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 http://home.roadrunner.com/~w8pu 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-4 TR-4C TR-4CW/RIT RV-4C MC-4 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 PRN1000
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