1222 Tuner VCO Module

[Sandelinos]

The frequency chart is not in Hz, but in timer counts per cycle. The timer runs at 38462 Hz. 38462 Hz / 440 = 87.4 Hz, which is an F.

[Cafetorium]

Ah, ok. I guess it was the use of “frequency” in the code that made me assume it was in Hz.
Thanks for your explanation.
I shall be adding comments to my copy of the code to that effect so that it doesn’t confuse me again when I look at it again 6 months or more down the line.

[TheOriginalMasty]

Ive just built a VCO and when I connect it to my midi keyboard they tunings all funny, its displaying the wrong notes on the display and when I try to manually tune it, the notes aren’t correct

[analogoutput]

General advice for troubleshooting a module and asking for help

(This is a wiki post, please add and correct!) Look and see if others have had the same problem. Use the Discourse search facility to find topics where the module you’re building is discussed. Inspect your work. Verify all components are correct values. Look for solder connections that are rough, irregular, skimpy, dull, or missed completely (we all do that!) and reflow them. Look for solder bridges between pads and remove them. Check your power connection. If it’s a Eurorack style power cabl…

Advice for troubleshooting 3340 based VCO

This is a wiki post; please correct and improve! This is written regarding the 1222 Performance Oscillator module, but similar things apply to the simple stripboard oscillator or other oscillators based on the 3340 chip. For the stripboard version also see If you are not getting good tones from the oscillator, do the following. If with the 1222 you are also not getting good behavior from the tuner, don’t waste time on trying to fix that yet. Get good tones on the outputs first — the tuner won…

VCO tuning

[krztoff]

Oh wow, I wish I’d have known you’d written this VCO tuning doc before. This is a much better procedure than the one documented.

[TheOriginalMasty]

After looking through the tuning document I have discoursed. voltage of -7 on my fine tune pot on one of its legs as opposed to the 1v?

[krztoff]

I have 3 1222 performance VCOs in my rack. Today while tuning them with my scope I noticed that one of them has about half the pulse width range as the other two. The PW pot is correct. Is there a specific resistor that could also be affecting the range of this pot?

[analogoutput]

The relevant part of the schematic is:

The voltage VPWM gets sent to the 3340 and determines the pulse width.

R10 and R13 relative to RV4 determine the lower and upper limits respectively of the voltage from the pulse width knob, and then -R14/R12 is the gain for that voltage. -R14/R11 is the gain for the control voltage.

If it’s just the pulse width knob whose range is limited, look at RV4, R10, R13, and R12; if it’s just the response to pulse width CV, look at R11; if it’s both, look at R14.

The voltage at the wiper of RV4 should vary from about 20 mV to about 2 V. With no CV going in, that should give about 4 to 450 mV at U2 pin 7. That’s not a lot of range. Per the CEM3340 datasheet (scaling to +12 V supply), VPWM can be in the range 0 to 4 V. If PWMIN is say 0 to 10 V then that becomes about 0 to 2.1 V at pin 7, still a factor of 2 shy of what it could be. You could lower R12 and R11 to get bigger ranges.

Honestly I have no idea what R15 is supposed to be doing. I can’t see how it does anything.