Recently swapped out old Scientific Atlanta 8300HD DVR for a new one. Since my remote (Silver with Red ok button) was getting along in age, I swapped it out at the same time. Big mistake.

The old remote worked fine with my Marantz SR7200 receiver. The manual for the new one lists only one (5 digit) code for all Marantz receivers, which doesn't work. I laboriously went through the manual's instructions for finding a code that would work, but no dice. Should I just give up, or am I missing something here?

BTW, I have ordered a URC R50, and expect it will allow me to retire that new Comcast remote.

Please try to blink back the remote's signature. We may be able to suggest a setup code from that, or perhaps to discover that there is a completely new Comcast remote running around.

1) Press Setup until 2 blinks.
2) Enter 983
3) Tap Digit 1, and count the number of blinks. No blinks mean zero.
4) Repeat step 3 for digits 2 through 5.

The standard red button Comcast will return 3039, but let's see what yours says. Probably only digits 1 through 4 will respond, but check digit 5 just in case.

It came back as 30391, so yes I did get a response from digit 5.

So that is a standard red OK/Select button remote. Comcast remotes have always only provided one setup code that will work with Marantz receivers, and it is 30189. This will control Vol+/-, mute, Power toggle, and perhaps a few other things.
A few issues to consider: A stock Comcast remote will route volume signals to the TV. To make volume signals go to the receiver, set up volume punch through, using the 993 command. That's explained in the instruction sheet, or see the 9XX command descriptions.  Similarly, the digit buttons, OK, and chnl+/- are sent to the cable box as default.  Use the 973 command to disable channel lock.

Another possibility is that someone has already programmed the remote to send custom signals on the AUX button.  Clear all of those by
1) Press AUX,
2) Press Setup until 2 blinks
3) Enter 994 (2 blinks)
4) Press AUX twice
The remote should blink twice.

If all of this fails, ask Comcast for a replacement.

Many thanks for the detailed reply! Looks like this one is going to have to go back to Comcast.