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258v Trimpot Layout
MUFF WIGGLER Forum Index -> Buchla, EMS & Serge  
Author 258v Trimpot Layout
My 258v's Square wave is all screwed up. It sounds weak and buzzy, more like a sawtooth than a square. Might try to recalibrate before I reach out to Mark V to get it fixed.

Does anyone know or have the layout of the trimpots on the circuit board and their function? if the waveshape is a trim pot it might have just drifted a bit.

As always thank for your help!
If you have four trimmers/osc look here:
Ugh I searched so much but somehow missed this, thanks so much!
The waveform trimpot should likely be adjusted like on other 258s. Set the front panel waveform pot to sine/minimum then trim the waveform trimpot to sine only. Now the front panel waveform pot should produce a clean square at maximum level; the waveforms are less "together" at lower levels than full. If it still sounds like less than a square at max front panel pot position there's something else going on.

There are two things to check if so. Q9 is very important; check for a square at Q9. If you can follow the square wave through the schematic and it's good, but still sounds bad at max level at the output, R46 should be adjusted up to 470K so the square is correct at the full waveform pot position. It's an offset for full level and full level is required for the correct waveform mix. You'll likely have to again reset the trimpot to null to sine. chematic_358.jpg 0_1_200.jpg

For reference, it appears that Roman made R46 a trimpot on his V3 258 build.
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