By taking the drastic step of actually reading through ALL of the install instructions. 
In my misguided attempt to run my ROKU 3 through the AVR 154 HDMI switch, I had run an optical audio cable from the TV to an available OPT input port on the AVR. This (now understandably) was a futile (dumb) idea since there was no connection to the ROKU but it had no effect...I thought. It clearly stated in the manual that NO video outputs from the TV should be ever be run into the AVR 154 or video anomalies could result. After removing the optical audio cable, the AVRs HDMI Monitor Out now functioned as advertised. ALSO found by reading the manual (Mr Smarty Pants) was the information (hidden in the actual system specs) that the HDMI #3 output (unlike #1 & 2) was 'switchable'.So...there was the mystery solved:
Running optical audio output from TV also confuses the AVR as to handling of HDMI signal (sometime it works; sometimes not)
The HDMI #3 IS different from #1&2 and can act as general HDI input from the AVR to the TV (display).
In closing I would second an ironic but valid comment someone posted in a discussion group about the HK AVR 'issues': It used to be (20+ years ago) that audio systems were pretty much plug and play and PCs were complicated and time consuming to configure at installation. Yet, now, decades later, Harmon Karden, the inventors of stereo, now present the inverse to their users. Incredibly complex (to the average consumer) setup configuration protocols, wrought with caveats and warnings, while PCs are by and large plug and play out of the box.
How the worm has turned. Still...HK quality of both hardware and audio performance is outstanding ..once properly setup.
Have had the HK AVR 154 for a few years and had no functional issues until I tried to add a third HDMI component (ROKU3) to the system. Never realized that there was a dedicated HDMI Monitor OUT port and had been happily using HDMI 3 from the AVR to my Zenith (LD) HD while other HDMI devices (PS3 & SONY DVD player) were input through HDMI 1 & 2. Now, having actually read the user setup guide, I found that I was supposed to run all HDMI into the AVR ports and out to the TV from the HDMI Monitor out port It acting as a HDMI switcher). Problem is NOTHING on HDMI gets video to the TV now that I am 'correctly' configured; the HDMI Monitor Out port seems dysfunctional. Nothing in 'OSR' setup addressed this. I have just seen another user comment on the fact that only HDMI 3=> TV allows HDMI pass through for other devices. Very weird..as if port 3 were wired as 'Monitor Out' and the actual Monitor Out port was ..well..out. THEN when I switched TV HDMI cable from 'Monitor Out' port back to HDMI 3 port...now HDMI did t pass video through anywhere! On a hunch without reason I switched the TV HDMI cable back to Monitor Out port and now HDMI is working all around!?! For now...HDMI to TV seems to be unstable and changed by undetermined 'conditions. There is some reference to a default based on wherever previous successful setting was but dont recall specifics. Any clues on this goofy HDMI observation on the AVR?
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