Korg 35 ic test circuit

[gkmaia]

Digging over the internet I come across this test circuit for the Korg 35. The test circuit has been draw by the same person who designed the IC for Korg decades ago.

On node for pin 1 there is a V1 or VI not sure. I am struggling to understand what that means. Is it a DC voltage source? Signal input?

[andy3055]

It looks more like V1. There is already a V-in in the circuit.

[ozcar]

It's used as a VCF isn't it? VI would be the frequency control voltage.

[LateLesley]

A quick bit of digging found this.

"The Korg 35 is a masterpiece of simplicity. Inside the chip there are five transistors and six resistors. By adding a small number of external components around the Korg 35 it can be configured as a high pass or low pass music filter. There is one in the MS-10 configured as low pass and the MS-20 adds a second Korg 35 for it’s high pass function. Thus was born the distinctive and quite aggressive Korg filter sound.

The Korg 35 is a form of Sallen Key filter that uses clever transistor biasing to enable the filter to become voltage controllable. Later MS-20’s for some reason discarded the 35 module and replaced it with a chip-based OTA transconductance filter."

https://secretlifeofsynthesizers.com/the-korg-35-filter/

So I would surmise that V1 is a voltage input to control the filter point. I'd assume Vin is the audio signal. I may be wrong though.

[andy3055]

It's used as a VCF isn't it? VI would be the frequency control voltage.

I thought so too, but then that could be controlled by the input signal frequency. The explanation from LateLesley makes sense.

[ozcar]

Well, other than I called it VI, rather than V1, I took what LL said to be confirming my take on it.

Do you have a sick one that you are interested in a test circuit?

[gkmaia]

I tried to inject a sine 500hz signal on VI and it produced some sort of modulation within the square signal but not idea what to make out of it.

What i produced resemble a bit what to expect, but it is still far from it.

I am a bit stuck here. Not sure if the whole circuit is working or not...

[Synthtech]

A quick bit of digging found this.

"The Korg 35 is a masterpiece of simplicity. Inside the chip there are five transistors and six resistors. By adding a small number of external components around the Korg 35 it can be configured as a high pass or low pass music filter. There is one in the MS-10 configured as low pass and the MS-20 adds a second Korg 35 for it’s high pass function. Thus was born the distinctive and quite aggressive Korg filter sound.

The Korg 35 is a form of Sallen Key filter that uses clever transistor biasing to enable the filter to become voltage controllable. Later MS-20’s for some reason discarded the 35 module and replaced it with a chip-based OTA transconductance filter."

https://secretlifeofsynthesizers.com/the-korg-35-filter/

So I would surmise that V1 is a voltage input to control the filter point. I'd assume Vin is the audio signal. I may be wrong though.

That quote is actually written by me, Secret life of Synthesizers is my blog. Thanks for doing the right thing and adding the link. I have some new old stock Korg 35’s in my spare parts inventory, I repair MS and PS series synths and have been meaning to try out that test circuit but I have to say that I have only ever come across a failed Korg 35 once and that was in a PS3300 that I restored so I never had need for a test rig. Perhaps I could build that test circuit and plug a 35 module in and take some measurements.

Pin 1 of the Korg 35 is the cutoff frequency control pin where the summing node for the various filter cutoff control voltages are connected. You can find the service manual for the MS-10 synth on the web easily and study the circuit.

[fzabkar]

You can find the service manual for the MS-10 synth on the web easily and study the circuit.

https://elektrotanya.com/showresult?what=MS-10%20korg&kategoria=&kat2=all

[gkmaia]

So this K35 is for my Korg 500 micro preset. I had several oxidised traces that killed most of the features.

After those were fixed I am still having issues with the traveler. When Synth 1 & 2 or Brass are selected and I move Traveler to ZERO I get no sound. Not sure if that is how it is supposed to work as the traveler feeds 0/+12 to pin 2 of the K35

When on Synth 1 & 2 I get a crackle noise on the background.  Sound is not clear.

Other very odd thing. I put the old K35 apart and replaced all components as per that manufacturer schematic. But I get no good sound when using them.

So I decided to make my own little PCB, using the same wiring and components and my one outputs sound for Synth 1 & 2. And i also could manage to get changes in sound when changing VR6 and Vr11 which are the ones that will dosage between 0 and -12v on pin 2 of the K35.

I read the old epoxy they used could get conductive... but not sure. I did test conductivity on these old K35 pcbs and got none.

I will do another DIY K35 today and try to run with 2 DIY ones and see if that changes things... that is odd.