Korg Poly-800 Chorus mod

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crafter57
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Korg Poly-800 Chorus mod

Post by crafter57 » Sat May 25, 2019 10:42 am

So, while looking at the poly-800 chorus board circuit I realized it closely resembles some flanger circuits. So, I'm wondering if I can get a Flanger sound out of the Chorus board. The LFO can be modded with pots for rate/depth but is that all it takes or is there more for that flanger effect? How to slow down the chorus?
Mooger5 you know about those bucket brigades..
any ideas are welcomed..

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Re: Korg Poly-800 Chorus mod

Post by Mooger5 » Sat May 25, 2019 8:58 pm

The main difference is that flanger uses a shorter delay time.
It goes from shorter to longer delay times like this:
Karplus-Strong synthesis, flanger, chorus, slapback, 'long delay', looper.
IMO flanger is a more radical effect and needs to be tweaked while chorus can be set to 'one preset fits them all'...
So, in order to prevent every patch from sounding almost the same, controls for delay time, modulation rate, feedback and depth should be available...
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Re: Korg Poly-800 Chorus mod

Post by Mooger5 » Sat May 25, 2019 9:29 pm

C45 seems to be the timing capacitor for the modulation. R61 the timing resistor.
R64 and R65 form a voltage divider. Replacing them with a pot should provide some control...
Ditto for R58 and R57 for Depth...

Not sure which cap is responsible for delay time. C44 seems to be there to round off the edges of the modulation wave. Think it´s C46: 1000pF is a value suitable for chorus. Plus, it´s styrene, low tolerance and steady value. Try a pot wired as rheostat in place of R69...
There is no feedback so it'll have to be thru trial and error. Most probably the path will go right before the output FET and the input...
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Re: Korg Poly-800 Chorus mod

Post by crafter57 » Sun May 26, 2019 8:36 am

Ok, here's the first draft, no totally sure about it and the pot values and also if I need limiting resistors?
flanger.jpg
, it would also be nice to put some led's for a visual effect ;)

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Re: Korg Poly-800 Chorus mod

Post by Mooger5 » Mon May 27, 2019 12:07 am

Your first try should be making sure C46 is the de facto clock capacitor. Temporarily replace it with a ceramic of a much shorter value, from say 10p to 180p and see if the chorus sounds like a flanger. It could sound glitchy.

Limiting resistors, always. For R61 for example, there must always be a resistor between opamp stages. It will prevent the pot from shunting the path between the stages. So use a value around 10K to 33K before adding the series potentiometer. The resistor will determine the slowest mod rate possible when the pot is fully turned left.

Leave the voltage dividers for now.

The feedback is in the audio path, actually. It´s also known as regeneration. A good place to start is at the junction of F2, R15 and C20. Then find a good spot to inject an attenuated, AC coupled portion of the wet audio signal back in the delay section.
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Re: Korg Poly-800 Chorus mod

Post by crafter57 » Mon May 27, 2019 11:28 am

I put a 10p as that's all I had on hand but did not hear much of a difference. I'll have to head out to buy some bigger values. I'm lost on the feedback, I'll try to research more on it..

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Re: Korg Poly-800 Chorus mod

Post by Mooger5 » Mon May 27, 2019 10:26 pm

Feedback in a delay line works much like resonance in a filter. Although it can be either positive or negative. When the delay time is long enough, feedback determines the number of audible repetitions. Same as echo. Because the effected signal gets processed over and over. If the delay is too short, it makes up for the ringing, metallic tone so characteristic of flanger effects.

So I just read the service manual more carefully as well as the MN3102 datasheet. Pins 5, 6 and 7 are meant to control the clock frequency. But pins 5 and 6 were left floating because Korg decided to run the 3102 in separate excitation mode.
There has to be a timing capacitor of a small value for the clock alone, leading to pin 7. And a resistor, later turned variable, to change its time constant at the player's will.
So try replacing C44 with the 10p capacitor and report back, please.
Turns out there's a comparator after it to convert the modulation triangle into a pulse wave followed by a saw converter of which C46 is at its hart, which is then followed by another comparator that converts the saw wave into pulse signals for the clock divider (thanks Korg).
If that doesn´t work, try C43. Doubtful but anyway. Then, pins 5 and 6 will have to be put to work.

Good luck!
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Re: Korg Poly-800 Chorus mod

Post by crafter57 » Fri May 31, 2019 2:45 am

sorry for delay I had to move a behemoth (casio Privia px830) off of my bench as I did not have any space left.
So, I replaced C44 with the 10pf and still sounds the same, should I try bigger values? or perhaps C43..

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Re: Korg Poly-800 Chorus mod

Post by Mooger5 » Fri May 31, 2019 11:09 am

No problem. Please replace c43.
Must be missing something here. The darasheet clearly ststes pins 5 6 and 7 determine the clock speed by mrans of rc network. If pins 5 and 6 arent usef then pin 7 alone makes the 3202 run under external time control.
There must be still something that sets the minimum or maximum delay time as the lfo alone wont make it. Msybe pins 5 and 6 floating determine the longest time by default so youll have to add the rc to override it...
More later.
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Re: Korg Poly-800 Chorus mod

Post by crafter57 » Tue Jun 04, 2019 6:30 am

Unfortunately nothing happened when I replaced C43..
hmm..I'm wondering if this is at all possible?

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Re: Korg Poly-800 Chorus mod

Post by Mooger5 » Tue Jun 04, 2019 3:49 pm

It is possible, it is just korgs own way that i m not used to and its a bit frustrating.
Try r69.
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Re: Korg Poly-800 Chorus mod

Post by crafter57 » Tue Jun 04, 2019 9:47 pm

Mooger, I found some info on this mod (2 ways), I'll work on some pics and get back to you before I attempt any changes as I don't know which will work best.

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Re: Korg Poly-800 Chorus mod

Post by Mooger5 » Tue Jun 04, 2019 11:04 pm

Eager to see them. The saw core resembles a lot the relaxation oscillator of the Polysix VCO, so making R69 adjustable could probably vary the delay time. My last bet is adding a resistor and capacitor to pins 5 and 6 of the 3102 to offset the time, as per the datasheet.
Damned complicated stuff. I think before Korg was bought by Yamaha they used to do things unlike everybody else for uniqueness sake (and costs saving).
Would rather see them ditch the complicated stuff and just use simpler clock gen and dual BBDs, for true stereo output, instead of their usual (at the time) arrangement.
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Re: Korg Poly-800 Chorus mod

Post by crafter57 » Wed Jun 05, 2019 5:50 am

Mooger, sorry could not get to the drawings, but yes, 1 talks about cutting off the input to pin7 putting a potentiometer between pins6&7 and a cap between 5&7, the other example removing R67 & R68 and install a potentiometer as a voltage divider between D+ and D- with a 10K resistor between D+ and the pot? Don't know what to do with the diode D6 ? and in the first example where do you re-insert the LFO signal once you cutoff the input to pin7?
what do you think?

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Re: Korg Poly-800 Chorus mod

Post by Mooger5 » Wed Jun 05, 2019 3:16 pm

Your questions are pertinent. Would like to know the source of those mods.
Q8 is a constant current source. D6 is there for thermal compensation. Removing it from the circuit would make the current fluctuate, affecting the saw wave.
I *think* I got it (someone please correct me otherwise):

Q9 is always either on because the base is positively biased by R70, or off because of the comparator IC8b.
When Q9 is on it shunts C46 and a current flows immediately from -D to the non-inverting signal input of IC8b at pin 5. At the same time a constant positive current also flows from Q8 to the same input, therefore what arrives is close to zero.
Now at pin 6 we have the reference voltage for the comparator. It is continually changing, but is kept above zero volts. Since pin 6 is the inverting input, the difference in potential between pin 6 and 5 makes the comparator go into negative saturation. This negative output is nulled by R70, opening Q9. This in turn stops the current at -D from flowing. It will have to build up through C46 that starts charging. This means the positive current from Q8 is at full blast. A positive pin 5 and 6 stops the comparator, triggering Q9 again, until C46 is fully charged and pin 5 returns to zero, and so on.
As the reference voltage is modulated by the LFO, the two positive charges at pins 5 and 6 coincide sooner or later, affecting the rate at which the comparator stops saturating and therefore the ramp time. If we offset the LFO output to a higher voltage, pins 5 and 6 will coincide earlier. R57 and R58 form one of the voltage dividers mentioned earlier and a small value pot in their place could probably do it (along with limiting resistors). Are you sure R67 and R68 were not a typo?
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