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Date Posted: 22:04:06 01/11/02 Fri
Author: Chris
Subject: The Mother of All Degaussing Information
In reply to: Lance Bernard 's message, "Source for new/used CC CRT? Likely pricing?" on 20:09:46 01/07/02 Mon

Just got two more CCs in this week, numbers 5 and 6. One had a very splotchy screen due to a degaussing problem much like the one Lance describes here. I figured it was a simple matter of the degaussing coil being unplugged.

Not quite. Once I cleaned the cubic foot or so of dust out of the two CCs, I got out the trusty multimeter. The degaussing coil on CC with the blotchy display was plugged in perfectly. It simply wasn't getting any voltage through the degaussing coil at all.

(Note to the more electrically inclined: the a/b only supplies voltage through this circuit when the CC is powered on but the motherboard is NOT being used. In other words, turn on the switch in the back but DO NOT press the power key on the keyboard if you need to test this circuit.)

So it wasn't the connection. What was it?

Well, the more observant reader might notice the paragraph above - specifically, that the coil is only on when the computer itself is off - implies the existence of a relay or relay-like device in the circuit. I'm reasonably sure it's a relay, and in my case, that relay was faulty. I'm not sure what about it is "broken," but it gets VERY hot after only a few seconds of power-on time. No other components get that hot in such a short time, nor does the same component in the other CC I was working on tonight.

That leaves one CC with a very blotchy screen and a bad degaussing circuit. How do you fix that, short of replacing the faulty component?

Ah, that's where a bit of ingenuity on my part came in. Materials needed: one length of lamp cord with a plug on one end and two exposed leads on the other, and one cow magnet or other reasonably strong magnet. Hook the AC/mains lead to the degaussing coil and give the screen a few short zaps. Don't overdo it or you'll really cook the degaussing coil - the relay has a current-limiting device either built into it or as another part of the circuit, while your wall can probably provide at least 15 amps, which will get the coil VERY hot after 15 seconds or so. Use the cow magnet to spot-fix the colour distortion.

If you're like me, that still didn't fix it all, but at least it got the tube "loosened up" for a real degaussing. If you don't have a second, working CC, this is where you stop reading and find a TV repair shop that can professionally degauss your tube.

Take a second CC with working degaussing circuitry and swap analogue boards with the funky CC. Hook everything up, plug it all in, and turn the CC on in the back, but DO NOT POWER IT UP WITH THE KEYBOARD.

Leave it for 24 hours like that. The degaussing circuitry will be sending a low-level current through the degaussing coil the entire time it's turned on. If 24 hours doesn't fix it, you need a professional, because that's a damn stubborn magnetic field you've got there :)

(Yes, some/all of this post is probably going to make it into the CC FAQ at some point.)

cl

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