Korg Poly 800 coil replacement??

agustindead · Posted: Tue Jul 19, 2016 9:37 am Post subject: Korg Poly 800 coil replacement??

Hello everyone, I need some help with a dead poly 800... I'm not sure this was the problem, but the -5v rail wasn't giving me the expected voltage, I saw a coil in bad shape but definitely ruined it when taking it out.

The thing is... what is this? How / where do I look for a replacement? What does it do in the circuit? Is there any way to work around it? I asked around a bit but had no luck... They usually only had single coils, with two terminals and had no idea what this was. I don't know how to specifically ask for it anyway...

I'm attaching a picture plus the schematic.

Thanks!

coil.JPG

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poly 800 coil.png

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agustindead · Posted: Tue Jul 19, 2016 9:48 am Post subject:

Full schematic page:

poly800 schem.jpg

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blue hell · Posted: Tue Jul 19, 2016 12:45 pm Post subject:

Yes coil .. actually two coils wound on the same core, which makes it a transformer.

In this case it is used in a switching power supply to generate a more or less stabilized -5V supply. The exact value of that voltage probably is not too critical, but will have a look at the schematic and when my assumption was wrong I will write a bit more on it.

You probably only melted the plastic a bit which is not essential for the functioning, and could try to solder the wires into the PCB again and then stabilize the while thing with some sort of glue.

Replacement would need some experimentation, as the transformar is a bit od a critical component in this configuration.

Alternatively you get all of the -5V generator out and buy a DC/DC converter to do the job for you.

blue hell · Posted: Tue Jul 19, 2016 12:52 pm Post subject:

Ok, what I see in the schematic is that the -5V is used on some analog switches and on the big divider chip to allow for signals around zero .. so it is not too critical indeed, like -4.5 or -5.5 would work too. Don't know how badly it deviated tho ...

agustindead · Posted: Tue Jul 19, 2016 1:35 pm Post subject:

agustindead · Posted: Wed Jul 20, 2016 2:08 am Post subject:

I was able finally to make the synth start. I did a really ugly job winding and gluing that thing in place, but I was able to get -5.5v and then it worked beautifully. It also took me some time to realize I should tie two wires to use that as the center tap... now I understand the sizes of the primary and secondary in the schematic...

Again, thanks for the help! Sweet having that thing back and running. Very Happy

blue hell · Posted: Thu Jul 21, 2016 1:51 pm Post subject:

Brilliant!

Was just about to ask some questions, but you found and answered 'm already :-)