TX-NR646 problems almost always fall into three buckets: it blinks and shuts off (protection mode), it shows "NG" during speaker setup, or it drops HDMI video — and that last one matters, because the TX-NR646 is one of the Onkyo models with a well-documented HDMI-board weakness. Work out which bucket you're in first.
It blinks and shuts off (protection mode)
The receiver sensed a fault on the amp output — almost always a speaker-wire short (a stray strand bridging the terminals), sometimes overheating.
- Unplug and check every speaker terminal for stray strands; re-strip and re-seat.
- Test with all speakers disconnected — if it stays on, reconnect one at a time to find the bad cable. If it trips bare, it's internal.
- If it only trips when hot, improve ventilation and dust the vents.
- Clear a latch: power-cycle (unplug at least 30 seconds), then a microprocessor reset per the manual.
"NG" during speaker setup
During AccuEQ, "NG" for a channel means the receiver couldn't detect that speaker. Re-seat the speaker wires (+/− correct, no shorts), plug the setup mic fully in at ear height pointing up, run it in a quiet room, and confirm the speaker actually plays. If all channels show NG, the mic isn't seated or the room's too noisy. The NG: L R front-channel walkthrough covers this result in detail.
No HDMI video — and the board issue
First rule out the easy stuff: confirm the input assignment, reseat and swap the HDMI cable for a certified one, set the power-on order (source → receiver → TV), and update the firmware. Connect the source straight to the TV to confirm the source and cable are fine.
But if HDMI inputs progressively die — video drops while audio still works, or ports stop passing video one by one — that's the TX-NR646's known HDMI-board fault, not your cable or settings. No reset fixes a failed HDMI board; it's a board-level repair. The TX-NR646 HDMI-out walkthrough isolates this from a handshake. The practical workaround: use the receiver for audio only, run video from the source straight to the TV, and return TV audio over ARC. The amp section is usually still fine, so the receiver stays useful.
FAQ
My TX-NR646 blinks and shuts off. Protection mode, usually a speaker-wire short. Disconnect all speakers and power on; if it stays on, reconnect one at a time to find the short.
HDMI video died but audio still works. That's the classic TX-NR646 HDMI-board symptom. Rule out the cable, but progressive HDMI failure is a board fault — run video direct to the TV and audio over ARC.
All channels show NG in setup. The calibration mic isn't fully plugged in, or the room's too noisy. Re-seat the mic at ear height and run it in quiet.
Is it worth repairing? If it's the HDMI board out of warranty, weigh the repair against a newer receiver. The audio amp is often fine, so the direct-to-TV workaround keeps it usable meanwhile. For codes on other Onkyo models, see the full Onkyo receiver error code list.