HP Princeton HP-21 - Manual (Page 14)

There is a touch response function to set the
default value for the keyboard touch.
Press the [TOUCH] button to adjust the touch
response level in the range of 1 to 3. 2 is the
default level.
To turn off the touch response function, press
the [TOUCH] button repeatedly until "OFF"
appearing on the display.
8. Record & Playback
Record
Press the [RECORD] button to record. The
record indicator lights up.
There are 2 tracks for recording - track A and
track B. Select and press [TRACK A] or
[TRACK B] button to save your recording.
To remove the recorded data and record again,
press the [RECORD] and [TRACK A] or
[TRACK B] simultaneously. Their indicators
light up and flash.
Play a key on the keyboard to start recording.
To stop recording, press the [STOP] button.
Playback
Select and press the track you want to
playback. Its indicator will light.
To stop the playback, press the [RECORD -
STOP] button.
Press the [RECORD-PLAY] button to start
playing back the track.