Its probably the AC-4 power supply connector to the back of the TR-4, it’s a 
common problem. The speaker and filament grounds are shared through the 
connector and if a bad connection develops between the AC-4 and TR-4 due to 
dirty or worn out pins the AC filament voltage will make its way through the 
speaker, thus a loud 60 hz hum. Try cleaning the power supply pins on the TR-4 
with alcohol and a stiff brush or if need be a ink eraser. 

73, Gary

R-4A T4-X R-4B T4-XB R-4C T4-XC MS-4 L-4B RCS-4
2A 2B 2C 2NT 2CQ 2AS 4-LF
TR-7 RV-7 CW-75 MS-7
WH-7 W-4 WV-4 MN-2700 MN-4C MN-2000
UV-3 PS-3 TR-22C TR-33C AA-10 AC-10 MMK-22 TR-6

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