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From: bill@crystal.UUCP (Bill Cox)
Newsgroups: net.micro.mac
Subject: Re: "Ultrasonic" hearing
Message-ID: <25@crystal.UUCP>
Date: Sun, 19-Jan-86 12:26:51 EST
Article-I.D.: crystal.25
Posted: Sun Jan 19 12:26:51 1986
Date-Received: Tue, 21-Jan-86 08:09:54 EST
References: <1280@brl-tgr.ARPA> <590@harvard.UUCP> <11370@ucbvax.BERKELEY.EDU> <464@tekig4.UUCP>
Distribution: net
Organization: U of Wisconsin CS Dept
Lines: 45
Summary: Mac noise problems and (?) solutions

> >I have had the same problem with my Mac, and also found that the "sharp rasp"
> >(read: "swift kick") solution is usually effective.  Why is this?  I never
> >had the problem before I blew my power supply, and had to replace the analog
> >board.  Can anybody suggest a possible solution (or a reason why this problem
> >would suddenly crop up, for that matter)?
> 
> I also am interested in a solution to this one!  My home Mac will (sometimes)
> suddenly take up this squeal (I assume 15.5kHz) after a half hour of silent
> running.  Of course, when I attempt to run it without the case so that I can
> identify the offender, it never heats up enough to start this up.

My mac is now on its third analog board.  My original problem was that the left
edge of the screen bowed in when text was highlighted (you could see a "dogleg"
effect, around 5 screen pixels).

Diagnosis: bad analog board; replaced.

After replacement, there was a LOUD whistle, either a high flyback whine
or something else (arcing?).  Noise disappeared when the disk drives ran.
It was so bad that I had to wear hearing protectors while using it.

The newer analog boards have a silicone anode cap (that fits on the CRT),
whereas my old one had a black rubber(?) one.  A non-[electrically]conductive
silicone grease on the silicone cap reduced the noise considerably.

Apple's technical people told my dealer that either
	(1) a line filter/surge arrestor should be used;
	(2) the anode cap should be relubricated; or
	(3) replace the analog board.

In my case, I use (1), (2) didn't do any more good, and the analog board
died when the HV adjustments were tried.

CAUTION:  If you attempt the lubrication, KNOW WHAT YOU ARE DOING ! ! !
The voltage at the anode cap could KILL YOU ! ! !
I assume NO responsibility whatsoever for any use of this information.

	bill cox

-- 
	William Cox
	Computer Sciences Department
	University of Wisconsin, Madison WI
	bill@wisc.crys.edu
	...{ihnp4,seismo,allegra}!uwvax!bill