Hi all. I am working on fixing up an old radio and have noticed that touching the top steel casing of a capacitor (electrolytic I believe) is causing a large increase in current to be drawn in the radio. It also makes a loud audible noise.

I understand most likely my body is acting as an antenna and is inducing a varying EMF that I am exerting onto the capacitor. However I did find it a little weird as the outer metal casing should be electrically isolated from the 2 pins of the cap (other than EMI of course)

So my question is does this seem completely normal? Has anyone tried lightly touching top of cap to a power amp (with finger or meta object) before to see if it produces a similar result?

Photo of capacitors (C309, C310)


Video of loud noise
https://youtube.com/shorts/JGHKUX80EKQ?feature=share

Power amp circuit with the cap locations highlighted in red (I believe the caps are absorbing the DC component of the input?)

 

Hi all. I am working on fixing up an old radio and have noticed that touching the top steel casing of a capacitor (electrolytic I believe) is causing a large increase in current to be drawn in the radio. It also makes a loud audible noise.

I understand most likely my body is acting as an antenna and is inducing a varying EMF that I am exerting onto the capacitor. However I did find it a little weird as the outer metal casing should be electrically isolated from the 2 pins of the cap (other than EMI of course)

So my question is does this seem completely normal? Has anyone tried lightly touching top of cap to a power amp (with finger or meta object) before to see if it produces a similar result?

Photo of capacitors (C309, C310)
View attachment 341176

Video of loud noise
https://youtube.com/shorts/JGHKUX80EKQ?feature=share

Power amp circuit with the cap locations highlighted in red (I believe the caps are absorbing the DC component of the input?)
View attachment 341175

You have rediscovered AC hum. Yes, completely normal when you touch input components to a sensitive audio amplifier.

 

You are "loosely" capacitively coupling your body antenna to the amplifier.

 

Cool. Thanks guys