The APM/Display board is available from an authorized Crown service center. The APM board is located in the front panel. I can't reliable say what could have caused this symptom. Though, two units that have the same behavior? Perhaps, if at all possible, try to confirm with another XLS owner, or with the service center that the APM board is really defective. Hopefully, it may be an issue with the LV psu!
I received this explanation :
" The 21/NN error code is display on the LCD not the thermal LEDS.
The 21 error code/NN error code is a DSP Boot failure, the NN = 99 means it has occurred 99 times which the max the error buffer will register.
The APM/Display board must be replaced to correct this error." (TY Rodney, for this explanation!)
The error codes should be read via the thermal indicators :
The amplifier’s processor runs through a series of Power On Self Tests (POST) when the amplifier is first powered up. If the processor encounters an error, it will communicate the error via the front panel Thermal LED’s. These error codes are unique and based upon a Major and Minor element. The Major element is determined by which portion of the amplifier failed POST, while the Minor portion is which part of the Major element failed.
They are displayed by CH1 Thermal LED = MAJOR, CH2 Thermal LED = MINOR. The MAJOR is blinked 1st, then the MINOR.
Error codes for Mm (Major-minor) lists 21 to 24, no 29! Try to read the thermal indicators and post them back.
You're on the right track : First, discharge the filter caps; Next, remove the power harness to both output module.
Make sure that you have +15V and -13V in the LV section!
Note : By default, if the amp detects 6 seconds of silence it will go into Sleep mode. This delay setting can be changed to anywhere between 6 and 60 secs.
Reconnect, one module harness at a time, to verify the output module's operation. Don't forget to discharge everytime you want to change harness!
Probably, one of the o/p modules is faulty. You'll notice it quite readily!
Make sure that the module's heatsink (live! at +VCC) is not grounded via the silpad! If a silpad is pierced and possibly, the module is making contact with the chassis, use Kapton tape, for example, over the hole.