SA-HT75 tray won't stay closed?

When I try to close the disc tray (with or without disc), only the STOP button closes it for a second or so, then the display says CHANGE and the tray opens again.
The OPEN/CLOSE button does not close the tray at all, instead the display briefly shows ERROR.
Usually, pressing the POWER button would close the tray if open, but not anymore.
We've had the unit for more than 10 years now and this is the first time ever it has given us any problems, would like to try to fix this before throwing it out!