Minimoog D Pitch Bend modification (solved)

[chris j]

Hello everyone,
I just bought a 9xxx Minimoog D, first one ever, I love the sound!
While modifying the pitch wheel zero point mechanism (which I found too hard to get out of, a thinner washer did the trick) I noticed there was an extra board added (bottom right on the picture).
I thought it was the zero band modifier, but the pitch changes whenever the wheel is moved in the slightest, so can't be that. Does anyone know what could this be for?
I was hoping the green and white trim pot could be used to change the pitch bend range (which I'd like to be able to change to +/-2, or +/-12, right now it's +/-6) but I don't want to take the risk of unsettling something by touching it.
This place seemed the right one to ask about it first !
thanks in advance!

[thealien666]

First, welcome Chris J to the forum !

Second, could you take another picture a little more to the right, and maybe a little closer to that board, so we can see which wires seem to go to, and come out if, this little board ? It's clearly a DIY board with possibly a 1458 dual op amp and a trimpot. The single-multiple trigger mod maybe ?

A closer pic should give a better idea...

As for the pitch bend range limit adjust, I'm not sure it's possible at all to add such a mod ? Except maybe to replace R1403 (pitch bend pot) by another linear pot with a different value ?

[chris j]

Thanks The Alien666!
the image is here http://chrisjoss.free.fr/DSC05804crop2.jpg (gets cropped if inserted here)

single-multi mod, I'm not sure what it is, each note re-triggering the enveloppe instead of legato ? Right now it's the classic lowest note legato mode.
too bad for the pitch bend, I would have liked a pot I could set and (re-set) to change easily the pitch bend range, without soldering, but it's a good idea cheers!

[thealien666]

The wires are connected to the pitch bend pot. That trimpot could possibly be for range adjustment... Have you tried turning it to see if it changes the pitch bend range ?

[Kevin Lightner]

This circuit is obviously a zero detent mod.
On the board there's the standard back to back diodes.
All the wires from the bend pot go to that board also.

The trimmer is likely to set the zero point, the width of the zero band or the bend point at which it starts working.
Either way, Rudy Linhard has done a very similar board on several Minis and this might be one of his earlier examples.
He later developed a true printed circuit board with a similar circuit.

[thealien666]

Chris wrote in his original post : "I thought it was the zero band modifier, but the pitch changes whenever the wheel is moved in the slightest, so can't be that."

Could it be that the circuit is non operational/defective/bypassed ? Or maybe the trimpot needs readjustment ?

[Kevin Lightner]

When installing these mods myself, it's often very hard to find that tiny point where it doesn't change.
I've used a scope to do it sometimes.
So it's possible he made a mistake or the trim has such sensitivity it's dialed far out of wheel range.

[chris j]

thanks so much guys!!
Alien666: I haven't touched the trimpot yet, you know, waiting for opinion first
Kevin: OK nice info, I need to set the pot properly,
I'll let you know,
cheers!

[chris j]

hello,
So I've tried turning the trim pot, turns out it's just a "tuning" pot, much like the tune button on the front panel.
But no zero band, any change on the trimpot will just change the pitch but any slight move on the wheel changes the pitch.
weird, no ? what's the point of such a mod ?

[thealien666]

I'm puzzled.
No idea...

[RL]

hello,
So I've tried turning the trim pot, turns out it's just a "tuning" pot, much like the tune button on the front panel.
But no zero band, any change on the trimpot will just change the pitch but any slight move on the wheel changes the pitch.
weird, no ? what's the point of such a mod ?

You have to measure on the cathode and anode of the diodes with an voltmeter. The ideal case would be zero Volts but +/- 100mV is also OK.
Have fun,
Rudi

[chris j]

I said like the tune button on the front panel, but the range is actually 2 or 3 octaves on that trimpot!

OK RL, so by getting zero V from the diodes, that would set the trim pot at the right position, correct ?

[RL]

so by getting zero V from the diodes, that would set the trim pot at the right position, correct ?

Yes, this trim pot is for the "dead zone" of the pitch bend center position and NOT for the range.

[chris j]

yes I know it's not for the range as it's just changes the pitch while the wheel still goes +/- 6 whatever the pitch is.
I'll try that , right now I just can't tune the oscillators...

[thealien666]

Before attempting to tune anything, try to solve the pitch bend dead band mystery. Because if the pitch is not what it should when the pitch bend wheel is in the center notch, you will have a hard time tuning the oscillators, as the pitch bend wheel offsets everything.

So make sure the pitch bend wheel is working normally for at least one oscillator first. Is there a dead zone at notch position or not ? If not, despite adjusting the trimpot, then maybe the circuit is defective ?