Make sure you're using DC trigger 1. The 990 has two 12V triggers. DC trigger 2 is an assignable trigger that you can assign to specific inputs. It's used for advanced functions described on page 25 of your manual.

I have a 990/7700 combo (same as the 7500 with 2 additional channels). I'm using DC Trigger 1 and it works fine, always has.

If this isn't your issue, call outlaw support.

Hope this helps,
Tony.
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Outlaw 976, Outlaw 7700, Pro-Ject Phono Box S
Sonus Faber Domus Grand Piano (F&C), Niles HDFX (Surr. & Rear Surr.), Outlaw LFM-1 Plus, Velodyne SMS-1
Sonos multi room audio
Video: Sony KDL-46V2500, OPPO BDP-103, TiVo Premiere XL4
2-channel: Outlaw RR2150, SF Concerto Home, Outlaw LFM-2