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My Raspberry Pi B+ is working well except: the HDMI only seems to work if the ethernet cable is plugged in. If I remove the ethernet cable the monitor goes black. I plug the cable back in - monitor works again. Every time.

Only the HDMI seems to be affected. Everything else works fine. Wifi adapter, SSH connection - all good. Just the HDMI/monitor somehow is affected by the ethernet being plugged in or out.

I tried powering the Pi with a very capable power adapter as well as an active USB hub. Doesn't make any difference.

What's going on here?

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The solution: using another HDMI cable (shorter and thicker). The monitor now stays on no matter what, ethernet has no effect anymore.

Weird.

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    Please accept your own answer with a click on the tick on its left side. Only this will finish the question and it will not pop up again year for year. – Ingo Mar 3 at 13:14
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Sounds like a ground loop side effect. I think that the Ethernet port on the 3B+ is not the isolated kind but just has common mode noise magnetics. So ground is DC connected through it and probably is making its way through a power supply in the Ethernet switch to AC power ground and thus to your HDMI monitor, then back to the Pi through the shield of the HDMI cable. My guess is there was noise being picked up causing the HDMI signals to be compromised causing the monitor to ignore them. Shorter cable means less noise pickup, and also it may be a better shielded cable, which would help also.

A quick experiment would be to float the monitor using a 3-2 power plug adapter and see what happens. Or float the Ethernet switch power supply the same way. There may be other paths as well but not knowing your full setup its hard to guess.

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