02-23-2018, 03:08 PM
I experienced some arcing last night when I tried attached my scope ground to the chassis, so I was happy to see this post (I'm playing with a 38-7). I figured it is was due the two 0.015 caps on the power line, which is exactly what is discussed above. The caps form a voltage divider for the 60 Hz line frequency, and the impedance, as noted above, calculates to 176kohm, so you would expect, as Dan observed, to find the chassis about 60V above ground, and able to pass a peak current of a milliamp through a good conductor connected to ground (like my scope probe).
This got me wondering, what is the purpose of those input capacitors? Is it from an older time when the power lines were a lot noisier? Is it to shunt RF signals trying to leave the radio and go back to the power grid? The latter possibility seems unlikely because the power transformer probably blocks most RF stuff, and also I doubt FCC regulations of RF emission even existed back in the 30s, at least not to the degree they exist today on things like laptops, so its doubtful Philco was trying to deal with that.
Does anyone know the reasoning behind these things? I can't figure out what advantage these caps offer when the "cost" is putting a high impedance 60 VAC on the chassis.
I have seen some schematics posted in the Phorum that the cap was only connected between the two power lines, and not to ground. For shunting noise on a power line (if that is the goal) this probably is good enough. I'm going to isolate the caps from the chassis and see if I can sense any changes. I'm also thinking, as discussed by Dan, of going to a smaller cap. I just can't see much advantage to this capacitor.
I'd really enjoy learning more about the reasoning behind the "bakelite capacitors", if anyone knows the story.
Thanks,
Clif
This got me wondering, what is the purpose of those input capacitors? Is it from an older time when the power lines were a lot noisier? Is it to shunt RF signals trying to leave the radio and go back to the power grid? The latter possibility seems unlikely because the power transformer probably blocks most RF stuff, and also I doubt FCC regulations of RF emission even existed back in the 30s, at least not to the degree they exist today on things like laptops, so its doubtful Philco was trying to deal with that.
Does anyone know the reasoning behind these things? I can't figure out what advantage these caps offer when the "cost" is putting a high impedance 60 VAC on the chassis.
I have seen some schematics posted in the Phorum that the cap was only connected between the two power lines, and not to ground. For shunting noise on a power line (if that is the goal) this probably is good enough. I'm going to isolate the caps from the chassis and see if I can sense any changes. I'm also thinking, as discussed by Dan, of going to a smaller cap. I just can't see much advantage to this capacitor.
I'd really enjoy learning more about the reasoning behind the "bakelite capacitors", if anyone knows the story.
Thanks,
Clif
