eyIELrv&algo_pvid=08685e77-8528-4150-aff0-94f4277a2178&algo_exp_id=08685e77-8528-4150-aff0-94f4277a2178-1

Typical values (depending on  the crystal used) are of the order of 6 to 22pf for both the trimmer and the fixed load capacitor.

I have seen low value "ghetto" capacitors, of the order of a few pF used on some really cheap bits of equipment. These consisted of two short pieces of insulated wire, which were simply twisted together to form a rudimentary capacitor (think cheap musical greetings cards and the like). 

Some watch and clock crystal circuits will actually run without the load capacitors, relying on the in built capacitance of the circuit traces presumably. i.e. removing them doesn't always kill the circuit, but will probably affect the oscillator frequency somewhat.

I would try a couple of 10pf smd caps or salvage whatever you have lying about from some other watch or clock, and see if it runs. Often what actually dies when the board gets affected by moisture or battery corrosion is the crystal, so if it is still dead, swap the crystal for a known good one. Also examine all of he PCB traces very carefully for hairline cracks and corrosion.

Obviously also test the continuity of the coil. These are usually of the order of a few k ohms if good, but dead short or open if damaged.
 

Edited July 12, 2021 by AndyHull

[steve1811uk]

Thanks guys for the input. I tried with 2 X 22pF but without success (no oscillation). I then tried another crystal again without success. I think the chip itself has died as it is getting power and there are no breaks in the PCB tracks. Coil is measuring approx 2k which I believe is correct. It's a shame as the watch dial is great (if you like that 70s vibe). Will put it away for now. Steve.

[JohnR725]

On 7/11/2021 at 1:02 PM, steve1811uk said:

1976 citizen quartz watch working.

I don't suppose the watch had a model number?

Then in the old days when quartz watches first came out there actually were people that would sell the quartz crystals and the trimmers.

On 7/12/2021 at 2:39 PM, steve1811uk said:

Coil is measuring approx 2k

then a coil resistance around 2K more or less is correct. Typically usually that's the first thing to go because somebody scratches it.