I'll assume you want to control a Marantz 1504 using a red OK/Select button Comcast remote. There are three scenarios. First find out which Comcast remote you have by using the 983 signature blink back which will most likely either yield a signature of 3039 or 25362.
1) Press Setup until 2 blinks
2) Enter 983
3) Press the "1" button and count the number of blinks. No blinks means 0
4) Repeat step 3 using buttons "2" through "4" (or "5" if the signature starts with 2 blinks)
Steps 3 and 4 should yield in succession each digit of the signature.
A) If your remote is a version 3 (e.g. 1067BX3) with signature 3039, then you can command the following inputs:
B) If you have a version 4 (e.g. 1067BC4) with signature 25362, you can command the above functions plus:
C) If you buy a $8 USB to Serial interface cable, the free software RMIR will allow to additionally command:
Spotify (some 1504 models)
SIRIUSXM (some 1504 models)
Media Server Select
Pandora Select (some 1504 models)