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The 4075 is a filter module that ARP used in many different synthesizers, including the Omni 1 and Omni 2. Sometime during the design phase, a resistor value was miscalculated; as a result, the 4075's cutoff frequency will not sweep up as high as ARP really wanted it to. This may be partially responsible for the commonly held view that ARP filters do not sound as "snappy" as Moog filters. Timothy Smith designed a modification which will allow the cutoff frequency to sweep up as high as ARP meant it to. The mod simply involves changing four resistors on the board from 4.7 to 2.2K. I have performed this mod on an Omni 1, and Omni 2, and a Quadra, and in all cases there was a very noticeable improvement in sound. I highly recommend it!
The original ARP synth documents don't contain schematics of the filter modules. I got this schematic from Joachim Verghese's ARP Tech page:
Mike Novak created this very cool 4075 board layout diagram using Joachim's schematic and the filter boards from his ARP Quadra. This makes it much easier to find the four resistors in question:
