Here’s one for the old timers, I’m working on a model 771 amplifier from a Wurlitzer 616 and the complaint is,
“ There is significant hum primarily at 60Hz as seen in the attached spectrograms. The hum is 30dB SPL over the noise floor, is constant regardless of volume and is still there when the input is disconnected.”
I set up the amplifier on my bench and played an iPod through it with field coil and volume box from the jukebox and I get perfect sound and dead quiet with or without input.
I rebuilt the amp a few years ago but was surprised when I saw the volume box modification, it just had straight wires to volume pot and back out again. I’m only familiar with the mods for the 1940’s volume boxes. Is it different for these early amplifiers? I’m at a loss as to what might be causing this hum in his jukebox but was curious about the lack of resistor or capacitors in the volume box. Anybody familiar with these? Thanks, Mauro
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