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Old 10-29-2019, 02:46 PM   #21
Dannyx
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Talking Re: Improving a slow/fast digital clock

Here's some more pics of the inside of this thing. I took one with a light source behind the board to make the traces more visible like an x-ray. Sadly, nothing too interesting going on in there. I managed to work out what all those pins are for and that the clock is indeed crystal-based.

I popped off the board which holds the buttons and the display to the face plate and I was able to spot the crystal tucked under the corner of the display there. The proprietary epoxy blob chip is directly under the LCD as well, so no need to take that out as well, since it might not line up properly again.

That J pin we saw connects to the base of the S8050 transistor, so it's the control signal for the relay. The PWR pin is a sense wire connected directly to the unregulated rectified DC via a resistor and tells the chip whether or not the thing is actually plugged in, since it won't pull the relay when it's not, even if you manually set it to "ON". VCC and GND are self explanatory: GND is GND and VCC is a zener-regulated supply for the IC AND the battery, so now we know what the battery does too: it's simply placed in parallel with VCC and has a ceramic cap between it and GND. There IS a resistor feeding its + terminal from VCC, but it's got a single black band on it, so it's zero ohms used as a jumper, isn't it ? The relay's VCC comes from the unregulated DC as well.

So that's about it....not much I can do to improve this thing. Funny thing is that it's sat unplugged for a couple of days now and it seems to be running SLOW now - when plugged in, it would run too fast, though I haven't done an in-depth analysis and actually watch the thing carefully, but that's the way it feels...
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Old 10-30-2019, 06:19 AM   #22
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Default Re: Improving a slow/fast digital clock

the 2 white ceramic caps tune the crystal,
so it or the caps may be temperature sensitive.
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Old 10-30-2019, 12:01 PM   #23
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Default Re: Improving a slow/fast digital clock

Oh yeah, that's another thing I forgot: temperature. I took the timer inside after swapping it with the other one, since it's normally plugged into an outlet in a balcony where it can get pretty cold when winter comes around, since there's no radiator there, though the behavior occurred during summer time too, so something is drifting there due to various reasons...
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Old 10-30-2019, 06:52 PM   #24
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Default Re: Improving a slow/fast digital clock

this is also interesting on how to fix this for me as i have a battery powered digital lcd alarm clock. it uses three triple A batteries as opposed to ac line voltage and no surprises, its china made and the time drift on it is quite terrible. its like 10 secs faster every day.

after about a week, its 1 min fast and i have to keep setting it back and its getting quite annoying to do that every week. well, on the bright side, i wont be late for work or my appointments with it running fast and making me move faster! lol!

i guess the chinese tuned it for their colder temperate climate and when its brought into singapore, due our hotter tropical climate, the higher temperature causes a drift in the circuit, making the seconds tick by at a faster rate.
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Old 10-30-2019, 07:32 PM   #25
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Default Re: Improving a slow/fast digital clock

my mobile phone gains time .
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Old 10-30-2019, 09:02 PM   #26
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Default Re: Improving a slow/fast digital clock

Quote:
Originally Posted by petehall347 View Post
my mobile phone gains time .
Your (cell) phone should resync with the network time servers each time it acquires signal.
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Old 10-30-2019, 09:04 PM   #27
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Default Re: Improving a slow/fast digital clock

Quote:
Originally Posted by ChaosLegionnaire View Post
this is also interesting on how to fix this for me as i have a battery powered digital lcd alarm clock. it uses three triple A batteries as opposed to ac line voltage and no surprises, its china made and the time drift on it is quite terrible. its like 10 secs faster every day.
You can purchase inexpensive clocks that will sync to WWV/CHU. I keep one for my bedside alarm. Unfortuantely, the clock isn't smart enough to know (or be told!) that we DON'T observe DST so, twice a year, I have to tell it we're in a different timezone.
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Old 10-31-2019, 04:29 AM   #28
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Default Re: Improving a slow/fast digital clock

Quote:
Originally Posted by Curious.George View Post
Your (cell) phone should resync with the network time servers each time it acquires signal.
only if its set to do that .mine isnt as i dont trust it
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Old 10-31-2019, 10:17 AM   #29
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Default Re: Improving a slow/fast digital clock

That thing looks like it uses a "tuning fork" crystal oscillator though you need to closely inspect it to be sure. Then go buy a bunch of them and replace them every week until you find one that's accurate enough
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Old 10-31-2019, 03:00 PM   #30
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Default Re: Improving a slow/fast digital clock

Tuning fork crystals are delicate. Maximum drive level of 1μW for 32.768kHz parts so they need a big series resistor ~300k is usually missing.
So most circuits overdrive them, and they age rapidly or fracture.

The oscillator is almost impossible to work on, because scope probe capacitance a few pF upset things.

Hard to tell what is causing the clock to be slow/fast, it can be VCC or temperature moving around too much, or a bad oscillator design that needs tuning (caps) optimized. I would try a new crystal - if it worked when new.

pic from http://nkg.com.hk/pdf/NKG-TIT_TuningForkCrystals.pdf
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Old 10-31-2019, 05:30 PM   #31
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Default Re: Improving a slow/fast digital clock

Note that 50ppm error is already about 30 seconds per week ... so don't get the cheap ones
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