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Old 24th Mar 2017, 5:07 pm   #1
Goldieoldie
Octode
 
Join Date: Jul 2014
Location: Petersfield, Hampshire, UK.
Posts: 1,043
Default LCD clock crystal

Hi
I have a couple of clock radios where the crystal won't oscillate
When sprayed with freezer it then works for a while
What's odd if I take the crystal out and put it in a another clock circuit it works fine !
If the crystal from the working clock is put in the faulty clock it won't run
Is there a way of persuading the crystal to oscillate with this chip ?
I have attached the circuit
Cheers Pete
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Old 24th Mar 2017, 5:24 pm   #2
Ambientnoise
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Join Date: Feb 2006
Location: Marlborough, Wiltshire, UK.
Posts: 915
Default Re: LCD clock crystal

Sounds like the capacitors each side of the crystal are suspect, perhaps the trimmer is at one end, or it could be the chip. The caps form a voltage divider so if their ratio is far out, it can fail to oscillate or overdrive. I suggest you try replacing both cap/trimmer with fixed 22p as a starter. If that is no go, then suspect the chip.

Ken
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Old 24th Mar 2017, 5:29 pm   #3
Goldieoldie
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Join Date: Jul 2014
Location: Petersfield, Hampshire, UK.
Posts: 1,043
Default Re: LCD clock crystal

Sorry I should have said I've dis the trimmer and put in two fixed caps
I also tried it with no caps
I was hoping it's not the chip as it runs on its own 1.5 v battery so nothing to upset it
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Old 24th Mar 2017, 5:42 pm   #4
Rubberfingers
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Join Date: Oct 2012
Location: Dorset, UK.
Posts: 240
Default Re: LCD clock crystal

Make sure the board is spotlessly clean around the crystal and the relevent pins of the chip, with no flux deposits (modern "no clean" soldering processes) the slightest leakage will stop the crystal oscillating
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Old 24th Mar 2017, 6:44 pm   #5
Goldieoldie
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Join Date: Jul 2014
Location: Petersfield, Hampshire, UK.
Posts: 1,043
Default Re: LCD clock crystal

I lifted the two osc pins from the PC to eliminate leakage problem ( connected xtal directly ) Still the same !
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