DESCRIPTION:
I've always been a fan of U2's music and got into them quite heavily when I was at school. I admire the Edge as a guitarist and musician. His use of delay has definitely influenced my playing and I love his tone. I recently purchased a Korg DL8000R which is a largely unknown rackmount delay made in the 90's. It's finding a sort of cult status now as people are discovering it has a great sound and is quite similar in character to the sought after TC2290 (used by the Edge). In actual fact, it's a lot more powerful in many ways than the 2290.
After integrating it into my rack I discovered I was able to get quite close to the Edge's tone from the 80's.....think Joshua Tree, Unforgettable Fire, Rattle and Hum.

So I decided to put up a video showing how I created the tone from the ground up and pointing out the things which are (in my opinion) key to nailing that classic tone.

EQ and compression
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Hard to specify exact settings as all compressors are slightly different. Use your ear here. Basically you want to increase presence and push the high end a bit more than normal. You really want the notes to hang in and really 'pop'. It's quite a harsh and 'glassy' sound.

Pickups
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As mentioned in the video postion 2 on a strat is key. This contributes to the glassy, sharp attack.
The way the Edge picks is also quite a big factor. He strikes the strings hard and seems to scrape more than actually pick. This contributes to the sharp attack on the notes.

Delay
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I'm running the Korg in parallel and 'kill dry'. Just a couple of repeats mixed at same level as dry signal. In the video the delay time is set to 450ms which produces 'dotted 8th' delay repeats in relation to 100bpm.
The key thing is the modulation. It's set to 4.2 on the Korg which might be 4.2Hz? I'm not sure. The modulation is the key feature of the tone in my opinion.