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Warm-up issues are usually related to electrolytic capacitors. Take a look at the power supply output voltages (5Vdc, 3Vdc, 9Vdc, etc…) with an oscilloscope and look for excessive ripple. I’d bet that one of those supplies will have an excessive ripple when cold, but it will decrease as the coefficient of the filter cap alters with heat.
If you don’t have a scope, use Freeze spray to find a dead cap. Let the unit warm up and get it playing. While it’s playing, spray each capacitor on the secondary side of the power supply one at a time. Give the sprayed cap time to cool (2 to 5 seconds). When you hit the dead cap, the unit will malfunction.