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Other Vintage Household Electrical or Electromechanical Items For discussions about other vintage (over 25 years old) electrical and electromechanical household items. See the sticky thread for details.

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Old 24th May 2021, 12:04 pm   #1
vinrads
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Default Clock IC?

Having forgot about this repair I am under pressure to get it done , I need an ic number mn6251 looked on line found nothing ,is there an equivalent ? Mick.
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Old 24th May 2021, 12:34 pm   #2
Beardyman
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Default Re: Clock ic ?

Found this link, any good?

https://www.datasheetarchive.com/pdf...=M&term=MN6251
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Old 25th May 2021, 8:57 am   #3
vinrads
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Default Re: Clock ic ?

Thanks for that , Still looking for the MN 6251 ic or an equivalent , Mick.
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Old 25th May 2021, 4:51 pm   #4
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Default Re: Clock ic ?

Quote:
Originally Posted by vinrads View Post
Having forgot about this repair I am under pressure to get it done , I need an ic number mn6251 looked on line found nothing ,is there an equivalent ? Mick.
Are you certain it is the IC that is dead, I have seen far more dead crystals than ICs in clock movements. May be worth checking out first.
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Old 25th May 2021, 4:54 pm   #5
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Default Re: Clock ic ?

Also clock timing crystals are much easier to find than specific obsolete ICs, although you need to pay attention to the capacitance figure (if any) specified in the data sheet and find a crystal with the correct spec otherwise it may run noticeably faster or slower than it originally did. I don't know if the circuit under discussion has a variable trimmer capacitor for fine adjustment of the timing.

If you have a comms receiver which covers the 4.194304Mhz frequency of the crystal try tuning around that frequency, ideally in SSB or CW mode. If the oscillator is running you will hear a steady note or whistle when you tune close to the right frequency: It should disappear when you remove the battery from the clock.

Last edited by SiriusHardware; 25th May 2021 at 5:01 pm.
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Old 25th May 2021, 5:08 pm   #6
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Default Re: Clock ic ?

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Originally Posted by SiriusHardware View Post
Also clock timing crystals are much easier to find than specific obsolete ICs, although you need to pay attention to the capacitance figure (if any) specified in the data sheet and find a crystal with the correct spec otherwise it may run noticeably faster or slower than it originally did. I don't know if the circuit under discussion has a variable trimmer capacitor for fine adjustment of the timing.

If you have a comms receiver which covers the 4.194304Mhz frequency of the crystal try tuning around that frequency, ideally in SSB or CW mode. If the oscillator is running you will hear a steady note or whistle when you tune close to the right frequency: It should disappear when you remove the battery from the clock.
If it is a more ‘modern’ movement, try tuning the receiver to 32.768 KHz, as that is the later clock xtal frequency.
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Old 25th May 2021, 5:18 pm   #7
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Default Re: Clock ic ?

I did originally think it would be that but the datasheet linked to in #2 shows a 4.194304MHz crystal in the application circuit. I checked.
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Old 25th May 2021, 5:30 pm   #8
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Default Re: Clock ic ?

A dead IC or xtal will result in nothing being detected on the receiver, I would still swap out the xtal first I think, just to eliminate it, if it sorts it then it can be trimmed up for time accuracy.
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Old 25th May 2021, 5:48 pm   #9
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Default Re: Clock IC?

Also for a go / no-go check of the crystal, you could try any other crystal with a nearby frequency just to see if that makes the clock run, although of course it would run at the wrong speed.

For example, a 4.000Mhz microprocessor clock crystal or a 4.33619Mhz PAL TV 'colour crystal' could be tried.
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Old 25th May 2021, 11:37 pm   #10
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Default Re: Clock IC?

I don't think you will find a direct equivalent to the MN6251 these days - here are two equally hard to find modern clock ic's - H1344 & EM1537.

Assuming it is faulty is there the possibility of:-

Replace the whole movement with a modern after-market quartz clock movement.

Removing the pcb from a modern movement and grafting that in the place of the old one.

If all else fails and the clock must be made operational, then there are diy projects out there to build the clock movement electronics.
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