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Compact Mac CRT EnergyDo this at your own risk - you're working with high voltages here!Low End Mac Reader SpecialsDownload Typestyler, still the Ultimate Styling Tool for Internet, Print and Video Graphics. Works great in Classic with a Native OS X Version on the way. Free Tryout: www.typestyler.com
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From: Rowland <rebecca@astrid.u-net.com> Subject: Re: Stored Energy in Compact Mac CRTs The book Macintosh repair and upgrade secrets by Larry Pina (which covers Macs from the 128K to the SE, including Lisa/Mac XL) says that you must use a resistor to discharge the CRT because of the high risk of damaging other components (a blown LAG chip on the logic board and a blown 74LS38 on the analogue board; no, I don't know what the LAG chip is) if you just spark the energy away. It recommends that you use a setup roughly like this:
This isn't quite right: the CRT can store *energy* in the form of electrical charge (electrons). A charge (extra or missing electrons on the conductive coating on the inside of the CRT and on the outside of the CRT) *in* the CRT results in a potential difference (voltage) *across* the CRT. Discharging the CRT will liberate this energy; the voltage will drive a current through the discharge circuit, liberating this stored energy as light, heat, and sound. This happens over a (short) period of time: energy/time = power. Macintosh repair and upgrade secrets says that the CRT has 13,000 V across (not in) it.
Personally, no, because playing this game is likely to damage a Mac. I'd want to use some decent measuring equipment, but I've got no convenient way of measuring anything above about 5000 V and no convenient way of measuring capacitance. I suppose I ought to borrow a capacitance meter to measure the capacitance of my Mac Plus's CRT when I get round to fixing it (real soon now, as they say...)
I don't know about the horizontal white lines, but a vertical white line might be due to a dead C1, J1, L2, or LAG chip. It's worth re-soldering any iffy looking joints and maybe replacing C1 (using a higher-voltage part). Note that C1 is unpolarised and low-impedance: replacing C1 with a normal electrolytic capacitor will result in catastrophic failure after a short while in service (you can expect a small explosion). Do try and get hold of Larry Pina's Macintosh repair and upgrade
secrets (Hayden Books, 1990, ISBN 0-672-48452-8); it's a long way
from perfect, but it's a damned good book in many ways.
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