I believe you are talking about an IR Emitter. If that is the case, on most receivers, it is to the right of the red record light. To test, you can put a piece of tape or something to block that area and see if your remote works. If it does not, that's where the IR sensor is. Hope this helps.
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A single thickness of tape will often not block IR signals into the IR receiverof the set top. In some cases, it actually only dampens it enough to improve accuracy of reception from a "too hot" remote control IR emitter.
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