This particular error, as you stated, can be 'unfixable' without replacing the board, but there are a couple of things you can try which may fix this.

The problem could be a faulty a/v cable.  You probably don't want to buy a replacement cable just to try this, but if you have someone who would let you borrow their cable, try testing your 360 with their 'known good' a/v cable.

If this doesn't do the trick, the problem is most likely a scaler chip or gpu problem, which will not be an easy or cheap fix - in this case, MS repair is probably your best option.  Keep an eye on xbox support forums though, as the E74 error is becoming more and more common, you may hear something official out of Redmond about this at some point.   In any case, good luck!
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A couple of ideas...

Turn off the console again and remove the hard drive then turn on your Xbox 360 console without the hard drive, if the console works fine without the hard drive attached, you can just call the Xbox support and ask for a new hard drive.

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I fix a console every day and put them on ebay, out of the 100s that i have fixed only 1 or 2 have ever not worked after doing this fix:

1. One of the main reasons that causes you’re Xbox to die is over heating. This can cause warping of the motherboard due to the X clamps that Microsoft use to hold the heat sinks in place, which in-turn can damage the minute solder points all over the motherboard. The first thing we need to do is open the 360 & remove all the internal parts. There are several tutorials for this, here is 1 of them: http://www.xboxchopshop.com/catalog/man_360_open.php

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I took my G/F's hair dryer, put it on full hot and positioned it to blow down at the Hana/Ana chip next to the GPU. It gets EVERTHING hot, besides the chip. Let it blow on the chip 5 mins. Then re-attached cables and it booted up. I reassembled the console then tested it again and it works.

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I can't believe it! It worked, it really worked! I opened it up and blew it with that spray cleaner that you use on computers. Then, I wiped it off gently with a cloth tissue. Put it all together and wala. Back in action soldier, see you on the battlefield......

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AV cable error... There is a problem with the AV cable, try using a different AV cable.

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Open the case and spray system out with canned air.

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Do not block any ventilation openings on the console or power supply.

Do not use the console in smoky or dusty locations. Smoke and dust can damage the console, particularly the DVD drive.
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Good Luck!
Source(s):
http://xbox360help.blogspot.com/2007/03/one-red-flashing-light-xbox.html

http://www.fixya.com/support/t302599-xbox_360_e74_one_red_light

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