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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:
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.
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