One of the components of a voltage controlled oscillator (VCO) is a linear to exponential converter, that converts the 1V per octave control voltage to a per-hertz reset pulse for the sawtooth waveform.
Such components can be done in various ways, but one of the most traditional is to use the exponential part of a transistor's voltage-to-current transfer function (not sure what the actual term is here). The issue is that this function is very sensitive to temperature, and so fluctuation in temperature on this part can easily detune the VCO by a very audible amount. Fluctuation is less of an issue at higher temperatures (around 70-80°C), so the solution was: heat!
The trick we implemented in the synth was to use a transistor array in a chip: use two transistors for the Lin/Exp converter, another as a heat source (we nicknamed it "the oven"), and another as a heat measurement device, to get some sort of PID feedback loop. I believe there were five transistors in the chip, so there was a leftover that got used for another purpose.
Surprisingly efficient, though we could still hear some pitch drop when blowing on the transistor array chip. Once in the enclosure, we figured only applying some ice on the front panel would have an effect.
This "heating trace" in the PCB would have been a good alternative, as some of those transistors arrays were particularly hard to source (we needed a particular transistor type, can't remember which).