Re: Apple //c monitor - tilted?!

[mjmahon@aol.com (Michael J. Mahon)]

Richard Kilpatrick wrote:

>On 4/8/04 2:15 am, in article cepdm101ske@news2.newsguy.com, "David Chiu"
><none@nowhere.org> wrote:
>
>> You need to pop the casing and adjust the yoke... Not for the light
>hearted.
>> Monochrome monitor doesn't quite use as high of voltage as colour ones, but
>it
>> will still give you quite a shock. My suggestion is to leave it to places
>like
>> TV repair shops that knows what they're doing than to venture inside of a
>> monitor.
>
>Yep - I'd suspected it was a yoke issue, but I Don't Like To Mess.
>Especially since getting that bit wrong can result not only in people
>jumping about uncontrollably and getting burns, but also, you can crack the
>neck of the tube if you get it wrong.

In fact, it is quite unlikely to get into trouble working on a CRT monitor
if you follow standard, simple precautions.

1) POWER OFF.  Open the case to gain access to the CRT.
2) Wait a minute, then discharge the CRT anode to chassis ground
   using a clip lead attached to ground (first) and a thin, stiff wire on
   the other end to slip under the anode connector.  There will usually
   be a spark "snap" just before you make contact--no worries.
3) Leave the grounding wire connected while the monitor is powered off.
4) Locate the yoke and, firmly but reasonably, test its ability to be
   rotated a few degrees.  A CRT is a strong glass envelope--just don't
   do anything to it that you wouldn't do to a test tube, for example.
   It is not unusual for a yoke to stick somewhat--that's a good thing
   unless it's stuck in the wrong position.  ;-)
   (Note:  some older monitors may have a wingnut and a slot to
   adjust yoke rotation--modern ones usually do not.)
5) DO NOT use metal tools around the neck of the CRT--they are
   seldom required and can only cause trouble.
6) After you've located the yoke and verified your ability to rotate it,
   remove the grounding wire, check that the monitor electronics are
   clear of any conducting materials on the bench.  (You must power
   it up with the case open to adjust the yoke.)
7) Make sure that you have a good, straight-on view of the display.
8) Ready the video source (a full-screen text display will do nicely)
   and connect the video input and the power cord.
9) Checking again that nothing is in danger of shorting to anything
   conducting, apply power and let the monitor warm up.
10) Using nonconducting gloves if you don't know what not to touch
    on the yoke, carefully rotate the yoke to achieve a straight display.
    If you had to loosen any fastener to rotate the yoke, re-tighten it
    now and verify that the raster is still straight.
11) Turn the monitor off, ground the anode again, and re-install it in
    the case.  Remove the grounding wire when there is no longer
    any liklihood of coming in contact with the anode or the HV lead.

The procedure is quite simple in practice, and was done regularly
by hundreds of thousands of people working on their own TVs in
the 1950s and 1960s.

A CRT is probably one of the strongest pieces of glass you will
ever find.  The danger of implosion is played up to keep people
from taking a wrench to the neck of the CRT.  ;-)  Actually, the
biggest danger is having someone with a heavy metal tool near
the neck of the CRT come in contact with the undischarged
anode, producing a rapid, uncontrolled movement of the tool.

>Just don't understand why the //c monitors I've seen have always been like
>this. Surely it's the sort of obvious flaw people notice when making
>something ;)

It is usually the result of vibration changing the alignment of the
yoke, often in shipping.  Some monitors use sticky stuff to try to
anchor the yoke to the CRT, but that has its own problems.

-michael

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