Ground to inductor to ground capactior mystery

[sausage]

Hi all.

I'm currently troubleshooting a dead Commodore 64C and I found what I thought was a short across C56 which is a ceramic type capacitor. I took the part out and it was fine. Then I looked at the schematic and discovered this:



The point of interest is highlighted in blue. The Power Input comes from an external transformer. Pins 1, 2 and 3 on that plug are ground.

The mystery for me is this: The incoming grounding path goes through an inductor to the PCB ground. Before the inductor is one side of the capacitor. Essentially both sides of the capacitor will appear as a short, or at least plus the resistance of the inductor.

I'd be interested to know what this technique is for, and what it might be called so I can read up on it.

[M0HZH]

L1 seems to be a common mode choke; the DC return will minimize core flux in L1.

C56 will mostly have an effect at high frequencies (transients?).

[retrolefty]

Looks like a high-pass filter (cap/inductor) on the ground line. Helps to filter out noise.

[sausage]

Thank you muchly to you both.