Slim VCO b waveshape issue

[Purveyor2]

From the ramp output, with the waveshape knob full ccw I get ramp, at 9 o’clock triangle, then from 11 o’clock onward the saw. On the pulse output I get no sound until about 2 o’clock then skinny pulse through to full and back in a narrow range, after 4 o’clock sound is gone again. The pot has 10/-10v on pins and registers 0v when at 12 o’clock, sweeping from -10 to 10. I’ve checked for incorrect components and confirmed power throughout using the schematic.

Tonight I also noticed a glitchy sort of instability in the sound but I couldn’t catch in on the scope. Replaced C14 and the SKA jfet without change.
Any thoughts?

[Synthbuilder]

From the ramp output, with the waveshape knob full ccw I get ramp, at 9 o’clock triangle, then from 11 o’clock onward the saw.

Check that you have a ramp wave on the output of U7 (pin 6). It should oscillate between 0V and +5V.

Then check that you have a sawtooth wave on the output of U4a (pin 1). This one should oscillate between -5V and +5V. I'd take a guess that it doesn't, so firstly check R7 is 30K, R6 is 7K5, and R5 is 15K.

Instability in the output may suggest a poor solder joint or dirty IC socket connection - probably around U4. It may be worth pulling U4 out of the socket and push it back in again to see if that solves the problem.

Tony

[Purveyor2]

Verified resistors 5-10, replaced U4. I can scope the appropriate waveforms at u7 pin 6 and u4 pin 1 and 8. I even confirmed that the pot was 50k. Only thing I can think of is D1&2 may not be 5v6, can’t read them but vaguely recall using something “close”. Could the diodes be the problem?

And thanks for the quick support, your assistance is greatly appreciated.

[eggpie]

Hi all. I've built four of these oscillators. They sound great and are so easy to use. The octave switching on the B version is a dream.
Your problem could well be diode related. The first one I built wasn't shaping the waveforms properly. Turned out to be one of the diodes the wrong way round, took me ages to find the mistake! It was a long time ago unfortunately, so I can't remember if it was D4,5,7,8 or one of the 5v6 zeners. Could be worth checking though. Hope this helps and good luck with your troubleshooting!

[Synthbuilder]

Could the diodes be the problem?

This is quite possible as Eggpie found.

Check the voltage on pin 7 of U4. It should go from -5V to +5V when the shape pot is turned. The diodes will have an affect on this signal. If either D1 or D2 are around the wrong way you'll get a much smaller range. And they really need to be 5.6V zeners.

Tony

[Purveyor2]

Thanks, ordered some fresh 5v6.

[Purveyor2]

Rebuilt main board, all good. Thanks for the input.