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 Forum index » DIY Hardware and Software » The Repair Shop
Mono/Poly noise in CV?
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agustindead



Joined: Jul 22, 2015
Posts: 33
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PostPosted: Wed Jul 27, 2016 1:30 pm    Post subject: Mono/Poly noise in CV? Reply with quote  Mark this post and the followings unread

Hi, I'm having some trouble with a Korg Mono/Poly and I wonder if someone could help me locate this issue... When I play the synth, I get a cyclic interference, like a low frequency FM that comes and goes. I thought I had some strange modulations going on at first, but its frequency and the period it takes to appear and disappear don't have anything to do with the LFOs or any other settings...

Attachment is a little recording of me pressing the keys without any VCOs mixed in (you can hear some background noise and hum... is this OK?), and then I fade in an arpeggio with only VCO1.

It seems like the noise is generated with the controlling CV in the keyboard section... I used the CV out with external gear and the same happened with those VCOs. Also, I was able to play with no interference through the CV and trigger inputs (well, I actually hear some interference but it's definitelly much quieter and static, it doesn't come and go).

Here is the service manual for the synth, the keyboard schematic is on page 10:
http://dl.lojinx.com/analoghell/KorgMonoPoly-ServiceManual.pdf

I don't have a scope sadly, but the +5v supply feeding the big 8049 IC oscillates slightly on my meter, in phase with the interference I hear. Also all four CV connections oscillate in the same way. Still, it seems like it's not a power supply problem, since it measures a steady 5v once I disconnect the KLM-356 board (which handles the keyboard input and CV generation) from the supply.

Can anyone point me in the right direction? Where should I check? Thanks in advance.


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