Korg Polysix Noise

Synth Repair
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1

[Korg] [PolySix] – [Noise]

Hi,

I’m working on a Polysix which has a very noticeable white noise issue when volume is set to normal levels. I traced it back step by step but can’t figure out the cause. Here’s what was done so far:

  • The noise can be modulated by the attenuator and LFO(VCA modulation) setting just like the normal audio.
  • It is NOT impacted by filter, envelope, effect, waveform , key assignment settings.
  • Once i disconnect the main line going from the voice to effect board, it disappears.
  • went back on the voice cboard and removed components starting with filter chips, decoupling caps, op-amp and finally the last adder op amp (before connector) but the noise didn’t go away. (i was removing the set of 6 similar components together at a time, e.g all filter chips together to be 100% sure. For the op amps, all 3 wehre removed at the same time.
  • When the voice-to-effect boards line is removed, i notice the ground voltage difference is about 11v on the two connectors ; however, i think one is main ground and the other is for audio. Not sure about what i should expect, i guess zero V.
  • I tried to put decoupling cap between the voice and effect board but there was no difference.
  • PSU voltage are perfect without any ripples or noise.

Any ideas or recommendations ?

2

Any signs of corrosion or battery leak on the boards?

3

Sounds like a capacitor has failed.

If there are through hole tantalum capacitors in that area, use a multimeter and test of any have shorted. Tantalum caps either explode spectacularly or short internally with no external signs.

4

It had a few but very minimal around the battery itself. The battery was replaced and the impacted lines were repaired.

5

Ok will check but wouldn’t these be on power lines ? Asking because voltages are perfect and the power transistor are running at normal temp.

6

I’d thoroughly test continuity around where the minimal battery leak was. When you say ‘repaired’ was the conform stripped the area neutralised and then repaired. The corrosion can keep going if the corroding agent is still at large.

7

The corrosion was very minimal and addressed properly and is no longer a concern. Did you experience an audio noise caused by a battery leakage? To the best of my knowledge, that board is handling the digital control part and has no audio signals passing through it. It does have some ADCs and DACs though.

8

It depends on how the tantalum caps failed. The power supply my be compensating for the failed cap if it failed open, but where that cap is feeding to is still affected. Its the biggest reason why the repair industry is replacing through hole tantalum caps with electrolytics.

That being said,

The SMD mounted ones do not seem to have the same issues, laptops using them rarely ever fail so its definitely something in their construction.