briand@tekig4.UUCP (Brian Diehm) (01/23/86)
Well, the net comes through again! I solved my video scream problem last night
with pointers provided from this newsgroup.
SAFETY PROCEDURE - to avoid getting zapped, do the following:
1) Unplug the Mac to tear it apart. DO NOT apply the power cord until it is
completely reassembled.
2) When the case is off, remove the one screw of the four holding the CRT in
place that has the ground wire.
3) Clip the ground wire to the shaft of a screwdriver, ONE THAT HAS AN
INSULATED HANDLE.
4) Holding the screwdriver by the handle ONLY, and keeping your other hand
behind your back (I'm cautious) probe the screwdriver under the anode cap
on the CRT until the screwdriver touches the lead underneath. If the Mac
had been powered on just before you tore it apart, you will hear a snap that
indicates a discharge spark. If the Mac had been off for some time, there
may not be any charge to dissipate.
5) You are now safe to play. Be careful of the nearby end of the CRT - the
shaft with the electron gun (with the yoke on the outside) CAN be broken
off.
6) If you DO get zapped by the charge, IT WILL NOT KILL YOU! It is about 25kV
but has no ability to deliver much current - it is about like the high
voltage side of an automobile ignition. You will, however, be awakened to
the fact that you have screwed up, in a way that would make Federal Express
just green with envy!!! :-)
7) When reassembling, before replacing the case, make sure that ALL connections
have been reestablished, ESPECIALLY THE HIGH VOLTAGE ANODE! This is not
just for your safety alone, it involves the safety of your Mac too!
I used insulating silicone grease and toothpicks. The flyback transformer of
the Mac has several parts that are not securely sealed, specifically a wire clip
that runs the length of the exposed transformer core. This I filled with
toothpicks wherever a gap was present, and I covered the wire with silicone
grease. Next, just for drill, I shoved silicone grease under the base of the
transformer and the board, on the unlikely chance that the vibration was there.
Lastly, I sealed the outside edge of the anode lead cap, where the anode is
connected to the CRT. I did not put any sealant inside this cap!
The silicone grease is insulating, and thick enough to provide some mechanical
damping. A better solution would be RTV Silastic, the stuff with which Apple
glues down various capacitors on this same board. It dries to a rubbery solid.
My local expert on video circuits gave good marks to the toothpick idea, but
thought the anode lead sealing sounded pretty flakey. That would only help if
the lead wasn't making a good connection to the CRT and you were hearing some
arcing - pretty unlikely. He suggested that if it did help someone, it was due
more to moving the high-voltage wire to a position where the strain it applied
on the flyback transformer was enough to dampen vibration.
Whichever cure fixed my problem, I know the intermittent is solved because I
could detect the old noise - if I got my ear up to the disk slot and the room
was ABSOLUTELY quiet! I'm satisfied!
Thanks again to the net for the help!
-Brian Diehm
Tektronix, Inc.roy@phri.UUCP (Roy Smith) (01/25/86)
[Brian Diehm describes safe practices for deal with high-voltage] > 4) Holding the screwdriver by the handle ONLY, and keeping your other hand > behind your back (I'm cautious) ... And smart. When I'm working with live high voltage (including 110) I keep one hand not just behind my back but in my pocket. You look foolish, but it's better than looking dead. If you zap yourself and get knocked over, you instinctively reach out to steady yourself with your other hand. Keep in mind that current flowing from hand to hand goes through your heart (I know about skin effects, but wouldn't count on it). If you happen to find that grounded water pipe... With your other hand in your pocket, the extra time to pull it out may be all you need to let go of the hot wire. I'll gladly take the bump on the head from hitting the floor rather than being fried. -- Roy Smith <allegra!phri!roy> System Administrator, Public Health Research Institute 455 First Avenue, New York, NY 10016