You can find them on Reverb and Ebay.
"The clone is fully drop-in replaceable and is able to coexist with the original vintage chips in the same synthesizer. It can function as VCA and current source for envelope controller on M-cards, or control voltages for TRG boards (and on any other board that uses that chip). It has the same control voltage cutoff point and similar non-linear control curve as the original, so all the control sliders work the same."
Additional details follow:
"The iG00151AP is a replicant of the rare IG00151 Voltage Controlled Amplifier (VCA) chip, which was used by Yamaha in their CS-series analog synthesizers and organs from the 1970s and 80s. The iG00151AP was carefully engineered from the original chip, retaining all of its characteristics and properties, including the peculiar non-linear control curve and cutoff, input and output impedances, and identical power consumption at the same gain setting. This makes the iG00151AP the only "clone" available today that is fully drop-in replaceable and is able to coexist with the original vintage chips in the same synthesizer. While the originals are getting more expensive and harder to find, the availability of the iG00151AP ensures that the CS-series analog synthesizers remain serviceable. One CS-80 synthesizer contains 210 of these chips!INSTALLATION
To install the iG00151AP, ensure that the marked PIN 1 location on the part matches PIN 1 location on the circuit board. With the pins pointing down, PIN 1 on the original chip is on the left when facing the part number, while the iG00151AP has PIN 1 on the left when facing the component side (not the part number). Carefully orient the part properly and solder it to the circuit board. Clip excess leads as necessary.
WARNING: As with the original chip, a reverse installation will damage the part when power is applied!
CALIBRATION
After installing the part, allow the instrument to warm up and stabilize for at least 30 minutes, then recalibrate the circuit following the procedures of the original manufacturer. Similar to the original chip, the gain may vary from unit to unit. Proper calibration will ensure that the new part performs the same as the original.
As with the original chip, the iG00151AP may have a similar input offset that varies from unit to unit. This manifests itself as a control voltage (CV) feedthrough to the output. Depending on the application, it may be necessary to trim the offset, but this is usually provided in the vintage circuit, if required. No additional offset trim circuitry is necessary if it is installed in the original circuit.
The iG00151AP is compatible with Yamaha CS-80, CS-70m, CS-60, CS-50, CS-40m, CS-30, CS-20m, CS-15D, CS-15, CS-10, CS-5, and other synthesizers and organs that use the IG00151 VCA chip."
