increasing impedance of the filter capacitors at low frequencies can mean that low frequency swings in the current drawn by output stages can cause voltage swings in the power supply voltage which feed back to earlier stages, making the system a subaudio oscillator. This is caused by inadequate power supply filtering or decoupling. The electrolytic capacitors used in equipment of 1960s vintage contained liquid electrolyte, which dried out over decades, decreasing the capacitance and increasing the leakage current, and these are often the cause. One solution suggested is a "capacitor job", replacing all the old electrolytic capacitors. A more radical but comprehensive solution is to add modern IC voltage regulators, or replace the entire power supply with a modern regulated one. In radio equipment In equipment that includes radio transmitters, motorboating can be caused by radio frequency interference (RFI), the strong radio signal from the transmitter getting into audio or receiver circuits. Receiver audio circuits with automatic gain control (AGC) have a long time constant feedback loop which adjusts the gain of the audio stage to compensate for differences in audio level from causes like different speaking voices. Squelch circuits used in two-way radios to cut out noise similarly have a feedback loop which turns off the audio when high frequency noise is detected. If the inaudible radio frequency (RF) transmitter signal is inadvertently coupled into the receiver's audio signal path, it can trigger the AGC or squelch circuit to reduce the gain. Then, after a delay time set by the circuit's time constant, the circuit increases the gain again until the amplitude of the radio signal triggers another gain reduction. This repetitive cycle is heard as motorboating. An example might be a 27 MHz Citizen's band radio in a car, connected to the car's 12 volt DC supply. If the decoupling capacitors which bypass radio noise from the power supply wires are missing or inadequate, or the long power leads pick up excessive RF from the antenna then it is possible for the RF transmitter signal to enter the radio's receiving circuits through the supply wires. This then causes the motorboating to occur. References Radio electronics