I have a query about calibration on the 901A controller.
I actually have both my 901A’s nicely calibrated, and have no complaints at all… it is just that the tuning pots (fixed voltage pots and not the rotary ‘click’ selectors) are parked in different places on both of the controllers. One is at ten o’clock and the other, at three o’clock - when they are both tuned to A440.
When I calibrated them, there didn’t seem to be any way to do the job via the internal presets, whilst leaving those tuning pots set at ‘0’…! Is this normal or am I missing something..?
Possibly!
What you describe is approximately a one octave difference in the knob settings between the two oscillator controllers. The “Fixed Control Voltage” knob is calibrated to zero volts fully CCW, one volt at 12 o’clock, and two volts fully CW. So roughly a two octave range.
I would make sure the little dots of the knobs on the rotary range switches are properly aligned. Check by making sure there is a jump between the -5 setting and 6 setting. The rotaries are set at 1 volt increments.
Probably just a simple knob alignment issue, since it is an octave offset?
CZ is right. The selectors can spin around completely with no stops.
Quite like the instrument selector on a TR808, you have to remember where it was set when putting back on the knob.
The only reason I’m adding to his post tho, is that sometimes the resistors mounted to that selector can short against each other.
So consider a visual inspection to make sure no leads are touching.
Thanks for the info… but I think I might have described my symptoms badly or haven’t explained the whole thing too well… OK, so I’ll try from a different angle.
On the Minimoog you “zero” the master tune knob - at 12 oclock - and then proceded to tune the first oscillator to A440. This can all be done with out touching the master tune again.. yes..?
So my question now is… on the 901A controller… should I be “zeroing” the tune knob (rotary not clicker) at 12 o’clock and then procede to tune the unit KEEPING that tune knob “zero-ed” throughout…?
I found that during the tuning process I HAD to move those knobs, to tune my 901A’s sucessfully… and they ended pointing all over the place.
By the way, the knobs on the shafts are right… they both end stop at in the right places.
On a Minimoog, the master tune is the ONLY way to tune the first oscillator. Only oscillators 2 & 3 have their own tuning controls.
I don’t know if there really IS a right way.
901Bs are going to have some offsets.
That is, to zero beat several together it’s very likely their tune knobs won’t always be pointed at exactly the same places.
Most people I’ve observed (including myself) tend to set the 901A selector to 1 or 2, zero the knob and then tune their 901Bs as they prefer.
After that, fine tuning of all 901Bs can be done with the 901A tune knob.
It’s a matter of taste. However… be aware that 901As and Bs may not track as well at some settings, especially extreme ones.
One of the things about 901s is that they’re not always that linear. That is, they don’t follow V/pitch in a perfectly straight line.
So you may tune several oscillators together fine and tightly, then transpose them via the 901A or by an input voltage and their tracking may waver.
Also please understand that 901Bs derive their octaves from selected capacitors, while a 901A selects its octaves via resistors.
Resistors can be chosen more accurately and tend to remain more accurate over time.
So when doing octave switches, sometimes the 901A is a better choice. This said, don’t be surprised that changing octaves on any 901B may not exactly hit the next octave. You may very well have to fine tune that oscillator again via its tune knob.
The short answer:
Tune the seperate oscillators to each other via the 901B Frequency Vernier controls. And raise or lower the whole bank of tuned oscillators via the 901A Fixed Control Voltage dial.
The long answer is more complicated:
It is important to note the actual CV range the 901A drivers are designed to work/tune in. This is what the two “Fixed Control Voltage” controls are there for.
From the 901 spec sheet:
The 901A drivers have a working range of -4 to +6 volts. With the optimal range of 0 to 5 volts. The “Fixed Control Voltage” can be used to get the 901A in the proper range by offsetting or adding to the pitch CV for optimal tuning range if needed. (Note: All the voltages are added together, the 7 one volt per octave CV inputs and the Fixed Control Voltage combo for the 901A working voltage range.)
With a 951 keyboard where the lowest “C” is zero volts, a 901A setting of -1 on the rotary and 1 on the dial will give a zero volt offset. (Or any combination giving zero volts, like zero on rotary and zero on dial.) The 951 five octave range of 0 to +5 volts, would then fall into the optimal tuning range of the 901A driver.
Likewise with a 952 keyboard the middle “C” is zero volts. So a 901A setting of 1 on the rotary and 1 on the dial will shift the 901A into the optimal tuning range.
The combination of the rotary -5 volts to +6 volts and dial of zero to +2 volts, gives you a large range to add or offset the other incomming CV’s to be in the proper range. Too much voltage added together and the 901’s will crap out and hit a wall. Easy to do if set the rotary to 2, the dial to 1 (12 o’clock position) and send 5 volts from a keyboard or sequencer. Adds up to 8 volts and you will notice only a three or so octave range before they crap out.
Just need to do a bit of math to decide where the best range setting is if you need tuning accuracy.
My optimal settings to get a seven octave range from a -1 to 6 volt keyboard is the 901A rotary on -1 and the dial set around 1. Both 901B’s set around the 5 mark.
The Frequency Verniers on the 901B’s here are somewhat close when set to unison. Above set to an A-440 sending a 4.748 volt “A” , and actually tuned to zero volts for lowest “C”.
The Frequency Verniers on the 901B’s are double ganged 10K linear pots. The response is not linear though like the 901A Fixed Voltage dial. There is more of a sweep toward the second half of travel. (Try it!) So at a setting of 5 like in that photo, they tune down about 7 semitones, but tune up a little over an octave. This might have something to do with the misalignments between two oscillators. The two 901B’s here match perfectly at 7, but are offset a little at other settings. The one seems to have a slightly wider range. They are close but not exact to answer you question. But there is no “Range” trimmer inside the 901B to calibrate, just the tracking (Scale) trimmer.
The official tuning proceedure from the Moog Modular Owners manual is close to the way I tune. It is important to tune with zero volts first, then dial in the scale.
From the Moog manual:
Many thanks again for the info guys… but I still think my question remains…!
You see it isn’t the 901B’s I am enquiring about here. My 901B’s seem fine and are perfectly in sync… it’s the 901A controller that I am trying to address.
If I put the 901A’s fixed voltage potentiometer (which has a properly centred knob) pointing at 12 o’clock… I then cannot tune the oscillators to concert. I need to move the 901A’s fixed voltage potentiometer quite some way away from 12 oc’lock - and then the oscillators tune perfectly - and are in concert. I would just like my 901A fixed voltage potentiometer pointing straight up for concert that’s all…!
My question is: should I be able to get the in concert tuning with the 901A fixed voltage potentiometer at 12 o’clock…?
Not without adding a voltage offset from somewhere with either a mod inside the 901A or patching in an offset so your 901A is at your needed voltage at the 12 o’clock position.
The fixed voltage venier is not a fine tune like on the Minimoog that is calibrated. It has an almost two octave range, whlie the Minimoog the fine tune has a range of a few semitones.
Either patch in an offset voltage to compensate, or make a small circuit inside the 901A with a multi turn trimmer to add/subtract the offset. Simple solution, and the only way you are going to get two 901A’s to match at the 12 o’clock position.
Looks like you have some modding to do. There is no trimmer to do what you want, so you need to add one internally or externally patched.
I was thinking, looking at the schematic, that I might add ‘a resistance’ to ‘whichever’ end the 5K Fixed Voltage pot there… that should ‘offset’ the pot’s position, bringing it more in-line…?
