Bad power supply

The unit has a bad power supply. One or more regulators may have failed and requires an extended power on period before it cam reach full power and close the circuit to power the final stages. One other fault could be a failed or toasted final amplifier transistor that won't output until hot. A can of freeze spray would find the culprit by spraying transistors/regulators while the unit is playing and the one the spray knocks out the sound is the one. There may be more than one so more searching may be needed.