[Hallicrafters] Hallicrafter S-77

[radiomanL]

A low level hum is a normal condition for line operated  receivers . it generally is not audible over 12" from the speaker unless there is a problem in the power supply filtering or a HK short or leakage in the final audio tube . The hum level is higher that what you hear in a transformer operated receiver due to the use of a half wave rectifier . a larger electrolytic at the point where the B+ connects to the 25L6 plate may help but be careful not to exceed the maximum current rating of the rectifier .  in a quiet room with no other background noise it may be audible . reversing the line cord may help .  BE AWARE that some , not all line operated receivers use the chassis as B - . if one that does is connected the wrong way there will 125 vac on the chassis and if the chassis to case insulation is not good it will appear on the case if  metal  . a small 300 watt isolation transformer is a good safety investment . 

andrew
kd5pnt