Any Doepfer A100 owners here?

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Any Doepfer A100 owners here?

Postby rachelp » Tue Jan 08, 2008 1:25 pm

I have my little Doepfer A100 mini system, that works OK, but I think the VCO's are not correctly calibrated.

The symptom is that to get an A440 tuning on any VCO, I have to turn the fine tune knob to its limit, but even then
I've now found it easier to transpose 2 semitones in my Logic seq for the A100 and set the fine tune that way.
That is OK for a workaround, but I am a perfectionist and like things to work the way they are supposed to.

I am sure it is the VCO and not the A190 MIDI interface scaling as it seems to scale alright when I use it to trigger and drive
other stuff.

The thing is, I am nervous about calibrating the VCO (especially while it is powered up) and I do not have the
right tools.

Have any of you guys done this and is it hard to do?!


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Postby Thirteen » Tue Jan 08, 2008 6:17 pm

I haven't worked on Doepfer before, but I assume it is 1V/ octave. So the first thing to do is to get a $15.00 digital multimeter from Jaycar, and measure the output of the CV out jack on your MIDI-CV converter. Play a low note and write down the voltage, eg: 0.885 or whatever, and then play a note an octave up, it should be exactly the same as the first reading plus 1 volt. For every octave you go up it should add exactly 1 volt. This will tell you if your MIDI-CV converter is scaled properly. If not, adjust it until it is.

Next you need a good tuner or frequency counter, plug it into your VCO, play a low note. Tune it right onto a note, then play a high note and see if it is exactly the same note. (DO not move the octave switch if it has one.) If it is sharp or flat then the VCO needs scaling. If the VCO has an octave switch, do the same test, but still holding the low note, switch the octave switch around. If it moves off pitch then the Octave control needs calibration. Either of these calibrations should be easy, once you do them once you can do your own calibration every 6 months as the air temperature changes.

If the only problem is that it won't come right onto the note before you run out of fine tune knob, then there will be a VCO offset (as opposed to scale) trimmer inside that need to be adjusted.

As I said, I haven't worked on a Doepfer VCO, but the adjustments I mentioned are the same for any 1V/oct. VCO.
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Postby chris p » Tue Jan 08, 2008 8:25 pm

I share 13's lack of experience on Doepfer, but I can confirm that the A110 has a VCO offset pot inside.

My suggestion was going to be to start there, with a dual oscilloscope (owned, borrowed, or ... well, not really stolen). The A110 output (pick you're fav wave) goes into one input, and a known good synth out (maybe with a different wave, to avoid confusing which is which in the display) goes to the other. You can then compare the 2 waveforms and fiddle with the VCO offset until they align the zero crossings. You could also tune it simply by ear if you wish. The A110 service manual (available online) shows you where the various internal pots are located.

Good luck.
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Postby Thirteen » Tue Jan 08, 2008 9:43 pm

chris p wrote:I share 13's lack of experience on Doepfer, but I can confirm that the A110 has a VCO offset pot inside.

My suggestion was going to be to start there, with a dual oscilloscope (owned, borrowed, or ... well, not really stolen). The A110 output (pick you're fav wave) goes into one input, and a known good synth out (maybe with a different wave, to avoid confusing which is which in the display) goes to the other. You can then compare the 2 waveforms and fiddle with the VCO offset until they align the zero crossings. You could also tune it simply by ear if you wish. The A110 service manual (available online) shows you where the various internal pots are located.

Good luck.


There you go, if you can get that manual you should have no problem shifting the offset so that your fine tune knob is dead center. Don't worry if you mess it up, you can bring it over and I will put it back on track ;-). Seriously though, tuning and scaling analogs is usually pretty easy, so don't be afraid to give it a go. Here's something that every analog synth owner should have...




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Postby rachelp » Tue Jan 08, 2008 9:46 pm

Ok,

I found the service manual. I think the scaling of the A190 is fine, because I used it to trigger my System 100
and I get a proper result, but I will have to get a meter to check precisely. I think I can suffer doing the VCO offset, because that
is what I think it is. The instrument plays properly across a 2.5 octave range but the fine tuning is almost off the scale. I think
it must've got bumped when we moved house.

Thanks guys - I now have a project for next week!


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Postby Thirteen » Tue Jan 08, 2008 9:54 pm

rachelp wrote:Ok,

I found the service manual. I think the scaling of the A190 is fine, because I used it to trigger my System 100
and I get a proper result, but I will have to get a meter to check precisely. I think I can suffer doing the VCO offset, because that
is what I think it is. The instrument plays properly across a 2.5 octave range but the fine tuning is almost off the scale. I think
it must've got bumped when we moved house.

Thanks guys - I now have a project for next week!


rachel


Go for it Rachel, you can't go wrong. The offset trimmer simply moves the VCO's pitch up and down the same as the front panel pot, the scale trimmer actually changes the Volt/octave tracking.
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Postby rachelp » Tue Jan 08, 2008 10:01 pm

BTW, Steve - you did a beautiful job on my System 100. Everything seems to work fine on it and it's got a huge sound. Very stable too.

That's a strobe tuner, isn't it? I've been thinking of a tuner, because at the moment I use the A440 test tones of the moog and the 100.
And both of those are out against each other too! I used to use a sine wave from the DX but that's off now.


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