The Fairchild Semiconductor μA726HC or UA726HC, has been used in a prolific amount of synthesizers spanning from the 70's up until the early 80's. Intersil (Renesas) then created the CA3046 to succeed it, which is now also an obsolete part and no longer actively manufactured. For some reason it's sometimes said not many synthesizers used this component (a staggering amount do), but considering it was one of the only solutions at the time for temperature stability, it found its way into many things.
The μA726HC is a matched transistor pair with both a heater (oven compensation) and temperature sensor. The IC resides in a potted epoxy compound to ensure thermal stability. In the context of an analog synthesizer, this is to provide pitch stability of the voltage controlled oscillator(s) as the machine undergoes changes in temperature; the oven compensation will keep the transistor pair operating at a higher reference temperature than the ambient temperature to reduce flucations that may affect tuning of the oscillators.
Obviously, the μA726HC was used in other devices at the time (generally military equipment), and in fact, I don't suspect its original intention was pitch stabilization for voltage controlled oscillators, it's likely this component predated the concept.
Here's a list of synthesizers I know that use the μA726HC along with the (suspected) component count needed. I couldn't find a list like this anywhere so started building my own.
Curiously enough, the way Moogs, Buchlas and Rolands all use this component varies a bit which makes building a 'universal' replacement more complex.
There doesn't seem to have ever been a reliable/consistent replacement created for the μA726HC, however, some alternatives exist:
Constantin Papageorgiadis (papz) created the 'PA726' which is a partial replacement to the μA726HC, with a handful of caveats to consider:
1. It can only be used for synthesizers leveraging pins 3 and 10 on the original μA726HC, if they leverage others this won't work.
2. It is dramatically larger than an original μA726HC and won't fit in all synthesizers, the Jupiter-4 is a good example of that as the voice cards are so compact.
3. It is built around the CA3046 which is now, too, an obsolete discontinued part but infinitely more available.
Chinese μA726HC clones
For awhile some clones were being produced (notable from the fact they don't have gold plated legs as that is needlessly expensive to do in our modern time), there were reports of some working very well, but others destroying the synthesizer(s) they were installed in (oops).
dr_korg μA726HC clone - https://www.instagram.com/p/_iBObRBnr4/?hl=en
Martin Janson was working on a compatible μA726HC replacement using a highly compact circular PCB and SMD design to ensure size compatibility. However, there were some issues with them running in Buchlas over Moogs (and presumably no testing done on how Rolands respond), no update since then.
Rob Keeble took the System 100m oscillator design and rebuilt it to exclude μA726HC, I'm not certain how this was accomplished but in theory it may be possible to build something to retrofit existing synthesizers to omit it.
On the later IR3109-based Jupiter-4 voice cards, they actually used *selected* μA726HCs with a specific range. This is why later Jupiter-4 voice cards have painted marker on the top! That means if you do replace a μA726HC on a later Jupiter-4 voice you will most likely need to measure and test the component... which make it astronomically worse because just getting a μA726HC at all is tough. The values tested for are explained in the manual (and it's bizarre nobody servicing these has ever brought this up as this has to be the most terrifying μA726HC repair yet):