The IQS 401 spectrum analyzer, a commercial addition to the Apple II and III personal computers, can be modified for low‐cost power spectrum measurements of random noise signals and similar waveforms. The IQSbasiccontrol program is changed to perform low‐frequency sampling, spectral averaging, and spectrum scaling. External analog filtering must be provided on the lowest sampling rates; however, no change is made to the original IQS hardware. Resolution of 0.5 dB is obtainable from 10−4Hz to 20 kHz.