Hold on, not so fast…
If the recorder plays analog tapes but not DCC’s, it may just be that the head is dirty.
While there is a cassette in the recorder (preferably one that’s not a prerecorded cassette but does have something recorded on it) and the recorder is powered on and in STOP mode, hold both the DOLBY and REC PAUSE button while pressing PLAY. This makes the recorder go into service mode. Then use the PLAY button to start playing the tape. You should see some numbers on the screen like “z xx yy ffff” where z is the selected head, xx and yy are the error rates for head z and head z+1, and ffff is the number of frames on tape that didn’t have any errors on it. Use the TIME button to switch from head 0 (and 1) to head 2 (and 3), 4 (and 5), 6 (and 7), and 8 (auxiliary track).
Normally the xx and yy numbers should be 00, or may count up to a low number sometimes for some of the heads. The ffff number normally counts up at about 5 frames per second; it’s normal for it to reset on a regular basis except maybe on a brand new, perfect tape. If your head is entirely broken, you will see high xx and yy numbers for all heads, but if it only shows high xx and yy numbers for a few heads, there may be hope that you can clean the head with some isopropyl alcohol and that it will play fine again.
For more information about service mode, see page 25 of the service manual at https://digitalcompactcassette.github.io/Documentation/Service%20Manuals/philips_dcc600.pdf (This is the DCC-600 service manual; we don’t have a full service manual specifically for the DCC-300 but apart from the front panel, the DCC-300 is pretty much identical to the DCC-600).
To elaborate on what Ralf said: If a second-generation recorder tries to play a prerecorded tape but there is not Table of Content on the Auxiliary track or it can’t read the Aux track, it will start searching the entire tape to find another copy of the TOC. Only 2nd-generation recorders do this. You would also see this symptom if you put a user tape in the recorder and it wrongly detects that it’s a prerecorded tape. If the head is defective and the recorder correctly detects a user tape, it won’t go into search mode; it will simply think that it’s playing virgin tape. If the head is dirty or partially defective, it won’t be able to reconstruct the data from the missing tracks so it will just play silence. If you try to play a cassette that has the sticky-pad symptom, it will also not be able to reconstruct the music.
In other words: Your recorder is 27 years old, but it may still be possible to revive it. Don’t give up too quickly; if it plays analog tapes, it’s a good start! If there’s a lot of wow and flutter when playing analog tapes, it’s probably a mechanical problem.
===Jac