The 50 watt power down circuit will begin to operate if the cooling fan on the rear panel fails to operate after 15 minutes of continuous transmission in order to protect the final amplifier circuit. Failure of the fan will prevent the power down circuit from returning transmit power to normal levels since the transformer temperature remains high. This trouble can sometimes be traced to a pinched/burnt wire near resistor R17 of the AVR unit (X43-3070-01).
If the red wire attached to the connector CN2 of the AVR unit is pinched between the chassis and ground the insulation might be damage and allow this line to be short to the chassis. This wire supplies 15 Vdc to the fan motor thru resistor R17. Excessive current is drawn thru R17 under this circumstance, approximately 1.5 A rather then the normal .5 A. This can cause the resistor to become red hot and damage the surrounding circuit board, since R17 is a metal oxide film resistor.
R17 should be replaced with a ceramic resistor if this symptom is encountered. Replacing the metal oxide film resistor with a ceramic resistor will prevent damage to the circuit board should the red lead become shorted.
10 ohm 2 watt ceramic resistor, R92-1285-05