No picture from the Onkyo TX-NR646's HDMI output usually comes down to the output assignment, a handshake, or — and this matters on this specific model — the known HDMI board fault Onkyo receivers of this era are prone to. Work the quick settings/handshake checks first; if they all fail in a specific pattern, you've found the hardware issue.
Fix it
- Confirm you're using the right HDMI OUT. The NR646 has HDMI OUT MAIN and SUB. The TV must be on MAIN (or the output you've selected), and the receiver set to output to that one. Check the output indicator on the front display.
- Power-cycle the whole chain in order. Unplug the receiver, TV, and sources for at least 30 seconds; power the receiver first, then the TV, then sources — resolves most one-off handshake blanks.
- Test the receiver → TV hop directly. Swap in a known-good certified cable between the receiver's HDMI MAIN OUT and the TV, into a TV input you've confirmed works with another device.
- Bypass to isolate. Connect a source straight to the TV. If that works, the fault is the receiver's HDMI section, not the source or TV.
- Check Monitor Out / resolution settings. If the TV can't accept the receiver's output resolution, you get no picture — set the receiver's HDMI output resolution to Auto or 1080p to test (via the front panel/setup if the on-screen menu is blank).
The known TX-NR646 HDMI board issue
Receivers from this Onkyo/Pioneer generation have a documented HDMI board failure — symptoms are exactly this: HDMI out goes dark while audio still works, often intermittently, and it survives every power-cycle and cable swap. Tell-tale signs:
- Audio plays fine but no video from any source through the receiver.
- Bypassing the receiver (source → TV) restores picture.
- The problem is consistent across all inputs and both HDMI outs.
If that's your pattern, it's the board — repair shops re-flow/replace it, and it's worth weighing against the receiver's value. If instead it blinks and shuts off, or shows NG in setup, the broader TX-NR646 error-code guide sorts which fault you have.
FAQ
Audio works but no picture from my NR646. Classic of either a handshake or the HDMI board fault. Power-cycle the chain receiver-first; if it persists across all inputs and a known-good cable, suspect the board.
Which HDMI out should the TV use? HDMI OUT MAIN, with the receiver set to output to Main (check the front-panel indicator).
How do I know it's the board, not the TV? Plug a source straight into the TV. If that shows a picture but the receiver path doesn't — across every input — it's the receiver's HDMI section.
Can I still use the receiver? For audio, often yes. For video, route sources directly to the TV and use the receiver for sound (eARC/optical) until the board is repaired. The no-picture / 4K HDR passthrough guide covers the settings side if it's not the board, and the 9-step eARC & HDMI guide covers the full chain.