[sdiy] FW: Synthasystem Frequency Divider trouble shooting

[Oren Leavitt]

On 11/13/2010 9:29 PM, David Ingebretsen wrote:
> Hi group.
>
> I’m trouble shooting the Synthasystem Frequency Divider prototype PCB. Since
> these are all designs which were in actual production, I do make the
> assumption the original schematic is correct and that my capture may have an
> error. That said, I couldn’t find an error and Nyle said it looked ok. The
> schematic is here:
>
> http://www.xmission.com/~dingebre/SP_Freq_Divider_1.1.pdf
>
> Nyle’s original is here:
>
> http://www.xmission.com/~dingebre/SP_Frequency_Divider_0001.JPG
>
> The project page is here:
>
> http://www.xmission.com/~dingebre/Freq_Divider.html
>
> I understand the basic ideas of the circuit and everything seems to be
> working except the one-shot in the lower left side.  Basically, the four
> Flip-Flops in the middle are a binary counter. The 8 position “divide”
> switch is hooked to discrete AND gates which select the denominator for the
> divide. The way Nyle set it up, you get 3, 5, 7, 9, 11, and 15. The one shot
> is supposed to fire when you hit the desired count and it should reset all
> four FFs. The way I see it, the last FF should never even count if you
> select a divide by 3 or 5, but it does count. I also never see a reset pulse
> out of the one shot unless I force it by grounding the base of Q20.
>
> The primary symptom is the divide by 11 divides by 7. When I’ve looked at
> the scope, the first time I select the divide by 11, it divides the first
> cycle only by 11. Thereafter, it divides by 7.
>
> Secondarily, it appears the one-shot is not being triggered, therefore the
> FFs are not being reset when the count reaches the selected count on the 8
> position switch.
>
> I understand the one shot and simulated it for good measure. What I don’t
> understand is how Nyle is resetting it. I thought you need to pull the base
> of Q20 low. When I do that, it resets.
>
> I don’t get the little capacitor/diode/resistor connection to the pole of
> the divide switch.
>
> Any thoughts?
>
> I’ve emailed Nyle, but sometimes he takes a few days to answer and I hate
> pestering him.
>
> David
>


Hi David,

Does your SP/6T divide switch "break before make" when you change position?
It looks like the one-shot circuit depends on R56 briefly pulling D21 
"high" while the switch wiper is transitioning between contacts. If your 
switch is a "make before break" type, that will not happen.


- Oren