Jul 072013

Many Polysix’ will be hit by a sudden photocoupler death somewhen.
There are many “facts” around on the internet describing how do determine whether it has failed and how to substitute the unavailable IC.

Fact 1: it is not true that you can check the photocoupler by measuring the voltage drop on the 4k7 resistor inseries with the LED or even the voltage drop across the LED.
LEDs do fail without exhibiting any dramatic change in electrical parameters. You will find LEDs which drop 1.6 volts @ 20 millamps and emitting no light!
(That – and some scientific applications – is why some applications still require incandescent lamps, and that’s why I’ve designed a controller for a 1960s filament winding machine in 2010!)

In this special case, the LDR saturated at 5.4 megohms with full LED current, so either the LED has become dark or the LDR insensitive.
The only way to check the photocoupler is to remove it from the circuit and measure the LDR resistance versus the LED current and compare it to a known working part (see diagram below)

Some background: the photcoupler is used in a feedback loop to stabilize the expo converter which makes up the V/Hz scale for the VCOs from the octave/V control voltage.
To use only one expo, Korg has used multiplexers to feed the 6 VCO CVs to the expo, followed by a calibration voltage and then the CV for standard pitch (only for the new production voice board).
The two latter outputs are routed to two independent controllers; the last one introduces an offset to the expo input to shift the standard pitch to its desired value, while the converted calibration voltage drives a current through the photocoupler’s LED, while its LDR is placed where you would normally expect the 3300ppm tempco resistor in similar circuits.

This leads to an eady way to check whether a tuning problem is related to the photocoupler circuit at all: measure the voltage on pin 7 of IC18. Within regulation it should be somewhat between -2 and -6 volts, -3 are more normal.
When the loop breaks open due to failure of the photocoupler or related parts, it will be stuck on the maximum negative output, something around -13 volts. There must always be a small current through the LED, otherwise the loop does not work and the TUNE HIGH preset has no effect!

Fact 2: the types VTL5C2 and VTL5C3 which are widely discussed as replacments are simply the wrong choice. In order to make the regulation work, the LDR resistance needs to be in the range of below one to a few kiloohms. The maximum LED current is limited by design to slightly over 2 mA. According to the datasheet, a VTL5C2 has 2 kiloohms at 2mA – regulation will never take place! Even if it seems to work, the controller will work close to the margin and operation cannot be guaranteed. Furthermore, resistance variation is quite large for those devices; the VTL5C2 I have tested had about 20kohms at 1mA.

The diagram below shows a known good original part (blue graph) and a VTL5C9 (yellow graph, values from the datasheet)

As you may have expected from my previous text, the red line represent the data sheet values for a VTL5C2.
While the VTL5C9 closely matches the original part in the most interesting region, the VTL5C2 resistance is way too high for any current.

What about RoHS…?

Photocouplers with LDRs are, as any LDRs which are commonly based on CdS (cadmium sulphide), forbidden for new products within the EU for some years now.
There is an exemption for the use in electronic music instruments, because there is no practical solution to control varying ac voltages better with LDRs.
(MOSFETs, for example, become highly nonlinear introducing distortion).

In this special case – a closed control loop and very small voltage variation across the device – a photo-mos device would probably be possible.

But it won’t be a solution for the control of audio signals in many applications. The EU has decided that science will have developed and indutry produces an alternative from december 31, 2013 on.
Science hasn’t, and industry doesn’t – except from Macron, who even try to force the EU to withdraw the exemption. Unfortunately, their devices are now way available at major european distributors by now…

Although it is always a good way to reduce hazardous substances, eurocracy has lost any sense of proportion for this topic. While power toys (sorry, I refuse to claim such crap tools) with short life spans flooding the market at low prices are allowed to contain batteries with large amounts of Cd, small photocouplers containg micrograms of Cd typically used in high-priced gear which will last for decades and serviced in case of a problem are to be banned.



  12 Responses to “Polysix photocoupler myths”

  1. Thank you for posting this. I’m an electrical engineer that does analog synth stuff for a living. One of my biggest peeves is reading all of the bogus stuff that people say about electronics in the forums. It’s always refreshing to read something based on science. I found your posting useful. Thanks for the good work.
    -Rob Currier

  2. This is very interesting. I don’t think I have ever seen the original P1501 spec sheet with diagrams. I’ve used VTL5C2 as well as a couple of Silonex parts in Polysix’s. Of course, the value of the series resistor has to be decreased substantially to get high enough current to turn the LED on bright enough. I have seen a spec sheet that showed both P1501 and VTL5C2 with R-on= 500 ohm and R-off=1MOhm, but I expect the “on” value is at very different current levels. I don’t think you will find a forum post that mentions changing the optocoupler without also lowering the value of R93. Still, these high current levels might be decreasing the life span of the new part. I think I need to order some VTL5C9 and try them with the original resistor value next time. Speaking of forums, are you a member of the Polysix Yahoo group? That’s where you will find the most active discussions about optocoupler replacement, and many people grateful for this type of information.

  3. Great article, thank you. Absolutely right about bureaucrat regulators not seeing the forest for the trees, and failing to “keep it real”. Absurdity run amock. In conclusion, does the VTL5C9 work as a replacement?
    ps- how did you figure out the tuning algorithm Korg used in the Poly-6?

  4. Thank you for posting. On my Polysix I read -13 on pin 7 IC18. Just ordered VTL5C9 from Smallbear where he list VTL5C9 as replacement part for P1501 used in the Ibanez AF9 Auto-Wah

  5. Hi Stefan,

    I’ve found this post too late and i’ve already bought some VTL5C3/2 to replace the old HTV1501 assuming that the issue was there.
    I’ve changed also the resistor to lower the R value as read on P6 group, but nope.

    After reading this post, i’ve checked the IC 18 Pin 7 and I always obtain 0v.
    This happens with both configurations:
    HTV1501 and 47K R39 (original setup)
    VTL5C3/2 with 22K on R39.

    I’ve replaced the IC 18 also, but nope.

    Do you guess where i have to dig?
    My P6 is completely out of scaling, and each key note behave different from each other.

    Thank you very much.

    • Zero volts against circuit ground? Seems impossible, this control loop is always somewhat oscillating because of the large time constant, and whenever out of control, the op amp output should either be +Vcc or -Vcc, in this case -Vcc trying to push enough current through the photocoupler LED.

      • Yes, seems strange but that’s it, as you can see in these pics.

        That’s the pin poked with probe (GND probe clamped to GND point)

        That’s the oscilloscope reading (1V / 1mS div)

        Am i doing it wrong?

        I’ve replaced the PC1 with a VTL5C9 placing it upside down to match the anode / cathode, but issue is still there.

        I had already swapped these ICs on KLM366:

        I’ve changed the assigner Mask Rom 8049-217 on KLM 366 and now sounds much better, the scaling is not scrambled anymore, but it’s just out of scaling at same intervals, i.e.
        C4 = C#5
        C#4 = D5
        D5 = D#5
        and so on…

        I guess the issue is somewhere in the Assigner section on KLM366.
        But, where to dig?

        Thanks anyway for support, your help is much appreciated!

  6. Hello,
    Sorry, I’ve made a mistake about the value reading from IC18 pin 7, due to a wrong setup of the oscilloscope.
    The actual output value is +15V.

  7. Hi,

    I’ve put in a VTL5C9 with a 4K7 trimpot and still can’t get it scaling correctly. I get -2.3V on pin 7 of IC18. IC17, 18 & 19 have been replaced, also I have the Kiwisix CPU and power supply installed.

    Any suggestions on what to try? Or any measurements I can do with a multimeter or oscilloscope to identify the problem part?


  8. Hi,

    I am using a replacement circuit for the aged optocoupler. It is based on a FET (BF245) plus an OP-Amp to improve the FET linearity. If you are interested, have a look at:



    • Thanks for this interesting solution!

    • Hi Stefan,

      it seems, that I have the optocoupler problem too.
      Your solution looks very good. How did you realize the solution practically?
      Did you make a little PCB? If yes, could you give it to me?

      VG Rolf

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