Does the -2090 have a mode switch that will shift from stereo to mono?  On a -4010, there is and if the stereo light does not come on, then switching to mono and then back to stereo may bring stereo mode on.  When the stereo light is off even with stereo mode selected,  the receiver will be in mono.  Is the stereo chip an HA-1196 as in the -4010?  If so, the voltage on pin 9 of the 1196 will switch from aboout 12v to a low value when stereo mode is enabled..  The old -4010 works this way (and probably needs a tune-up).  If you get switching on pin 9, then look at the stereo indicator.  Also the voltage fed to it.  You mighy have a bad electrolytic capacitor.  Lets know what you find.  Ed