My Marantz SR-50 has developed a 50Hz hum thru both channels. It started as a fairly quiet hum and has gotten worse over time. The unit still works otherwise.

Looking inside I can see that a few components in the power amp stage have obviously become quite hot at some stage. There is a brown discoloration on the PC board underneath two pairs of small transistors (Q701, 702, 703 & 704). These transistors become quite hot to touch even when the amp is not loaded.

D810 (zener I think) on the power supply has also been quite hot at some time (dark brown colour on PCB)

My multimeter gives a reading of 39V across both of the 4700uF power supply caps.

Any help greatly appreciated.