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Discussion about the Korg PolySix synthesizer

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MG / LFO Q1 transistor replacement.

MG / LFO Q1 transistor replacement.

2014-03-09 by <josh.nursing@gmail.com>

What would be a good replacement for Q1 in the MG circuit?

The original seems to be 2SK30A.

Jed's updated parts list for the clone suggests a 2N7000 (not tested I think).

From some web research, a substitute may be the MPF102.

Would these last two work?

Re: [PolySix] MG / LFO Q1 transistor replacement.

2014-03-10 by <backshall1@bellsouth.net>

The 2SK and 2N parts have different pin-outs. The clone board is designed for a 2N transistors. To put one on an old original KLM-367 you would have to do the “crossed legs” thing. Actually, I think the Gate is still in the middle, so you would just put it in backwards, no bent legs, just reversed Source and Drain. I’ve been using 2N3819 without problems on the clone boards, and it still seems to be available at Mouser in the US. You could always be the first to try 2N7000 and let us know. I don’t know enough about FET characteristics to say what the 7000 would do in this circuit. DIYstompboxes.com has some discussion about what other people have tried  as a replacement for 2SK30A in some guitar pedals.
Don Backshall
 
Sent: Sunday, March 9, 2014 11:51 AM
Subject: [PolySix] MG / LFO Q1 transistor replacement.
 
 

What would be a good replacement for Q1 in the MG circuit?

 
The original seems to be 2SK30A.
 
Jed's updated parts list for the clone suggests a 2N7000 (not tested I think).
 
From some web research, a substitute may be the MPF102.
 
Would these last two work?
 

Re: [PolySix] MG / LFO Q1 transistor replacement.

2014-03-10 by Bob Grieb

According to the data sheets, the 2SK30A is  JFET.   The 2N7000 is a MOSFET.    I would suggest replacing a JFET with another JFET, not changing transistor types.

Bob


From: "backshall1@bellsouth.net" <backshall1@bellsouth.net>
To: PolySix@yahoogroups.com
Sent: Sunday, March 9, 2014 10:44 PM
Subject: Re: [PolySix] MG / LFO Q1 transistor replacement.

 
The 2SK and 2N parts have different pin-outs. The clone board is designed for a 2N transistors. To put one on an old original KLM-367 you would have to do the “crossed legs” thing. Actually, I think the Gate is still in the middle, so you would just put it in backwards, no bent legs, just reversed Source and Drain. I’ve been using 2N3819 without problems on the clone boards, and it still seems to be available at Mouser in the US. You could always be the first to try 2N7000 and let us know. I don’t know enough about FET characteristics to say what the 7000 would do in this circuit. DIYstompboxes.com has some discussion about what other people have tried  as a replacement for 2SK30A in some guitar pedals.
Don Backshall
 
Sent: Sunday, March 9, 2014 11:51 AM
Subject: [PolySix] MG / LFO Q1 transistor replacement.
 
 
What would be a good replacement for Q1 in the MG circuit?
 
The original seems to be 2SK30A.
 
Jed's updated parts list for the clone suggests a 2N7000 (not tested I think).
 
From some web research, a substitute may be the MPF102.
 
Would these last two work?
 


Re: [PolySix] MG / LFO Q1 transistor replacement.

2014-03-10 by Adrian Corston

2SK30 are still readily available...  They used to be very common and there are most likely millions of units of NOS out there.

http://www.smallbearelec.com/servlet/Detail?no=1206

http://www.vintageplanet.nl/pages/roland2.html

https://www.futurlec.com/Transistors/2SK30pr.shtml

http://www.taydaelectronics.com/2sk30-2sk30atm-y-n-channel-50v-6-5ma.html

http://www.ebay.com.au/itm/2SK30A-Y-Original-TOSHIBA-N-Ch-FET-2SK30-K30-K30A-x-20-/130579781608

If the part number suffix/colour matters then you might need to shop around a bit.

Cheers,
A.


On 10/03/14 1:14 PM, backshall1@bellsouth.net wrote:
 

The 2SK and 2N parts have different pin-outs. The clone board is designed for a 2N transistors. To put one on an old original KLM-367 you would have to do the “crossed legs” thing. Actually, I think the Gate is still in the middle, so you would just put it in backwards, no bent legs, just reversed Source and Drain. I’ve been using 2N3819 without problems on the clone boards, and it still seems to be available at Mouser in the US. You could always be the first to try 2N7000 and let us know. I don’t know enough about FET characteristics to say what the 7000 would do in this circuit. DIYstompboxes.com has some discussion about what other people have tried  as a replacement for 2SK30A in some guitar pedals.
Don Backshall
 
Sent: Sunday, March 9, 2014 11:51 AM
Subject: [PolySix] MG / LFO Q1 transistor replacement.
 
 

What would be a good replacement for Q1 in the MG circuit?

 
The original seems to be 2SK30A.
 
Jed's updated parts list for the clone suggests a 2N7000 (not tested I think).
 
From some web research, a substitute may be the MPF102.
 
Would these last two work?
 

Re: [PolySix] MG / LFO Q1 transistor replacement.

2014-03-10 by Bob Stewart

Think I got a load of 2sk30a s from Little Diode. Ebay is also a great source for discontinued components.



On Monday, March 10, 2014, <backshall1@bellsouth.net> wrote:
 

The 2SK and 2N parts have different pin-outs. The clone board is designed for a 2N transistors. To put one on an old original KLM-367 you would have to do the “crossed legs” thing. Actually, I think the Gate is still in the middle, so you would just put it in backwards, no bent legs, just reversed Source and Drain. I’ve been using 2N3819 without problems on the clone boards, and it still seems to be available at Mouser in the US. You could always be the first to try 2N7000 and let us know. I don’t know enough about FET characteristics to say what the 7000 would do in this circuit. DIYstompboxes.com has some discussion about what other people have tried  as a replacement for 2SK30A in some guitar pedals.
Don Backshall
 
Sent: Sunday, March 9, 2014 11:51 AM
Subject: [PolySix] MG / LFO Q1 transistor replacement.
 
 

What would be a good replacement for Q1 in the MG circuit?

 
The original seems to be 2SK30A.
 
Jed's updated parts list for the clone suggests a 2N7000 (not tested I think).
 
From some web research, a substitute may be the MPF102.
 
Would these last two work?
 

RE: MG / LFO Q1 transistor replacement.

2014-03-10 by <josh.nursing@gmail.com>

Don, true for being careful about the pinout: I did the cross-legged thing with a transistor in my Poly-800 but I don't like it much as between the legs and the sheath, there's not much clearance unless you do some careful leg twisting.

Bob, thanks for the pointer about using another JFET and not a MOSFET. The MPF102 seems to fit.

Tayda is very cool, although they don't have everything I would have liked. Futurlec beats them on price on the SK this time.

Thanks all.

Re: [PolySix] RE: MG / LFO Q1 transistor replacement.

2014-03-10 by Chromatest J. Pantsmaker

You can always buy small heat-shrink to put on the leads or some small plastic tubing.

In a pinch, strip the insulation from some wire and use that bit of insulation on the transistor leads.

-chromatest


On Mon, Mar 10, 2014 at 9:39 AM, <josh.nursing@gmail.com> wrote:


Don, true for being careful about the pinout: I did the cross-legged thing with a transistor in my Poly-800 but I don't like it much as between the legs and the sheath, there's not much clearance unless you do some careful leg twisting.

Bob, thanks for the pointer about using another JFET and not a MOSFET. The MPF102 seems to fit.

Tayda is very cool, although they don't have everything I would have liked. Futurlec beats them on price on the SK this time.

Thanks all.



Re: [PolySix] MG / LFO Q1 transistor replacement.

2014-03-15 by The Old Crow

  I used a 2N3819 when I made replacement boards.  These days, I would use a J112. --Crow

On 3/9/2014 10:51 AM, josh.nursing@gmail.com wrote:
What would be a good replacement for Q1 in the MG circuit?

The original seems to be 2SK30A.

Jed's updated parts list for the clone suggests a 2N7000 (not tested I think).

From some web research, a substitute may be the MPF102.

Would these last two work?


Re: [PolySix] MG / LFO Q1 transistor replacement.

2014-03-15 by <josh.nursing@gmail.com>

Thanks Scott,

By the way, how is it you were planning the 'second osc' on the Polysix? Are you using the inverted input used for Offset or just mixing it in at the non-inverted input like the additional Noise mod does?

Also, a friendly reminder to publish your info about that triple resonant filter mod! 

:P

Re: [PolySix] MG / LFO Q1 transistor replacement.

2014-03-15 by <chipaudette@yahoo.com>

Whoa, I didn't know that Crow did (or considered) doing a 2nd oscillator.  I've been working up to doing that, too.  In my trials, I've been using the inverted input on the SSM 2044.  

Being new to working with the 2044, I was surprised at how much you have to cut down the signal level before feeding it into the 2044 (down to the mV scale?).  For some reason, I didn't like how my wavetable oscillator sounded when i did that...it lost a lot of its lively ear-catching qualities.  I'm hoping that with some more optimization of my in-line voltage divider and AC coupling cap, I can get it to sound better.

Or, maybe if I switch back over to mixing my signal into the non-inverting side, it would sound better (though I don't know why that should matter). 

Thoughts?

Chip

Re: [PolySix] MG / LFO Q1 transistor replacement.

2014-03-15 by <josh.nursing@gmail.com>

Old Crow has crazy mods up his sleeves, but he's too busy playing with Oberheim and Yamaha CS-80 hacks and goodies now :P :P :P

My thoughts: the SSM2044 datasheet do suggest you should use the inverting input for a second oscillator, but you have to build a 3dB difference so that the oscs do not cancel completely when out-of-phase.

That's all fine, but I have checked all the schematics of synths using the SSM2044 and none of them use that inverting input, which is strange. I've looked at Waldorf, Siel/Opera, Korg Trident, etc...

Most use that to remove offset from the first input. I also more-or-less remember Olivier Gillet (of Mutable Instruments) writing about some issues with offset on the SSM2044 at the time he was designing that filter board for the Shruthi-1 (said filter board is since discontinued but I believe I saved schematics somewhere).

So we're faced with two things: the original SSM advice of using -IN for the second osc, and also no practical implementation as such, probably because of that offset issue.

So Chip, it's very worthwhile if you managed to test it. I actually was going to do the same with a single voice until I zapped my Arduino Uno!

In the meantime, I checked Johannes' Noise mod and he just mixes in with Osc 1 through a 33K resistor. If I'm not mistaken, in the Trident and some other synth with several 'oscs' before the filter stage, they do the same.

I guess both work, and the mixing one is very easy and we're 100% confident it should work since there are real-world implementations. The other solution of using -IN could be less than ideal despite the original recommendation because of that Offset issue.

I may test using the simple mixing solution, first with Noise, then with inputs to each filter. I should be getting some 33K resistors soon.

Re: MG / LFO Q1 transistor replacement.

2014-03-24 by <josh.nursing@gmail.com>

Got the MPF102 this afternoon, checked the pinouts, and decided to twist G and S, so that D is in mid position like the original DGS arrangement when looking at the flat side, the pins down.

Used chromatest's suggestion to add some wire insulator on these two legs. Just twisted them and pushed the transistor so that the tubing goes down to the PCB, the two friction forces (tubing vs tubing and PCB vs tubing) keeping the tubing put.

Just tested the LFO and it worked straight away.

Re: MG / LFO Q1 transistor replacement.

2014-03-25 by <josh.nursing@gmail.com>

You'd have corrected: "so that G is in mid position" :)

Side effect as I haven't re-done calibration: my LFO is faster than before. Some fun sound effects there.