Concertmate MG-1 CV pitch weirdness...

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allochthon
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Concertmate MG-1 CV pitch weirdness...

Post by allochthon » Tue Aug 12, 2014 4:53 pm

I am sending CV signals from a Doepfer Dark Link to an MG-1. I have the gate/trigger working correctly; however, I'm having problems with the pitch being wildly out of tune.

The Doepfer sends a 1V/Octave CV signal, and the MG-1 accepts 1V/Octave, so it doesn't seems like a Hertz vs Volts issue.

Is this possibly an issue with the MG-1's calibration? Is there somewhere else that this problem could arise?

Any ideas welcome.

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thealien666
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Re: Concertmate MG-1 CV pitch weirdness...

Post by thealien666 » Tue Aug 12, 2014 8:50 pm

A friend of mine has an MG-1 and he uses his Paia MIDI-CV interface with it without serious pitch issues. I say "serious pitch issues" because the tracking of the MG-1 isn't very precise, but not wild enough not to be playable. Only slightly off in the higher notes, due to internal calibration of the stretch tuning of both Sound Sources :roll: (oscillators) :wink:

So, maybe your MG-1 would benefit from being recalibrated ?
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allochthon
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Re: Concertmate MG-1 CV pitch weirdness...

Post by allochthon » Tue Aug 12, 2014 9:03 pm

Thanks for response. I'm not familiar with the term "stretch tuning".

I did recently have it serviced, but nothing was mentioned about calibration related to the CV pitch. I'll ring him up and see if it makes sense to him.

I would call this a "serious pitch issue", as it plays about 7 semi-tones off (or 5 depending on which way you count), but I can tune the MG-1. Once I tune it, if I then switch up an octave on the source (a monotribe), the MG-1 will be sharp. Tune that, then switch up an octave - sharp again, etc.

When it is relatively in tune, it sounds great.

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thealien666
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Re: Concertmate MG-1 CV pitch weirdness...

Post by thealien666 » Wed Aug 13, 2014 12:27 pm

Stretch tuning (or I should say stretch calibrating) an analog oscillator consist of adjusting the frequency of low notes against a reference (or tuner) with one trimpot, and then adjusting the frequency of high notes against a reference with a second trimpot. And going back to the first trimpot for touch-up since one trimpot affects the other slightly. (in the case of the MG-1 they are R1001 LOW, R1009 High, and R1008 SCALE for OSC1 and R1026 LOW, R1038 HIGH, and R1037 SCALE for OSC2)
That is the technique for calibrating many analog synths, which resembles stretching an elastic to precise lower and upper points (frequencies in this case) hence the name "stretch tuning" .

There's often a third trimpot called "linearity" (or "scale" in the case of the MG-1) which adjusts the notes in between upper and lower to try and reach an evenly frequency calibrated synth across the whole keyboard.

But in all those cases, the very first thing to check is the 1 Volt/Octave source controlling those oscillators. If it's off by even millivolts per octave, the resulting scale will be dramatically off especially in the higher octaves.

Have a look at the service manual for the MG-1, the calibration procedure is indicated and explained on page 17.

Found here :
http://manuals.fdiskc.com/flat/Realisti ... Manual.pdf
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allochthon
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Re: Concertmate MG-1 CV pitch weirdness...

Post by allochthon » Wed Aug 13, 2014 12:40 pm

That made complete sense. Thank you.

Does this stretch tuning apply to the MG-1, regardless of whether or not it is receiving a CV pitch? It was tuned when I had it last serviced, and it is definitely in tune to my ears when it's not being sent CV pitch.

Yes, I do have the service manual, and will look into whether this is indeed the issue. I do know that this issue exists regardless of the source, which means it could be MIDI-to-CV converter, but I would be surprised if that were the case.

Awesome help. Thanks again.

TOS
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Re: Concertmate MG-1 CV pitch weirdness...

Post by TOS » Wed Aug 13, 2014 1:07 pm

The first thing you have to set per the manual is the KEYBOARD CURRENT SOURCE ADJUSTMENT R2051 and R2057 with Glide set to zero. This sets the CV range for the keyboard. The frequency and scale adjustments are then made based on this. If this is not carried out in the correct order then the MG-1 can be in tune with itself but will not be correct when connected to external gear. Sounds like this may be the issue you are seeing.

Chris
This Old Synth, the new home of Synthfool's Opto-Key project.
www.thisoldsynth.com

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thealien666
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Re: Concertmate MG-1 CV pitch weirdness...

Post by thealien666 » Wed Aug 13, 2014 1:49 pm

"Does this stretch tuning apply to the MG-1, regardless of whether or not it is receiving a CV pitch?"

Yes. Because Pitch Control Voltage (CV) is what controls the frequency of the oscillators, be it internal or external. And the stretch tuning only adjust the frequency relative to that Pitch CV.

Remember that everything is relative in this world. The fact that your MG-1 is in tune when playing it stand alone, doesn't mean that it is tuned according to a faithful 1 volt per octave internal voltage control source.

That could explain why it is out of tune when trying to control it from an external Control Voltage. If the internal pitch CV source isn't well calibrated for 1 volt per octave, you can still manage to get the oscillators to play in tune with it. But then, any external properly adjusted CV source would make them play out of tune.

The Keyboard Current Source adjustment procedure on page 16 is very important to perform prior to adjusting stretch tuning of the oscillators. Maybe yours needs recalibrating ?
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allochthon
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Re: Concertmate MG-1 CV pitch weirdness...

Post by allochthon » Thu Aug 14, 2014 1:05 am

thealien666 and Chris - thank you immensely for your help. Best forum help I've had.

The good news is that it stays pretty much in tune over wide octave ranges when using a CV pitch in!! The bad news is that the "reference" tones do not match (i.e., when I play an 'A' on the source, the MG-1 plays a 'D').

I performed the KEYBOARD CURRENT SOURCE ADJUSTMENT - It was slightly out on both ends of the keyboard, but I got it to within specs.

I then performed the "stretch tuning" - took a while, but it wasn't tedious, as I could really sense what it was doing.

So.. Unless there's something else I can tweak in the MG-1, I'm left with either the MIDI-to-CV converter or the aftermarket MIDI device in the monotribe (MIDItribe). The fella who makes the MIDItribe is responsive, so I'll hit him up if there isn't any more sage advice from here. He did boldly state that the problem was Hz/V conversion, which was abjectly wrong, as far as I can tell.

Thanks again, I now have a dirt-cheap 16 step sequencer controlling my MG-1, and it sounds killer (as long as I remember to adjust for discrepancy in key!).

John

TOS
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Re: Concertmate MG-1 CV pitch weirdness...

Post by TOS » Thu Aug 14, 2014 12:01 pm

John,

That's because these synths scale from the left hand key. The Source starts at C giving 0v CV out. The MG-1 starts at F with CV of 0v giving that note. This leads to a C on the Source playing a note 5 higher on the MG-1, and that is what you are seeing here.

You need some sort of converter such as a MIDI to CV one that can deal with this offset.

Chris
This Old Synth, the new home of Synthfool's Opto-Key project.
www.thisoldsynth.com

Wesley
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Re: Concertmate MG-1 CV pitch weirdness...

Post by Wesley » Thu Aug 14, 2014 10:01 pm

The Rogue has a similar issue when using CV input. I use a signal processor (dotcom) to offset the CV signal and bring things back in tune.

allochthon
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Re: Concertmate MG-1 CV pitch weirdness...

Post by allochthon » Thu Aug 14, 2014 11:34 pm

Wow, well at least I know where things stand. $80 for the Q125 signal processor isn't too egregious. More gear!

koleeko
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Re: Concertmate MG-1 CV pitch weirdness...

Post by koleeko » Fri Feb 23, 2018 3:05 am

I'm having similar pitch issues on my MG-1 as the ones described in this thread. I read through the manual and found where it talks about setting the Keyboard Current Source Adjustment. It says to monitor the voltage at pin 1 of U1007A. I can't find anywhere in the manual that explains where U1007A is. If someone could point it out for me or upload a picture, it would really help.

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