Model D Saw Ramp Up on Oscilloscope Weirdness

[nealkaiser]

Hello, I hooked up a Korg NTS-2 audio oscilloscope to a Minimoog D (1975) and was surprised to see that the Sawtooth (ramp up) actually seems to ramp down (on all 3 oscillators). And the reverse sawtooth on oscillator 3 actually ramps up instead of down. Yet, if I use the sawtooth or reverse sawtooth as an LFO, it clearly Is modulating In the right direction. Is this the intended behavior, I'm just wondering. I have tested w/ other synths and the saw is correct., so I don't think it's the scope? Thank you.

[stiiiiiiive]

Hi there!

Might seem a stupid question, but have you double-checked you haven't inverted the scope inputs?

[nealkaiser]

I have checked that, yeah. Pretty strange?

[sdwillingham]

Your observations are correct. The signal path from the OSC to the output is non-inverting and the path through the modulation mixer is inverting.

The sawtooth/ramp graphics on the panel correspond correctly to the direction expected in the modulation path. However, the pulse graphics are inconsistent. For the narrow pulses as modulation it is the “low phase” that is actually shortened. Conversely, the saw/ramp graphics are inverted with respect to the audio output, yet the pulse graphics match the audio output (IIRC).

[stiiiiiiive]

Now, that's accurate!
Thanks

[nealkaiser]

Thank you! Very helpful information

Your observations are correct. The signal path from the OSC to the output is non-inverting and the path through the modulation mixer is inverting.

The sawtooth/ramp graphics on the panel correspond correctly to the direction expected in the modulation path. However, the pulse graphics are inconsistent. For the narrow pulses as modulation it is the “low phase” that is actually shortened. Conversely, the saw/ramp graphics are inverted with respect to the audio output, yet the pulse graphics match the audio output (IIRC).

[KushnomGhatak]

The behavior you're describing with the Mini Moog D's sawtooth waveforms is not the intended behavior, and it's good that you noticed it. Here's what's likely happening:

Possible Issues:

Faulty Oscillator Circuit: There could be a problem with the circuitry generating the sawtooth waveform in one or more oscillators. This could be due to aged components or other issues.
Incorrect Calibration: The Mini Moog D can be calibrated, and your sawtooth waveforms might be out of calibration, causing them to appear inverted.
Scope Settings: While unlikely, double-check your oscilloscope settings to ensure it's displaying the waveform correctly and not inverting it accidentally.

Additional Notes:

It's good that the LFO sawtooth behaves correctly, as it indicates the issue is likely isolated to the oscillator section.
Comparing the Mini Moog D's sawtooth waveform to other synths is a good troubleshooting step.
Remember, tampering with vintage electronics can be risky. If unsure, professional help is the safest and most reliable option.

[till]

...
Faulty Oscillator Circuit: There could be a problem with the circuitry generating the sawtooth waveform in one or more oscillators. This could be due to aged components or other issues.
Incorrect Calibration: The Mini Moog D can be calibrated, and your sawtooth waveforms might be out of calibration, causing them to appear inverted.
...

Not true.

[stiiiiiiive]

the LFO sawtooth behaves correctly, as it indicates the issue is likely isolated to the oscillator section

Neither this. Inverter circuits can be involved in there.

If unsure, professional help is the safest and most reliable option

Now this is true. Do you have a short list in our region please?