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I'm trying to fix a Motorola 8 track player model Quadraline TM912, and I came across this component which was new to me, the so called feedthrough capacitor. On the image above, if the fuse E6 is not broken and the S2 switch is open, what should I get upon measuring resistance from the +Vcc node (12.6 VDC) to ground? Should the feedthrought capacitor behave as an ordinary capacitor, thus, having a zero resistance at first and then increase the resistance?

enter image description here

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    \$\begingroup\$ Short answer: yes, it's just a capacitor with lower ESL & ESR \$\endgroup\$
    – Mike
    Commented Jun 10, 2022 at 20:47

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The capacitance of feedthrough cap is ".002" units.

Assuming it means 2nF, the multimeter would measure that as open circuit.

But measuring that assumes that you have disconnected the component from other circuitry, as you can't measure capacitors in circuit, because other components on VCC and GND will affect the measurement.

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  • \$\begingroup\$ I'll measure it with no 8 track cartridge, thus S2 is open, and, with the +Vcc wire not connected to the car battery. I will measure it from the battery wire to the the ground wire. \$\endgroup\$ Commented Jun 10, 2022 at 20:52
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If you want to measure it, you should remove the fuse, otherwise you will be measuring the power supply as well.

It should act like a normal capacitor. At that value, 0.002uF, you probably won't see it charge when when connecting an ohmmeter because it will happen very quickly, so you should just see an open circuit. You could measure it with a capacitance meter if you have one.

What you should not see is leakage from the center lead to ground.

Feedthrough capacitors are used for high frequency bypassing, usually they are mounted through a shielded enclosure so that the connections from the circuitry inside the shield to outside of the shield are decoupled, keeping RF from passing through the shield on the wires.

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