Vintage Synth Repair

Your previous mail came, too...


I have SK20, do repairs of my equipment, but my instrument is OK so  
there was no need to open it and start to work on it. I also didn't  
study Service Manual more deeply. So I'm afraid I can't help much...


The only thing I can say is that Roland volume pedal has different  
connection, different pot inside, so generally it's not recommended to  
use it for Yamaha instruments (or Yamaha FC7 for Roland instruments -  
when I wanted to use it with my Roland V-piano, I had to change  
internal connection).


I'm not sure, but maybe this could cause some problems you describe.


Daniel Forro




On Feb 26, 2017, at 4:58 AM, Karl Knuttel pinksoir@...  
[vintagesynthrepair] wrote:


>
>
> Hi,
>
> My SK20 is exhibiting strange behaviour.
>
> The first issue is that the 'Strings' section signal is audible in  
> the 'Polysynth' section mixer output.
>
> To explain a little better, there is a mixer section for Organ,  
> Strings, and Polysynth before the Master Volume and main output.  
> When the String section fader is set to its minimum it can still be  
> heard through the Polysynth section. So when I have the Polysynth  
> fader up and play the keyboard, the Strings section can be heard  
> underneath the Polysynth output at a low volume. It also appears to  
> overdrive the output so that it slightly distorts.
>
> The bleed occurs only on the Polysynth mixer slider. If I set the  
> Polysynth so that it outputs nothing (ie. ADSR all set to 0, filter  
> set to lowest, etc.) and set the Polysynth mixer slider up and all  
> the other sliders down, when I depress any keys I can hear the  
> String section coming through the output.
>
> If I do the same with the Organ section, the problem doesn't occur.
>
> I checked the volume pedal input. There's strange behaviour there. I  
> plugged in a Roland EV-5 foot pedal and at first it altered the  
> volume of the respective sections (Strings or Mixed). Then it ceased  
> affecting the volume. There was a smell of solder/component burn  
> coming from the synth. There was a similar smell emanating from the  
> synth last night when I was checking it as well.
>
> So now when the volume pedal is plugged into either of the pedal  
> input jacks, the respective sections are muted. Very odd.
>
> I opened up the synth and took out the JK board but visually it  
> looks fine. No obvious damage to either of the ICs and the caps all  
> seem to be ok.
>
> The second issue is that the transformer now seems to be  
> overheating. I left the synth plugged in but powered off and the  
> transformer began to smoke and there was a smell of melt coming from  
> the area.
>
> Any help would be greatly appreciated. Here is the service manual: https://elektrotanya.com/yamaha_sk20_synthesizer_sm.pdf/download.html
>
> (Apologies if this is a double post - I posted directly to the group  
> but it didn't seem to go through)
>
> Cheers,
> Karl