Hello, Here is what likely happened. After 20 years, the electrolytic capacitors in the power supply have started to dry out (common life for these is 15 years tops). With the power button left on all the time, the caps never fully discharged and maintained a charge. Normally when you shut the main power off, the caps drain and upon applying power they recharge. The thing here is that they have not discharged in quite some time, due to the power switch being left on. When you shut it off, they were discharged for the first time in a long time and being as they are 20 years old, they are now failing to recharge, which is showing the receiver a short.
Fortunately, capacitors are both inexpensive and easy to replace. The first thing I would do is have a Tech check the caps for shorts and leaks and then replace them with caps of the same rating in Uf and Voltage.
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