Built a dynakit st70 last year. Bought a PAS 3x (1969) to go with it. Has some hum issues but found that the PAS had about six cold solder joints that I fixed and hum subsided but still there. It's very low but gets louder as the volume is increased. Tried isolation transformer on the mains, put everything on the same outlet. Made no difference. When I connect the PAS to my Harmon Kardin 230 no hum. When I connect the PAS-NGT to the Harmon Kardin no hum. When I short out the inputs to the st70 lno hum. When I connect either PAS to the st70 there's hum. Doesn't matter if I connect an input to the PAS or not it hums the same. I measured a 300micro volt DC difference between the chassis of the st70 and the PAS but when I jumper them together the hum gets much worse but the difference in voltage disappears. I do not understand how the PAS preamps can work fine with the Harmon Kardin but not the st70. And why would the hum increase with I jumper the chassis together? Shouldn't that have eliminated any ground loop? I could not measure any AC difference between the st70 and PAS as the meter I have isn't sensitive enough and I haven't gotten around to putting my scope on it yet. I was hoping I could figure this out but obviously, I can't. So if I do hang my scope on it where should I look? And for what. The hum is obviously 60 cycle. That's for sure. Some times the hum varies with the volume control, other times it doesn't? What's that about? Could this all be a problem with the internal wiring of the st70? The wires from the rca inputs go right to the grids on the 6GH8A's and aren't very far from the power transformer. Maybe a couple of inches (3-4). Would shielded wiring help? This is soo frustrating. Been trying to eliminate the hum for months. Any help would be greatly appreciated.
Best Regards,
Paul