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 Forum index » DIY Hardware and Software » Circuit Bending
Hohner String Performer / portamento circuit
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Analogman



Joined: Sep 22, 2012
Posts: 4
Location: Vienna

PostPosted: Sat Sep 22, 2012 6:35 am    Post subject: Hohner String Performer / portamento circuit Reply with quote  Mark this post and the followings unread

Hello,
I am new here and hoping someone can help me out.
I have a Hohner String Performer which is basically a nice string ensemble keyboard using a top octave synthesizer with dividers, sound shaping boards for each note and bucket bridge delays for the ensemble effect. It has however also a not so spectacular mono synth which one can mix in to the string sounds. The mono synth has its own top octave synthesizer and dividing/shaping circuitary. The funny thing is that there is a slight portamento to the mono synth (mostly on the first note played) but there are no portamento 'controls' what so ever. Recently I was playing it and the portamento dissapeared. At the same time the vibrato on the monosyth stopped working (there is also no vibrato control on the syth but inside there is a pot for vibrato speed). This leads me to believe that the 'portamento effect is a by-product of the vibrato ciuitry. So i started thinking, " it would be really cool to have a real portamento cicuit, and it would also be cool to have real vibrato speed control as well as bypass circuitry and intensity. Hmmm?"
Let me state here that I am a beginner as far as electronics go, but that does not stop me from fiddling around. It is the innate curiosity in me that drives me to learn and experiment. So i started studying portamento circuits and decided upon a simple slew limiter circuit. But then i stumbled upon the problem where to put it. I have read that it should be between the cv in (so from the keyboard cv) and the oscillators.
So here are my questions. First please study the schematic, and i would appreciate answers that use the terminolgy, pin #s and parts that are in the schematic to avoud any confusion.
1. What part of the circuitry is creating the 'portamento effect'?
2. What kind of true portamento circuit could I put in here?
3. Where exactly should the portamento circuit go?
That's it for now.
any advice and tips would be greatly appreciated.
Schematics here; just click on the first blue button

http://www.sendspace.com/file/poir4x
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Analogman



Joined: Sep 22, 2012
Posts: 4
Location: Vienna

PostPosted: Thu Sep 27, 2012 3:21 am    Post subject: strange noises in the dark
Subject description: using your hands for diagnoseing analog organs.
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Hello again,
Since no one has answered, I decided to do some more diagnostic tests, and as this story proves, sometimes a hands on approach will quickly lead to the problem, or at least help to find the problem.
To recap i had opened up the hohner string performer to repair some dead notes. The notes in question were the 2 lowest B notes in the string section but only for the 8' and 4' sting sounds. The B sound card is marked H on the hohner, and A# is marked B, (German standards). So on the H card I eventually found that there was a hairline crack on the board that interrupted some of the etched channels. Having fixed that I then noticed that the mono synth had lost its' vibrato and 'portamento effect'. I call this portamento effect because there are no portamento controls and the effect only happens when you hit the first note. If you leave enough time between releasing the note and triggering again then it will happen again.
Anyway I could not find anything wrong with the voltages, contacts or parts.
But then I started touching elements to check for loose connections and sometimes the vibrato and potamento came back. Of course this means sticking your fingers in there while the machine is on (needless to say caution is advised, but the maximum voltage is only 15v dc, so not really dangerous) and having a constant note playing to hear the difference. Then I found that when I turned the circuit board over, the vibrato and portamento also came back. What the heck is going on here? Confused
Any guesses? no, ok
Then I got my lamp a little closer and decided dammit I'm gonna find that loose part. As I moved around touching and wiggling parts the vibrato and portamento always came in and out and it seemed to be in a specific area, and then I realized wait. I am making a shadow with my hand. No way, it can't be Surprised
So I turned out all the lights and yes the vibrato and portamento effect were back. Then I took a flash light and waved it around the board, and sure enough it dropped in and out and the pitch also changed. Cool
The part in question is a photo resistor. On the schematic it is marked with M228 (maybe M22B) and has this (early german standard) symbol
[/img]
So the reason why the vibrato and portamento effect were not working was because I had the top off and light was falling on the photo resistor.
This could indeed be an interesting element to use for some mods. but I am still not sure why or what this resistor is doing in the circuit. Is it creating some sort of sine wave that modifies the LM566? or is it separating two different circuits voltage wise but letting a signal thru? I did notice that the ground connects directly to the volume slider ground, which is not shown on the schematic.
Anyway, back to fiddling with my organs in the dark Wink


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forshee



Joined: Dec 16, 2010
Posts: 41
Location: Hopkins, MN

PostPosted: Thu Sep 27, 2012 10:48 am    Post subject: Reply with quote  Mark this post and the followings unread

My guess is that it just wiggles the voltage controlling the master clock. While a portamento would be quite difficult in in a divider based design it looks like a pitch bend would be pretty easy just throw another little light in the case.

I just glanced at the schematics so there might be an elegant way to get a portamento but this is much simpler to do with a VCO based designs. I'd expect it to be really complicated in this case.
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Analogman



Joined: Sep 22, 2012
Posts: 4
Location: Vienna

PostPosted: Thu Oct 04, 2012 2:34 am    Post subject: update Reply with quote  Mark this post and the followings unread

Thanks for your reply forshee,
too bad that no one else has taken interest in this subject.
So what I have figured out so far is this.
The key trigger signal comes in to the solo voice card on pin 4, goes through two inversion gates, and a transistor T5, then to a red light diode that lights up. The optical resistor then picks up this light signal and brings it into the vibrato circuitry between T8 and T7. The fact that the diode takes some time to light up, creates the 'portamento effect'. Before the diode lights up completely, the top generator solo takt coming in on pin 5 and fed into the LM566 LFO, is pulled to ground creating a lower frequency pitch. As the diode slowly comes to power, (about 1 second) the pitch reaches its' proper state, and the vibrato circuitry does its job. If the diode or an external light is too strong then the portamento effect does not take place as the contact between diode and optical resistor is immediate. In this case the pitch is a little bit higher than normal and needs to be adjusted. Why the vibrato circuitry also drops out in that case is not clear to me at the moment. It seems to be that a strong light source creates a bypass which then cuts out the vibrato signal from LM566 pin 4. It would be nice to have an on/off switch for this effect. It would also be interesting to see what happens if the diode does not light up. Maybe I will try to make a switch between the 2 inverters (4049B) or should I make a switch between the 12v power supply to T5?? I assume that if not lit, the pitch will be too low or lower than normal,
which can be adjusted with the pitch fine tuning pot R3, but then if turned on again the pitch will be wrong again (too high) so maybe this is not a good place for an on/off switch. Maybe for the switch I need to bypass the optical resistor with a fixed resistance that matches the optical resistor when the diode is on. Then the vibrato would work normally but there would be no portamento effect. Hhhmmm, how can turn I the vibrato on and off then??
Sorry, it seems that I am rambling on. please reply with tips and tricks, that way I don't have to talk to myself Smile
Ciao for now.
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forshee



Joined: Dec 16, 2010
Posts: 41
Location: Hopkins, MN

PostPosted: Fri Oct 05, 2012 5:36 am    Post subject: Reply with quote  Mark this post and the followings unread

I haven't looked at the schematics that deeply but it looks like we are dealing with a bias issue on the led. We need to break the signal coming into the LED into two pieces. 1 the vibrato LFO and 2 the gate/envelope that biases the LED. If this bias (which should be somewhere in the middle of the swing) is held high we won't see that glide just the vibrato. and if we let the gate through and remove the LFO we should just hear the glide.

There are a few things to note the LDR (light dependent resistor) will be slow and act as a slew rate generator/ LP filter. The LED will 'saturate' quickly. By that I mean if the bias is high enough the LED will stay bright regardless of the LFO. Look closely at the light while its running it should tell you a lot. And there is no reason you can't have more then one light!

Don't take any of this as true but that's how I see it after a quick look hopefully it's at least helpful.
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Analogman



Joined: Sep 22, 2012
Posts: 4
Location: Vienna

PostPosted: Wed Oct 10, 2012 3:58 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks for the tip forshee,
I watched the light closely, which is by the way difficult to do because the whole LED - LDR set up is encased in a sealed black box, but the light is still visible from one side, and it did the following.
I hit a key- little red light goes on- portamento effect occurs- light goes off after about 0.5 seconds- pitch settles in and vibrato kicks in.
If I continue to press a key and hit a second higher key (high key priority) the led does not turn on, nor does the portamento effect occur-
If I continue to press a key but shine i flash light onto the led side of the black box, then the portamento effect occurs, the pitch settles at a lower frequency (maybe a semitone) but the vibrato does not kick in.
So a continuous light source prevents the vibrato circuit from activating but also pulls the pitch down.
interesting..... Confused
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