Maestro FSH-1 / Oberheim VCF / Tonepad Filter/Sample/Hold  [schematic]

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Re: Maestro FSH-1 / Oberheim VCF / Tonepad Filter/Sample/Hol

Postby chicago_mike » 05 Sep 2014, 16:46

I have a jpeg around here somewhere... showing a mod for the ARP Odyssey. They replace the noise transistor in the same same sample and hold, for a zener diode..

I'll try it and see what happens. Cause I aint got shit to do.. :(
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Re: Maestro FSH-1 / Oberheim VCF / Tonepad Filter/Sample/Hol

Postby okgb » 06 Sep 2014, 00:40

I adjusted my trimmers to work , but it doesn't have the noise part so this just maybe what I need, thanks
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Re: Maestro FSH-1 / Oberheim VCF / Tonepad Filter/Sample/Hol

Postby chicago_mike » 06 Sep 2014, 01:21

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Re: Maestro FSH-1 / Oberheim VCF / Tonepad Filter/Sample/Hol

Postby aquataur » 08 Sep 2014, 15:50

Mike,

if you just have the hood open (or anybody), try soldering base and collector of the noisemaker transistor together.
If you scan the internet for "transistor noise generator" you will find exactly the same circuit, except the collector is left open or connected to the base (which will not make any difference)

The base-emitter junction of a transistor, if used in reverse, exhibits an excellent zener diode breakthrough at about 6 Volts. Any open pin (such as the collector) just introduces noise :lol: .
Joke aside, they should be joined. What an open base is supposed to do, I don´t know.
Naturally, a 6.x zener diode will work too.

So, if many internet borne schematics use the very same circuit with the base and collector joined (or the collector open) for a white noise generator, it does not seem outworldly that this is a tracing error that has been propagated over and over. This should also elimiate the mojo associated with this transistor.

I just started contemplating to build this circuit and currently am in the state of information acquiring, so I don´t have a board at hand.

Let me know if any of this works.

-helmut

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Re: Maestro FSH-1 / Oberheim VCF / Tonepad Filter/Sample/Hol

Postby aquataur » 08 Sep 2014, 15:57

(cannot edit the previous message)

Note that in the circuit you linked to (Odyssee) the transistor is a PNP, in which case the diode acts like a normal diode with 0.7V limit. Don´t know if this works as a noise generator. Replacing this with a 6.8V Zener will not be the same. If it were a NPN the replacement would be 1:1.

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Re: Maestro FSH-1 / Oberheim VCF / Tonepad Filter/Sample/Hol

Postby aquataur » 08 Sep 2014, 16:03

(Sorry for the multiple messages)
I found a source for the beforementioned synthesizer that uses exactly the same circuit the FSH-1 uses - namely an open base. It thus cannot be a tracing error.
Try connecting the pins.

-helmut
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Re: Maestro FSH-1 / Oberheim VCF / Tonepad Filter/Sample/Hol

Postby chicago_mike » 08 Sep 2014, 21:29

I'll try that out! :D

Thanks for the info.
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Re: Maestro FSH-1 / Oberheim VCF / Tonepad Filter/Sample/Hol

Postby chicago_mike » 20 Sep 2014, 12:53

HOLY SHITZKA!!!! :applause: :applause:

After some swaps,.I am90% there. I ordered LOTS of stuff of eBay and finally had a chance to get back to this.

Fets are 2N5457's Work fine
4558 for filter opamp.....I'll change this out later.
1458 S+H Works fine.

The other transistors are 2N5087/88 respectively.

THINGS TO CORRECT / FIX

1. Ticking. Adjusting trimmers helps. But is not 100% there yet. That's the first goal.

2. Volume output..is low. I need to correct this. Bypass is a bit louder

3. Filter sweep....seems a little swishy sounding. Like a whistle in the highs. Not bad but I am looking for more of a gurgle

4. BP output. SInce I play bass..I might add a Low Pass option. I tried it once, but I did not have the pedal working as is to really get the low pass part to go.

5. Blend pot. This will also be Very useful for bass. BUt I did build an FX blend pedal..so it's not that important.

For now I am happy. :D
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Re: Maestro FSH-1 / Oberheim VCF / Tonepad Filter/Sample/Hol

Postby aquataur » 20 Sep 2014, 15:08

Mike,

did you get around to try the noise transistor mod?
I have three other projects to get off the table before I can look into the FSH-1 so I have not chance to try myself.
The noise circuit itself is easy enough to breadboard, but I would not know if it makes the whole problem any better.

-helmut
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Re: Maestro FSH-1 / Oberheim VCF / Tonepad Filter/Sample/Hol

Postby chicago_mike » 21 Sep 2014, 01:52

I'm gonna do that mod tonight. :D

I am curious to how it behaves. And I think I know how to fix the other issues I am having.
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Re: Maestro FSH-1 / Oberheim VCF / Tonepad Filter/Sample/Hol

Postby aquataur » 12 Oct 2014, 11:11

I have made a PCB for the LFO and S/H section in lieu of driving my own version of the Colorsound Dipthonizer (with LM13700).
I managed to fix the noise generator, it runs with the mod I outlined earlier like a breeze with the first old transistor I found.

It does run with the Dipthonizer, but does not sound too interesting in combination with the format filters.

Read more on my web pages.

have fun,

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Re: Maestro FSH-1 / Oberheim VCF / Tonepad Filter/Sample/Hol

Postby oldrenegade » 24 Oct 2014, 20:22

Finished building this a couple of days ago and have been playing with the tuning ever since. If you try to tune as per the Sharkfin instructions - forget it!
Tune trimmer 1 (100k) in the filter mode and set it to work so as to give you some wah at both ends of the range scale then set trimmer 2 (10k) in the SH mode.
Down side is that the up/down now wont work unless you compromise on the Range setting so that you get no wah in the first quarter of the Range and that will then give you reverse sweep in the last quarter of the range when in down setting.
My build is bog standard except that I'm using JRC4580D chips but I think anything similar will work. Loving the pedal, great fun.
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Re: Maestro FSH-1 / Oberheim VCF / Tonepad Filter/Sample/Hol

Postby chicago_mike » 24 Oct 2014, 21:28

I want to try a couple mods. I almost had one mod working great, but never got around to finishing it.

Take a tap off the low pass filter and switch between that and the normal band pass.

Buffered blend control. This one I havent tried yet.

The low pass, I got but the volume output was way too low. I'll try it again and see what I come up with,
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Re: Maestro FSH-1 / Oberheim VCF / Tonepad Filter/Sample/Hol

Postby Nocentelli » 17 Jan 2015, 14:59

Finally got this working on the breadboard after far too many attempts over the last few years. I used a 2N2222A for the noise transistor, the first one I tried was the best, and I couldn't get any 2N3904s to work anything like as well (possibly why I had problems in the past). Other than that, it is almost identical to the Tonepad schematic, but using a 13700.

One thing I would definitely recommend is doing as Aquataur suggested, and connecting the noise transistor with BASE to -9v, and collector hanging (emitter to the op amp inverting input as per schem). With the base hanging, and collector to -9ve, i find the 100k trimmer needs to be set very carefully indeed to get any sort of decent random voltage blips, but with the Aquataur mod, I got decent randomness instantly, the 100k trimmer now seems to act as a frequency range control and the 10k trimmer acts as a depth control. I will probably box it with these controls as external pots - Video to follow.
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Re: Maestro FSH-1 / Oberheim VCF / Tonepad Filter/Sample/Hol

Postby Nocentelli » 17 Jan 2015, 16:10

brownwhopping wrote:How can I learn by reading threads an making circuits, when some day I can see a lawsuit or somebody beat me in the face for that?

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Re: Maestro FSH-1 / Oberheim VCF / Tonepad Filter/Sample/Hol

Postby aquataur » 28 Jan 2015, 16:09

Nocentelli wrote:(...) but with the Aquataur mod, (...)


Thanks for the roses. Good to know that one´s findings help others.
But "mod" is too much honor. IMHO it´s just an error that was traded over and over, and since most people don´t understand the background,they are hesitating to question the error. And I spotted it and it seems to work.

It did the job for me the first time I tried it, with the first tired transistor that happend to fall into my hands.

-helmut
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Re: Maestro FSH-1 / Oberheim VCF / Tonepad Filter/Sample/Hol

Postby grrrunge » 07 Apr 2015, 19:35

chicago_mike wrote:1. Ticking. Adjusting trimmers helps. But is not 100% there yet. That's the first goal.

Mike! Are you still working on this one?
I sold the ones i had, so i can't test it out myself, but perhaps i've got a solution for the ticking issue:
Try soldering a 47nF capacitor directly across the voltage supply pins on each of the op amps and the LM13700. It's a commonly used noise reduction technique in digital electronic circuits, on components that are prone to pick up clock noise via the voltage supply.
Let me know if you try this, and if you do: whether it worked for you or not ;)
Best of luck mate!
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Re: Maestro FSH-1 / Oberheim VCF / Tonepad Filter/Sample/Hol

Postby chicago_mike » 07 Apr 2015, 20:19

I'll give it a shot. Never really did finish these anyways.

I wonder if a higher capacitance would work too, like 10pf or 100pf
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Re: Maestro FSH-1 / Oberheim VCF / Tonepad Filter/Sample/Hol

Postby grrrunge » 07 Apr 2015, 20:56

The pF range is generally considered lower capacitance than nF ;)
The local supply rail decoupling caps are usually between 47nF and 100nF. Caps in the pF range will discharge to fast, and not give you the desired charge reservoir effect.
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Re: Maestro FSH-1 / Oberheim VCF / Tonepad Filter/Sample/Hol

Postby grrrunge » 29 Dec 2015, 21:54

I made a layout for this one, Using board mounted pots and switches. It fits snugly inside a Hammond 1590BB enclosure.
This layout includes decoupling caps on all op-amp supplies and multi turn trimmer footprints for the random S/H filter biasing. With the right IC's there's no ticking with this one :)

There's a handful of spare boards, if anyone's interested? I'll sell them for $5 plus shipping by regular mail. PM me for details.

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