I would first check to see if the signal is getting to the tape head. The Denon double-well decks are wonderfully laid out so you can easily see the connections for both playback and recording. open the case and you will see cables running from the transports to the board. Both playback and recording cables are marked. Identify the recording cables. Unhook one to test. Put the deck in record mode (preferably with a 1khz test tone at 0dB) and see if your multimeter measures AC voltage at the pins where you disconnected the cable. If you have no signal you'll have to go further back in the signal path. If you have signal that means the tape head is not receiving the signal. There could be broken wires close to the head where it has to move up and down. Do both transports fail to record? That points to a problem further back than the tape head. Hope this helps.