[sdiy] Korg Poly61 a peculiar design!!

Tom Wiltshire tom at electricdruid.net
Thu Apr 7 12:23:19 CEST 2011 

You've really got to do *something* to it. I've just been reading the user manual. It's interesting to compare with the service manual.
The main problem is definitely the incredibly low resolution on most parameters. (Osc detune in 7 steps, filter cutoff in 63 steps, resonance in 7 steps, etc etc). Dumping the presets and adding pots for the CVs isn't a bad option for this particular synth, I'd say. Certainly it's a lot easier than building a new master uP board, although that wouldn't be impossible, if you had half-way decent schematics. Neil J has such a project on with his Opera 12 voice.

One thing that struck me was that although the two oscillators are quite different internally this isn't exposed to the user at all (one being a genuine ramp-core-with-timer-reset (Juno style) DCO, the other is a sum-of-octave-squarewaves (OSCar style?) DCO. I wonder why they did them differently.
I also wonder why they used a fairly complicated filter design (although only 12dB/oct) when they must have had thousands of SSM2044s sat around the factory that they were using in virtually everything else they built around this time.

All in all, I agree with Karl's assessment; "a peculiar design".

T.

On 7 Apr 2011, at 01:25, karl dalen wrote:

> Nice but why not move the knobs ontop of panel print.
> 
> KD
> --- Den tors 2011-04-07 skrev Doug Terrebonne <dougt55 at yahoo.com>:
> 
>> Från: Doug Terrebonne <dougt55 at yahoo.com>
>> Ämne: Re: [sdiy] Korg Poly61 a peculiar design!!
>> Till: "karl dalen" <dalenkarl at yahoo.se>, synth-diy at dropmix.xs4all.nl
>> Datum: torsdag 7 april 2011 04:09
>> I'm temped to do this to mine. Dump
>> the digital control for pots - 
>> 
>> http://www.flickr.com/photos/37595587@N00/sets/72157622283778293/
>> 
>> Doug
>> 
>> 
>> 
>> ----- Original Message ----
>>> From: karl dalen <dalenkarl at yahoo.se>
>>> To: synth-diy at dropmix.xs4all.nl;
>> Paul Cunningham <paul at cometway.com>
>>> Sent: Wed, April 6, 2011 3:45:43 PM
>>> Subject: Re: [sdiy] Korg Poly61 a peculiar design!!
>>> 
>>> Uses two 8049 micros, a major rebuild are needed new
>> DAC+mux board.
>>> Could be done but needs lots of work.More work then to
>> build
>>> new different from scratch. 
>>> 
>>> KD
>>> --- Den tors 2011-04-07 skrev Paul Cunningham <paul at cometway.com>:
>>> 
>>> my Poly-61M suffered some severe battery damage,
>>>> and i fear that my motherboard is toast. too many
>> of the
>>>> traces are corroded and as well as the pins of
>> several
>>>> resistors and the CPU... I've seen how some
>> people fix this,
>>>> and perhaps I'm just not up for it.
>>>> 
>>>> I would love to just replace the 1980s
>> motherboard with an
>>>> arduino or something similar. Does anyone know
>> actually how
>>>> dependent the synth board is on signals from the
>>>> motherboard? I've seen some impressive Poly-61
>> mods that
>>>> basically add real-time control to several
>> parameters, but
>>>> some other oscillator signals are generated by
>> the CPU?
>>>> 
>>>> The Poly-61 seems like an awesome sounding synth
>> hobbled by
>>>> a simple programming model. I'd love to augment
>> it with 21st
>>>> century software, or in the least a decent
>> analog-based
>>>> control system.
>>>> 
>>>> I think I have the schematics somewhere, but I
>> struggled a
>>>> little bit to understand how all the circuits
>> were related.
>>>> Is this what's in the service manual PDF? I'll
>> have to
>>>> figure out what I have...
>>>> 
>>>> i'd love to find a working replacement KLM-509A
>> board if
>>>> anyone knows where to find one. -pc
>>>> 
>>>> 
>>>> On Apr 6, 2011, at 6:02 PM, Dan Snazelle wrote:
>>>> 
>>>>> I actually like the sound of the poly61 for
>>>> SOMEthings!!!
>>>>> 
>>>>> 
>>>>> 
>>>>> 
>>>>> Sent from my iPhone
>>>>> 
>>>>> On Apr 6, 2011, at 5:46 AM, Tom Wiltshire
>> <tom at electricdruid.net>
>>>> wrote:
>>>>> 
>>>>>> Have you got a link to some decent
>> schematics for
>>>> the Poly 61, Karl?
>>>>>> 
>>>>>> I've got a service manual PDF for it,
>> but it's
>>>> more-or-less unreadable.
>>>>>> 
>>>>>> Thanks,
>>>>>> Tom
>>>>>> 
>>>>>> On 6 Apr 2011, at 01:44, karl dalen
>> wrote:
>>>>>> 
>>>>>>> VCF:
>>>>>>> 12db LP LM13600 using a MS20 VCA
>> with limiting
>>>> diodes in resonance
>>>>>>> feedback but before VCA also the
>> feedback not
>>>> only goes to the input
>>>>>>> of the filter but to first cap like
>> half way
>>>> S/K and no local OTA
>>>>>>> gain loops but one fixed over both.
>>>>>>> 
>>>>>>> I have checked trough all other
>> analog Korg
>>>> products and this VCF
>>>>>>> are unique to the P61.It should have
>> a
>>>> peculiar Bw hump because of
>>>>>>> this.
>>>>>>> 
>>>>>>> VCA:
>>>>>>> MS20 final VCA. One SSM2056 EG split
>> use VCA
>>>> and VCF.
>>>>>>> 
>>>>>>> DCO1:
>>>>>>> It's a complete saw VCO core, a
>> regular Op amp
>>>> integrator/comparator
>>>>>>> design. The ramp reset comparator
>> also feeds
>>>> another comparator that
>>>>>>> sends this signal back to *both* 1/3
>> of DCO1
>>>> and DCO2 8253 timers gate
>>>>>>> input, then one of the 8253 timers
>> sends a
>>>> pulse back to the integrator
>>>>>>> NPN ramp reset transistor in the
>> VCO!! Odd!
>>>>>>> 
>>>>>>> Could this perhaps mean that half of
>> timers
>>>> are actually used to measure
>>>>>>> the frequency of the VCO and if
>> wrong in
>>>> realtime restart it on a per
>>>>>>> cycle basis?! It could work if the
>> value are
>>>> compared to a predefined
>>>>>>> LUT. Or is it just a bogus thing to
>> distract
>>>> anyone viewing the schematic?
>>>>>>> 
>>>>>>> The output of the ramp integrator
>> are sent to
>>>> a comparator to create
>>>>>>> PW wave.
>>>>>>> 
>>>>>>> DCO2:
>>>>>>> DCO2 are made out of 1/3 8253
>> driving a 4520
>>>> divider who's output's
>>>>>>> are selected by a switches and
>> summed by a
>>>> weighted resistor net
>>>>>>> acting like a DAC to create saw. A
>> redundant
>>>> signal goes to divider
>>>>>>> reset to actually turn off the
>> chip..
>>>>>>> 
>>>>>>> Apparently resolution of wave are
>> likely
>>>> dependent of octave selected
>>>>>>> otherwise only one switch would be
>> needed.
>>>>>>> 
>>>>>>> VCO ramp integrator note CV are a
>> 7bit R2R
>>>> generated by the same CPU
>>>>>>> that scan's KBD that then feed a
>> common expo
>>>> who are muxed out to
>>>>>>> each VCO. (Polysix/Trident)
>>>>>>> 
>>>>>>> The driver for the timer DCO's are a
>> VC RF
>>>> multi vibrator who in turn
>>>>>>> are driven by bend and vib voltages,
>> the CLK
>>>> signal are then driven
>>>>>>> into a prog divider to create
>> semitone offsets
>>>> for DCO2, finer detuning
>>>>>>> presumably by 8253 timer itself.
>>>>>>> 
>>>>>>> There are no regular muxed DAC for
>> the
>>>> parameter CV's instead each
>>>>>>> has its own DAC trough a bag of 8255
>> driving a
>>>> even larger bag of
>>>>>>> weighted resistor net's.
>>>>>>> 
>>>>>>> DAC for Cf of VCF are only 6 bits.
>> However
>>>> PWM, VCF, Vibrato modulations
>>>>>>> are full resolution from the analog
>> triangle
>>>> MG.
>>>>>>> 
>>>>>>> In other words an awful sounding
>> poly but a
>>>> great, peculiar and funny
>>>>>>> VCO/DCO design.
>>>>>>> 
>>>>>>> Disclaimer: I might be utterly wrong
>> in all
>>>> that said here.
>>>>>>> 
>>>>>>> Thanks for your
>> attention....G'day...
>>>>>>> 
>>>>>>> KD
>>>>>>> 
>>>> _______________________________________________
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>>>>> 
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