The Sine-Square Audio Generator IG-5218 was produced as a kit from 1977 till 1990. IG-5218 was the replacement of the IG-18 (1969-1975) and IG-18A (1975-1977). They nearly differed in style and cabinet color.
The generator can provide a sine wave signal from 1 Hz till 100 KHz and a square wave signal from 5 Hz till 100 KHz with less than 50 ns rise time.

The oscillator uses a small  90V lamp in the positive feedback loop to stabilize the output, oscillator voltage at the lamp is 8,3 VRMS. As the output voltage increases, the lamp resistance increases, which in turn lowers the gain of the amplifier, reducing the output.

This kit was assembled more than 30 years after production. Many components where highly corroded and had to be cleaned before soldering. All lectrolytic capacitors where replaced with new ones.

Heathkit always provided very detailed step-by-step assembly manuals, often with multiple fold-out pages.

To improve the already low Total Harmonic Distortion (THD) even more, an "IG-18 Meter Mod Kit" from DataPro was installed. The non-linear loading of the meter rectifier diodes can create harmonics in the analog signal path and affect the THD of the final output.  The modification includes a buffer amplifier to isolate the meter rectifier diodes from the output signal path.

The original line cord, strain relief and internal fuse have been replaced with a standard IEC 60320 connector (in this case a line filter) and an external fuse holder. All hot parts are insulated with heat shrink as electrical shock prevention.