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Compact Mac CRT Energy

Do this at your own risk - you're working with high voltages here!

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:

  • Wire a 10 Megohm resistor to an insulated lead. Crimp a ring or spade connector to one end of the lead and fit an insulated crocodile clip to the other end; use heatshrink or other insulation to cover the resistor.
  • Then undo a suitable chassis screw on the Mac and screw the ring or spade connector down, and connect the crocodile clip to a screwdriver with an insulated handle. Fit a length of insulating tubing over the screwdriver shaft so that as much the metal as possible is hidden.
  • Now you've got the lead connected to the chassis and screwdriver, gently lever the screwdriver under the HT lead boot on the CRT and ease it under until it touches the metal contact. Hold it there for a few seconds and the job's done (you should apparently wait until the soft crackling has stopped).
>Ok, now on to the experiment. I started by discharging the CRT with the Mac
>Plus Power supply attached (less than a minute after unplugging the computer).
>I saw no spark whatsoever from the anode. Now I realize the CRT may still
>have had power in it, even though it didn't spark, but common sense tells me
>that even a 9V battery will spark, and you guys are saying it could have tens
>of thousands of volts in it.

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.

>Ok, I then tried the same thing with the Mac 512kE power supply attached, also
>under one minute after unplugging it. This time, I saw a spark. It was a
>very small one, but still a spark. This tells me that there is some power in
>the CRT. But, one must realize that this power supply was giving power to the
>CRT a lot more than a normal power supply.
>
>Now, one could make the conclusion at this point that it depends on the power
>supply, but I would like to see one of you more experienced guys try this with
>a Compact Mac with a working power supply. Rowland, Neon, Moderator?

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 would like to see the results from a normal power supply. Preferably just
>as quick after unplugging it as I did. Also, somewhat unrelated, can anyone
>tell me if the Flyback transformer is what is wrong with the 512kE power
>supply? I see one bad solder joint on it which is from one of the smaller
>capacitors, but I haven't had a chance to resolder it yet. I am thinking that
>if that doesn't fix it, then I can put the 512kE's flyback on the Plus power
>supply.

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. LEM

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