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.
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:
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!
Best Answer
It's probably a 3 pin EMI filter like this but pin 2 isn't used: -
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): -
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.