Several of Don's VCO designs use the µA726 temperature controlled exponential pair. These are now rare and hard to find. MEMS designed a replacer PCB which works in all of their modules. It consists of a AS3046 transistor array with transistors used for the exponential pair, chip temperature sensor, and heater.

Here are a pair of them installed on a 258MC.

This replacer does need to be calibrated. These are the instructions I wrote for calibration.

µA726 Replacer calibration instructions PDF

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