Home Music Synth Stuff Store Contact

Fairchild μA726HC

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.

What the hell is it?

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.

Synthesizers That Use μA726HC

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.

Replacement Solutions

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:

Modifying VCOs to Omit μA726HC

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.

Jupiter-4's Dirty Secret

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):

  • *Excerpt from Roland's manual*
  • μA726HC (IC2) Factory selected with color dot. Resistance of R8 is to be determined according to the color on IC2.
    • Red -- 82K
    • Green 100K
    • Non --120K

spotify bandcamp soundcloud twitter itunes