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While perusing the schematic for the Sega Genesis model 2, I spotted these "EM1", "EM2", etc. components near the joystick inputs and video/audio outputs.

Schematic showing "EM1" and "EM2", both with values "101", across pins 1 and 3

Looking at a photograph of the board, the components themselves remind me of the older-style package for inductors, though it's a little hard to tell what's going on from above:

motherboard photo showing mysterious brown through-hole components

The same schematic also has several inductors, all using the usual "L" symbol prefix. (L2 is visible in the photo.) So these "EM" parts are presumably something different. The prefix itself suggests something like a ferrite bead, but do those usually have a value ("101" in the schematic) associated with them?

Worst of all: they're labeled as using pins 1 and 3 (where some unused pin 2 can just barely be made out under EM3 in the photograph) and I've never heard of a triple pin ferrite bead.

I can imagine they're almost certainly for some sort of protection or isolation or filtering, but I haven't been able to find anything about the symbol prefix or the components themselves. Could someone please help shed some light? Thanks!

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It's probably a 3 pin EMI filter like this but pin 2 isn't used: -

enter image description here

There's just a hint of circular clearance around the middle pin where the copper plane is removed (within the silk screen rectangular outline for EM3): -

enter image description here

It's possible that it's a C-L-C type where the centre-pin is a common point for the two outer pin capacitors and also there's an inductor between the two outer pins as well hence C-L-C.

If you could show a side view picture it might become clearer.

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They are EMI filters.

They are just ferrite beads or inductors, and they do exist in 3-pin versions as well.

The three pin versions have a capacitor inside them, with one leg connected to middle of two ferrites/inductors, and the other leg is pin 2 for ground connection. In this case, the part value of 101 would mean an EMIFIL with 100pF capacitance.

If it is simply a 2-pin ferrite bead, the 101 would mean 100 ohms impedance at 100 MHz.

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