Seventy-five years ago this month, the September 1946 issue of Radio Service Dealer
https://worldradiohistory.com/Archive-Radio-Service-Dealer/40s/RSD-1946-09.pdf
addressed the issue of how to deal with intermittent problems in radios. The magazine called them time wasters, because there was really no way to service the set until the problem manifested itself. And Murphy’s law being what it is, that generally meant that the set would work just fine when the serviceman was ready to work on it.

One solution to the problem was the circuit shown here. The set would be put on the bench hooked to a signal generator. The leads to the set’s speaker would be disconnected and hooked to the input. By carefully adjusting the control in the circuit, and the set’s volume control, the set would play while the serviceman worked on other sets. But when the signal strength started to drop, the bell would sound, and attention could immediately turn to the problem set.

The magazine even suggested that this circuit could be used as a final check for all sets serviced in the shop, to make sure there was no hidden fault before the set was returned to its owner.