[sdiy] 7555 as TOG for organ clone.

[Tom Wiltshire]

Hi Jon,

Yes, it'll be temperature sensitive, and yes, it'll be usable. I  
don't know the figures, but if you're cloning something from the 60s  
using modern technology, it's unlikely to come out with worse tuning  
than the original. I realise this isn't a scientific basis for the  
judgement - perhaps someone else can provide the numbers.

 From what I've read there's a trim on the oscillators on the Vox  
Continental which helps keep them oscillating. Plenty of Connies have  
spent their life with half the screws missing so you can get to the  
trims easily in an emergency. So you won't do worse than that!

Finally, it's worth remembering that if the instrument gets hot (or  
cold) the *whole* instrument gets hot or cold, so the oscillators  
will *all* drift. Consequently, it's the differences between them  
that are really going to be important, even more than their actual  
individual performance.

If you wanted to keep the chip count down even further, there's a  
4060 ripple counter with an internal oscillator. Might be worth  
looking into. Details are here:
http://www.kpsec.freeuk.com/components/cmos.htm

Hope this helps,
Regards,
Tom

On 23 Sep 2009, at 06:33, Jon Philpott wrote:

> Hi List,
>
> I'm looking into creating a transistor clone-organ using the
> 'classical' top octave synth + divider network. So far I'm looking at
> using a 4024 for the dividers and a 7555 to generate the top octave.
>
> The 7555 circuit I'm looking to use is fig 1a in:
>
> http://datasheets.maxim-ic.com/en/ds/ICM7555-ICM7556.pdf
> (1 resistor, 1 cap astable multivibrator, 50% duty.)
>
> For the resistor i intend to use a base resistor(1.5k-5k with 0.22 -
> 0.47 caps) + 5k or 10k multiturn trimmer for the tuning, and i'm
> looking into polyproperline for the caps.
>
> Now I understand that with this design the frequency will be
> temperature sensitive, but can anyone give me an idea of how
> sensitive?  will it be usable? or should I look into a different top
> oscillator source? Suggestions? :)
>
> Jon.
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