Neutron versus maxon Af9

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I build the neutron about 3 yrs ago (see other posts)
it sounds really good. I didnt use optocouplers, but one green lee and 2 ldrs
Besides that it has mods as a blend pot, and a volume pot

Because of the size of it's housing, and the fact I use it a lot in the studio, i bought a af9 from maxon as backup.
It simply doesnt compare to the neutron. Sweeps les dramatic, distortion us an issue ans simply a less luscious sound.
I do love the firmfactor of the 9 serie, so i rather mod it to sound like my beloved neutron.

I can't seem to find a schematic, but at first glance  it's a mutron clone. If anybody has midding tips and a svhematic it will be most aprreciated. I'll podt my progressions.

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First question. What ibviously are the optocouplers, the parts with 437 on it. CAn't seem to find the specs for them

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The plot thickens. According to the webside of maxon its a hd 837 from hamamatsu.
The datasheet shows a similar looking component, but with a different name
http://www.digchip.com/data/190/190-01469-0-P873_series.pdf

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Replacing the optocouplers with a led ldr combi, just like in my neutron.
The neutron has a normal green led and 2 ldr's
Seems it doesnt give enough sweep. Now testing white, and green brught leds, anyone experences with this?

P.s. Found that the print trces are letting loose fromthe print thats annoying..... Maxon doesnt take mods into accounr...