External Display No Longer Working


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After mistakenly upgrading my MacBook Pro (15-inch, Mid 2012) to Catalina (which was a disaster), I had to upgrade from Mountain Lion to Sierra (10.12.6), as this was the latest version in which Photoshop CS6 would work without any issues and have the latest browser security.
Now it seems my external display no longer works.
I used to be able to use a Thunderbolt-to-HDMI cable and see the image on my TV, but this no longer works.
I've tried multiple cables and adapters, but with no luck.
Sometimes, if I shut down the computer, then power it up again, the display will work, but this is pretty rare.
The cables do work on an Imac I have, so I'm thinking it has to be the software causing the issue.
Any solutions?
 
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Will this affect anything else?
I am loathe to mess with anything, because we all know how that usually goes.
Don't think it's the cable, as it happens with several, though it may be the port.
I read something where it explained that the computer needs to see a signal before it will output to the external device.
This only happens at certain times, which would explain why it usually works when shut down then started again.
 

Cory Cooper

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Hi,

These are pretty standard resets, and are used to fix issues similar to what you are experiencing. I have performed them thouands of times without any issue. I haven't seen them affect data at all, but it is always a good idea to have a current Time machine or other backup of your data at all times.

The SMC is the one which specifically addresses hardware issues with power and ports.

C
 
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Did the SMC reset, no changes.
If I plug in the adapter, it will usually work, but if I take it out and re-insert it, it will never work.
I have to shut down the computer for it to work again, so there must be some kind of signal that in only sent upon turning on the computer.
 

Cory Cooper

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Hmmm...

It'll be tough to pinpoint, as it could be the port, adapter, cable, or the TV itself. I wonder if it may be a setting on the TV - something related to Consumer Electronics Control (CEC)?

C
 
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Here is the post I was referring to:

>to get a Mac display to become active, you need the Mac to query the display, and the display to answer with its name and capabilities. Otherwise, no data will be sent to the display.

This query is only sent at certain times:
• at startup
• at wake from sleep
• at insertion of the Mac-end of the display-cable, provided everything on that cable is ready-to-go
• on invoking Option-(Detect Display) button in Displays preferences

so try doing some of those things and see if the display comes alive.

Modern Displays with multiple ports are sometimes busy scanning the other ports, looking for an input, and miss the query from the Mac. They need to pay attention to the port you are actually using, or they will miss the query.

Some displays have On-Screen Display settings that can be used to tell the display a computer is attached on a certain port, or a certain port should be highest priority. Changing those may make your display more responsive.
 
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Not the cable or TV, as it works on my other mac - port might be faulty.
 

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