[sdiy] FW: Synthasystem Frequency Divider trouble shooting

[David Ingebretsen]

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