How can I calibrate a retail digital clock that uses a 32.768 kHz crystal?

UPDATE 1 YEAR LATER: The clock is fine tuned well now, it lost about 5 seconds in 1 year

SOLVED: SEE BELOW ORIGINAL QUESTION FOR IMPLEMENTED SOLUTION

I have two Sony Dream Machine clocks. They are different models; one seems to use the mains frequency and is keeping its time precisely, while the other (model: ICF-C414) seems to be using the onboard crystal to keep the time. This leads to running too fast 1 second per day (30 seconds per month and 6 minutes per year).

If possible, I'd like to modify this clock to be more accurate, since the drift is known (1 second per day). Is there anything I can do to reduce this drift?

I considered just buying a new crystal and seeing if that has better accuracy, but I can't find any 32.768 kHz crystal with better than 20 ppm accuracy.

Here are the circuit board pictures:

================= SOLUTION ================

Thank you all for your help, this wouldnt be possible without your education.

Problem: Clock gains ~1 second every ~24 hours

Attempt1: Attempt 1 was to simply change the crystal to a high quality 5ppm CITIZEN crystal (CFS-20632768HZFB). This did not yield a better result. Clock was still gaining ~1 second every ~24 hours.

Attempt2: Check continuity to make sure correct capacitor is used from one of the legs of the crystal to one of the sides of the capacitor C204. After confirming I attached 10pF capacitor (FG28C0G2A100DNT06) directly to C204. So far 48 hours later there appears to be no gain/loss. I will observe over the course of a few weeks to determine if there is a slow gain/loss and adjust accordingly by going either to 11pF (10pF + 1pF caps) or 9pF (6pF + 3pF caps).

Attempt 3: With 10pF capacitor the clock lost 1 second after 4 days. Means loss of 0.25seconds every 24 hours. This is 1.25 second swing per 24 hours (was too fast now slow) from before any capacitors were attached. If capacitance (pF) influence on the crystal is linear this means 1pF slows the clock down 0.125 seconds per day. Meaning that by lowering down to 8pF capacitor I should achieve 0.25 faster clock speed, which would bring it closer to 0. Replaced 10pF capacitor with 8pF (6pF + 2pF). Will report on results.

Attempt 4: With 8pF cap the clock lost 1 second per ~6 days. This means each 1pF does not influence the clock linearly. Removed the 2pF (from 6pF + 2pF) and will test it at 6pF. Will update with results. It seems like I am getting close. Even as it is 8pF improvement was ~1 minute off per year versus unmodified ~6 minutes off per year. Hoping to achieve ~1 sec per month/~12 per year or better, but 1pF increments may not allow for this. Will get as close as I can and report here.

Attempt 5: After much experimenting I found the balance, it is somewhere between 6pF and 7pF. To be more granular Id have to order sub 1pF capacitors. Currently with 7pF (6pF + 1pF) I achieved 1 second loss (slow) every 2 weeks or so. Which is sub 30 second drift per year. I am more than happy with the result. If I come across some sub 1pF caps Ill attempt something like 6.5pF (6pF + 0.5pF) to see if I can nail it even closer. Thanks all for your help!

• Wiktionary says "To check or adjust by comparison with a standard." So as long as you expand your view of what "adjust" means -- sure, it's calibrating. Commented Sep 10, 2022 at 18:38
• Who says that desoldering a component and replacing it isn't an "adjustment". The procedure -- from Ford -- for adjusting the valves on a model A or flathead V8 engine is to measure the lash, take the valve out, grind the stem down, then put it back in and repeat. If Ford can call that an adjustment, then the definition is pretty broad indeed. Commented Sep 10, 2022 at 18:55
• What about buying a better crystal? 5 ppm for instance : eu.mouser.com/c/ds/passive-components/…
– Uwe
Commented Sep 11, 2022 at 3:17
• @jonk: What's funny is that John Harrison was able to build marine chronometers that were more accurate than that about 250 years ago. Commented Sep 11, 2022 at 16:51
• @SiHa "we have two Sony clock radios" --> Segal's law. 😉 Commented Sep 13, 2022 at 4:08

It is difficult to match the long-term accuracy of power line frequency. It is continually adjusted to maintain network grid performance. Crystals outperform the power line short term.

simulate this circuit – Schematic created using CircuitLab

C203, C204, and R214 are from the image that you provided. Within the crystal equivalent diagram, the components with subscript "m" are the motional values. Co is the shunt capacitance between the pins.

The load capacitance $$\C_{L}\$$ required for the crystal to function is:$$C_{L}=C_{0}+\frac{C_{203}C_{204}}{C_{203}+C_{204}}$$

The datasheet for the linked 5 ppm capacitor has $$\C_{L}=12.5\ pF\$$ and $$\C_{0}=1.2\ pF\$$, so:$$\frac{C_{203}C_{204}}{C_{203}+C_{204}}=11.3\ pF$$. If they are equal then:$$C_{203}=C_{204}=22.6\ pF$$ The stray capacitance are probably 1 to 2 pF. Cin is harder to estimate, perhaps 2 to 5 pF. Perhaps others here can provide better estimates. $$\C_{203}\$$ and $$\C_{204}\$$ must be reduced to compensate.

The drive level for the 5 ppm device must be $$\<1\ \mu W\$$ for others $$\<0.5\ \mu W\$$. Typically they operate at 20% of the maximum. For series resonance, the power can be calculated using the motional resistance and the voltage drop across the crystal.

For the 5 ppm crystal the voltage drop should be less than 187 mVRMS. Probably closer to 50 mV. Because a scope's capacitance will load the crystal, I would increase R214 to the point where the oscillations stop. Then decrease until they start again. Then a further reduction of 10% to 20% will allow operation over temperature.

What are the chances that the new crystal will just work? Try it and see. But making sure the capacitance is right will put it at the right frequency.

To trim the frequency either one or both of $$\C_{203}\$$ and $$\C_{204}\$$can be adjusted to get the frequency.

Use NP0 ceramic capacitors. X5R, X7R and all other ceramics are severely voltage and temperature dependent.

• thank you for this, going to pick up both 5ppm and 10ppm ones, will try soldering them on and see if its more accurate. I dont need it to be atomic clock, but would be nice if it deviated a few seconds a month instead of a few days. What I dont get is that I have wall clocks, simple Chinese ones run off AA battery, those seem to not lose or gain any time over 6 months until DST change, how are they so much more accurate?
– Duxa
Commented Sep 11, 2022 at 5:43
• @Duxa: Don't forget to change the load capacitors to the values specified for the ones that you purchase Commented Sep 11, 2022 at 6:24
• @RussellH OP needs to know how to calculate the correct load capacitor values from the crystal datasheet load capacitance and stray capacitance numbers. And probably adjust after that to get the advertised accuracy since stray will be a guesstimate. Commented Sep 11, 2022 at 7:58
• hmm, I need to know the current 'stock' load capacitance right? How do I figure that out? What are chances that current one matches the one I order? So in that case I can just swap the crystal and it would work?
– Duxa
Commented Sep 11, 2022 at 8:25
• I was going to order this assortment of caps, would they allow me to do the adjustments or should I consider adding some more? digikey.com/short/8nv05v8v
– Duxa
Commented Sep 11, 2022 at 9:22

First, refer to this post for details of (we hope) the underlying oscillator schematic. You'll probably just see the crystal and the caps, with traces going into a chip.

• Verify that you've found the crystal and its load caps.
• It looks like the capacitors are surface-mount, so they won't be marked and you'll need some good soldering skills. Don't trash the board.
• (that is a weird board layout, BTW -- "value design" rules, I guess).
• If the clock is running fast, adjust one cap value up slightly -- I'm talking like 1pF at a time here, so be patient. If that doesn't overshoot, but doesn't get you where you want, adjust the other one up by 1pf. Repeat as necessary until things work.
• If the clock is running slow, adjust one cap value down slightly, i.e., do the reverse of the step above.

Chances are that to start from a known point you'll have to replace the caps on the board anyway. I'd be strongly tempted to replace one or both caps with variable capacitors whose ranges center on 12pF, then tweak them. I'd also be strongly tempted to use 10pF fixed caps in parallel with variable caps whose range is centered on 2pF -- this will mean that you'll get less change in frequency for the amount you tweak the cap, which will, in turn, make your life easier.

Before you even launch on this quest, though, you may want to take the clock and put it in a space with a markedly different temperature than what you've been testing it -- chances are that if they didn't care about one second per day when they were designing it, they also didn't care about variation of frequency with temperature. If that's the case, then you could get lost in a very deep rabbit-hole trying to get the thing adjusted and stable over temperature.

• Or figure out which pin on the chip is the clock input, and make an external GPS-disciplined 32768Hz clock... Commented Sep 10, 2022 at 18:58
• Would it be worth gluing a half-watt resistor to the crystal and permanently running 1/4 W into it to see if a little heat would fix the problem. It's a long shot. Commented Sep 10, 2022 at 21:30
• I don't know which directions typical tuning-fork crystals swing when you warm them up. Ovenize the whole radio? Commented Sep 10, 2022 at 21:36
• Hmm, according to @Spehro the frequency will drop either side of room temperature. Commented Sep 10, 2022 at 21:51
• @Transistor If you keep it thermostatically controlled, this is called an OCXO and it's just about the silver or bronze standard of electronic timekeeping. You may be able to buy an OCXO and wire it into the circuit. Commented Sep 11, 2022 at 23:30

Since the clock is running fast, you could try increasing the capacitance on the crystal by using a "gimmick", which is a capacitor made by two insulated wires twisted together. Magnet wire would probably work best because of the thin insulation.

The Wikipedia article says capacitance is around 1 pF/inch (0.4 pF/cm). You might try a couple of inches (5 cm) to see if it runs slow, and then just snip off pieces until it is a close as possible.

• can I just use a twisted pair from CAT5 or CAT6 wire? its already pre-twisted, or is that too thick of a gauge?
– Duxa
Commented Sep 11, 2022 at 10:50
• It's worth a try. You'll get more capacitance with a tighter twist. Commented Sep 11, 2022 at 19:29

You can add a TCXO such as Maxim's DS32kHz to the board if you like.

1. Glue the chip down, preferably on a ground pour (dead bug style- legs in the air).

2. Remove R214 (0Ω resistor).

3. There are four connections required to the chip. Attach (using AWG30 Kynar wire-wrap wire or magnet wire) GND and Vcc connected together (to pin 11 on IC101), Vbat (to pin 14 on IC101), and 32kHz (to pin 13 on IC101, where one side of R214 went).

Or just muck with either one the caps (C203/C204). They're 22pF and you can get a trimcap and a few values of smt capacitor to get more values. I'd aim for 10pF +/-5pF more than the 22pF as just an educated guess based on estimated pullability of a random 32kHz tuning fork crystal and your 'running fast' number. So just connect the trimcap in parallel with one of the existing caps.

• A solution with a TCXO offers a much better precision.
– Uwe
Commented Sep 11, 2022 at 3:16
• how are you counting legs on IC101? I see the dimple, but which side is pin1? And which way does it increment?
– Duxa
Commented Sep 11, 2022 at 5:32
• by the way this is in-house outlet plugged in clock, so temperature variation is minimal +-5 degrees C.
– Duxa
Commented Sep 11, 2022 at 5:49
• The advantage of the TCXO is also that it's a complete oscillator and the correct load capacitors have been taken care of (and the oscillation frequency digitally calibrated, in this case, saving quite a bit of time fiddling with capacitors). IC101 is numbered from the bevel corner CCW, also marked on the PCB , and I've added an image. Commented Sep 11, 2022 at 8:01
• @Duxa the typical drift in ppm of an uncorrected tuning fork 32kHz xtal is shown in the graph above (the convex parabolic curve). The xtal cut is optimized to minimize drift near room temperature/body temperature. Commented Sep 11, 2022 at 12:47

Watch crystals are rarely more accurate than the 20 ppm you say. So replacing the crystal likely makes little difference. It might tick at wrong rate too, in that circuit, because you don't know the load capacitance rating of the crystal to replace it with a crystal that has a matching load capacitance rating.

But crystals are tuned by adjusting the load capacitance. You could replace crystal load capacitors with slightly smaller or higher capacitors, depending on if you want to slow it down or speed it up.

One of the capacitors can be replaced with an adjustable capacitor.

It can be difficult to determine the exact frequency accurately with home equipment, but measuring the drift over time as you have done so far is a good method.

If you really want it to be in time with a powerline clock, you could always tick off the powerline. Fortunately you have a transformer, so isolated zero-crossing detection is easy (clipping diodes and a large resistor to the high side of the secondary: with old microcontrollers people used to use the internal protection diodes); then you'd need some kind of PLL, either in software on a microcontroller or in hardware (fortunately this is easy as 32.768 is chosen to divide down nicely!).

Whilst I suspect this is more than you want to get involved in, are you sure it's not already possible? I do wonder if @Harper's comment isn't correct. If you compare the board from the powerline-disciplined clock, can you find the powerline circuitry? Is it present? If it isn't and it feeds a pin on that MCU as I suspect it does, what happens if you feed that pin on the non-working clock? There's a chance it's not disabled in software, particularly if the powerline clock also has a crystal present---it would be sensible to have put failover in there.

If you don't go for powerline disciplining, the two clocks are going to get out (even if you go for something better like GPS disciplining, they'll be out most of the time by average). But you can probably trim for better than 1/s day as per the other answers.

• The AC powerline frequency should probably not be counted on for the best possible accuracy. See electronics.stackexchange.com/questions/57878/… and kccscientific.com/… Commented Sep 13, 2022 at 15:15
• @spuck the instantenous accuracy is indeed variable: in fact it is almost gauranteed not to be correct (hence a gps-disciplined clock will be out of time, at any given instant, with a mains-disciplined clock). The long term accuracy of mains frequency is excellent. In this case the OP wants the two clocks synchronised with each other. That said I wouldn't advise anyone to use instantaneous mains frequency as a source for anything. You likely don't care if a clock is out by a second now, providing it's in again soon. You do care if a frequency counter is out... Commented Sep 13, 2022 at 15:26
• That said the kosovo/serbia situation caused a sustained frequency loss in europe which stole six minutes before it was corrected... so I guess the marketing should say "your clock will be accurate to a few seconds over a year, unless there's a war.". The long term stability of mains clocks is still much better than watch crystals. Commented Sep 13, 2022 at 15:28
• how does one calculate ppm? Is it logarithmic? Im wondering what kind of ppm is gaining 1 second every 2 weeks? versus gaining a second every month versus gaining a second every year?
– Duxa
Commented Sep 15, 2022 at 3:58
• @Duxa it's a pure linear dimensionless ratio, see wiki Commented Sep 15, 2022 at 8:21

The pragmatic way to do this is to twist some solid insulated wire together, perhaps 15 twists and solder that across the crystal. That is a crude but easily adjustable capacitor. If you have one of the little multi testers for parts just make sure you have > about 10 pf. If the clock runs slow, snip a little bit off the end. Keep doing so until it is dead on. You also have some adjustment with how tight the twists are.