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Jupiter 4 pitchbend problem
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the19thbear



Joined: Apr 19, 2007
Posts: 114
Location: denmark

PostPosted: Fri Feb 22, 2013 1:10 am    Post subject: Jupiter 4 pitchbend problem Reply with quote  Mark this post and the followings unread

Posted Image, might have been reduced in size. Click Image to view fullscreen.
Basically i cant get the offset right, to do octaves pitchbend. No matter how i position the pitch bend sensitivity pot, i cant get the pitchbender IC to output (via Pin7) equal plus/minus voltages when moving the actual pitch bender knob to its two extreme settings.. Its always +2,5/-1,8 (just an example) and never, say, +2,5/-2,5. On the audio side of things, that means that i cant get a normal pitch one octave down/pitch one octave up, its always a perfect octave up, when i pitchbend to one side, and then less than an octave down when i pitch bend to the other side... I already change IC1, but no cigar. Could C1 and C2 be the problem?
The PSU is right and the pot is getting +/-15v.

Thanks!

Last edited by the19thbear on Tue Feb 26, 2013 5:43 am; edited 2 times in total
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elmegil



Joined: Mar 20, 2012
Posts: 1535
Location: Chicago
Audio files: 14

PostPosted: Fri Feb 22, 2013 11:26 am    Post subject: Reply with quote  Mark this post and the followings unread

I would assume it has to do with some interaction between the two sensitivity pots and the op amps up at the top, but I can't really make out the schematic very well. Do you have a larger size you can attach rather than putting in line?
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bubzy



Joined: Oct 27, 2010
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PostPosted: Sat Feb 23, 2013 1:22 am    Post subject: Reply with quote  Mark this post and the followings unread

have you tried buffering the "bend signal" before you split it out to the two trimmers before the op amps at the top?
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the19thbear



Joined: Apr 19, 2007
Posts: 114
Location: denmark

PostPosted: Sat Feb 23, 2013 2:37 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks alot for suggestions. I reuploaded the image now.

The thing is, this circuit is showing the pitchebender schematics from a jupiter 4 that i am restoring, and it has worked earlier, i just cant seem to find out whats wrong. On the schematic you can see that you can select the pitch bend pot to modulate, VCF, VCA and VCO. Just ignore that part.. since the schematic is so simple i can only see that IC1 could be bad...
Here is what i have checked:
-Checked plus/minus 15v på IC1 -OK
-Checked plus/minus 15v on both sides of pitchbender pot. - OK
-swapped IC1: 4558- OK
-Check pins 7 på IC1 and see if i have equal plus/minus coming out when adjusting the bend sensitivity and pitchbending to either side= Impossible! (im guessing i have to get, for instance +5v when doing positive pitch bend and -5v when doing negative pitch bend in order to get an octave. - It changes when i vary the sense pot but its always offset wrong. A full octave up and say, half an octave down.
Sad
Any suggestions? Its just so dead simple that even I understand it... but it still doesnt work.
Thanks again!
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bubzy



Joined: Oct 27, 2010
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PostPosted: Sat Feb 23, 2013 5:00 pm    Post subject: Reply with quote  Mark this post and the followings unread

this is what i meant, but i could of course be totally wrong (its happened once before !) Very Happy


PitchBend2[1].png
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PitchBend2[1].png



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elmegil



Joined: Mar 20, 2012
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PostPosted: Sat Feb 23, 2013 5:24 pm    Post subject: Reply with quote  Mark this post and the followings unread

Is there a reasonable way to disconnect that pitchbend input and measure whether it is what you expect *before* you get to the pots up at the top?

I'd think if this worked once before that would be a good thing to know before modding the circuit. That said, i agree with Bubzy's circuit as a way to isolate the sections of the circuit....
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the19thbear



Joined: Apr 19, 2007
Posts: 114
Location: denmark

PostPosted: Mon Feb 25, 2013 1:31 pm    Post subject: Reply with quote  Mark this post and the followings unread

Ok. I spent the last 2 hours trying to get at this thing...
As you can see from the original diagram, the 100k pot (the bender) is a dual thingy..
It has 4 terminals. Pitchbend up i can adjust via "bend sens" pot, and pitch bend down i can adjust via the screw inside the pitchbender (see attached picture).
Posted Image, might have been reduced in size. Click Image to view fullscreen.
The 2 pots influence/offset each other making it very hard to calibrate it.
I have been adjusting and adjusting for about an hour, but its impossible to get it to be completely equal. Sometimes it is, sometimes it isnt.. because of the mechanical part of the pitchbender (an old spring that determines the exact position where the pitchbender will snap back too.... And when i have adjusted the octaves to be 95% correct, i have to screw in another screw to hold the pitchbender in place, and that sets the whole thing off. ( because you have to hold the whole thing tight and that somehow offsets the calibration - again.. this is caused by moving parts - moving them tiny bits.

Will buffering in any way help with this? What IC would be good? Im a newbie when it comes to electronics, and i cant really see what it will help here. Or can i connect a pot somewhere around the buffer and do easy calibration from there? (after i have screwed in all screws etc - so i dont offset the whole thing - i'll just calibrate it afterwards via the new pot).
Is that possible? I would be more than happy with a 97% correct pitchbend - the last uncertain 3 percent will be caused by the old spring i think.

Thank you SO much!!!!!!
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the19thbear



Joined: Apr 19, 2007
Posts: 114
Location: denmark

PostPosted: Thu Feb 28, 2013 8:16 am    Post subject: Reply with quote  Mark this post and the followings unread

bump Very Happy
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the19thbear



Joined: Apr 19, 2007
Posts: 114
Location: denmark

PostPosted: Mon Mar 04, 2013 9:59 pm    Post subject: Reply with quote  Mark this post and the followings unread

Anyone?Smile
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bubzy



Joined: Oct 27, 2010
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PostPosted: Wed Mar 06, 2013 10:36 am    Post subject: Reply with quote  Mark this post and the followings unread

if its a resistance difference, you could find out what the difference is and pad one out with an additional resistor. although this isnt a perfect method, it may give you a more usable range. just a thought. additional buffering isnt going to do you much good, as if you have used the mod i suggested along with the original circuit, theres already enough buffering happening.
other than that, you may want to just try with a "normal" potentiometer, and see how you get on. Smile

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the19thbear



Joined: Apr 19, 2007
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PostPosted: Sat Mar 09, 2013 9:20 am    Post subject: Reply with quote  Mark this post and the followings unread

thanks alot!
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