4 Pole Mission Distortion
  • Hello, I have some troubles with my Shruthi 4-pole M
    In LP1 and LP2 modes, the sound level is very high and very distorded. It sounds like it's passed thru a Guv'nor distortion pedal..
    Pichenettes took a look at my filter board and it seems looks ok.
    I need some help to fix it please...
    I am not an electronics and my skills are very limited .. However, I have an oscilloscope to make measurements if needed.
  • Too much resonance? it will cause the signal to clip if the resonance is too high.

    Pichenettes is the expert, he designed it.

  • Does the filter sound normally with the (default) LP4 mode?

    If you have a scope, boot the synth, leave the init patch, and post scope pictures of the signals at pin 1 of IC1 ; and at those points:

    If you suspect the problem has something to do with resonance, remove the LM13700 (without the LM13700, no resonance).

  • hello, I have removed the LM13700 and it causes no sound from the Shruthi' Out .
    I've replaced it with a new one, and it makes the same overdrive in LP1 and LP2
    take a look at the pictures I've done and let me know what you think about it
  • not tested IC 1
  • Yes, removing LM13700 breaks the audio out, but it does not break the filter. I would have prefered seeing the scope traces without LM13700. It’s not because the audio out is broken that the filter is broken!

    Anyway, your traces look normal, so the problem is somewhere else.

    Check all the solder points around the LM13700 ; and please show the signal traces at pin 8 of IC2 ; with the filter set to LP4, LP2 and LP1.

  • here are some new test with pictures
    the three first pictures are from IC2 Pin8
  • now from the 4 test points
  • Test point 2 without LM13700
  • TP3 without LM13700 ( the line are not dual, this is due to my poor oscilloscope and my old cam )
  • same as the previous, there were not two traces... too long shooting time I think
  • LP4 is OK.

    Can you confirm that the signals on the 1, 2, 3, 4 pads shown above are identical when you switch the filter from LP4 to LP2? And doesn’t change much when you switch the filter from LP4 through LP1?

    If this is the case, the filter itself is working, the problem is in the pole mixing circuit – some nodes are working, some others are not. Check the soldering of the big row with all the resistors (above the 3 logic ICs). Maybe you could post a photo of those. Check the soldering of the 4051.

  • I don't know if I can really confirm, in fact it changes a bit as you can see on pictures.
  • I don’t have any pictures to compare the signal on the 1, 2, 3, 4 with the filter set to LP4 and LP2.

  • here is a picture from the board Assy where you can eventually see all the resistor, maybe I’ve made a mistake ?

    ( don’t know how to put a link for an Url here . hummm ) ( edited : thank you

  • check out the textile help menu at the bottom of the comment box. very helpful!


  • I’ve checked the resistors several times and they are OK. And the traces of the filter points 1, 2, 3, 4 are correct.

    Can you post an image of the signal at pin 8 of IC2?

  • I think you already asked me for it. ?
    there are on the 1st page
    IC2 Pin8 LP1

    IC2 Pin 8 LP2

    IC2 Pin 8 LP4

  • I don’t remember if it was with or without LM13700 sorry
    I’ll do the either tomorrow if you agree
    how can I test the relay IC ? ( I think the Ic of the bottom row are kind of relay ? isn’t it ? )

  • Oh wait, I think there’s something fishy with the 4th resistor on the big row of resistors between the rows of ICs.

    82k. It’s supposed to be grey, red, black, red, and it appears to have on your board another value, can’t say if the 4th ring is brown or gold. Would totally explain your problem, because this resistor sets the gain of the LP1 and LP2 modes.

  • ok, I’ll take a look at it tomorrow, I think it is gold..
    maybe it’s a 82R one ?? shit so..
    Thank you for your eyes to linx

  • Hummm… you’re right.. ! the 4th resistor is a 82R and all my 82k resistors are 82R ( thank you Klaus … musikding )

    Very good sound. I replaced the 82R resistor by a 82K and now that's ok !
    thank you Olivier
    the sound of the Shruthi is always very good. Now with the 4PM it has new futures.
  • I know this is an old thread, but I’m having the exact same issue but on poles 3 and 4 so I assume it’s the gain on 3. Anyone happen to know which resistor controls this?

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