Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!mit-eddie!uw-beaver!uw-june!gordon
From: gordon@cs.washington.edu (Gordon Davisson)
Newsgroups: comp.sys.mac.hardware
Subject: Re: broken Mac-Plus: HELP
Keywords: Dark screen, diagram wanted
Message-ID: <10441@june.cs.washington.edu>
Date: 22 Jan 90 02:30:48 GMT
References: <2350@swi.swi.psy.uva.nl>
Reply-To: gordon@june.cs.washington.edu (Gordon Davisson)
Organization: University of Washington, Computer Science, Seattle
Lines: 36

[In the interests of conservation, this posting has been made with
recycled materials. :-)]

In article <2350@swi.swi.psy.uva.nl> jansweij@swi.psy.uva.nl () writes:
>My four year old MacPlus is broken. Symptoms are:
>	- a dark screen
>	- s soft "tsjilp-tsjilp-tsjilp" sound from the transformer
>	  or its immediate neigbourhood. The transformer has the
>	  identification: "Coilcraft Taiwan 157-0047-A" and "F 4704-A".
>	  The place on the board is marked: "B20-0107-C". It's on the
>	  bottom of the sweep/power-supply board.
>
>Can anyone tell me from these symptoms what's wrong? or else is there
>a diagram available from this board so I can test it myself?

Sounds like the power supply is shutting down due to incorrect loading
(and then restarting, and then shutting down, and ...).  The most common
causes of this are a pair of diodes (CR1 and CR5) in the video circuit
that tend to fail as short circuits.  They can be tested in situ, but
it's a bit chancy.  You're better off removing them first (be sure to
remember which way around they go).  CR1 is the one at the top of the
analog board near the back, and CR5 is the one attached to the big heat
sink just forward of the flyback transformer.  Use an ohmmeter to see if
either of them conducts in both directions.  If one does, you've found
the problem.  You may have to look around a bit for a replacement --
call local electronic supply stores and ask for GI854 diodes (or GI824,
a higher-current version, or MR854 or MR825, the Motorola equivalents,
or...).

If all else fails, you can send it to me (see .sig).  I do component-
level repair, which doesn't cost nearly as much as an Apple board-swap.

--
Gordon Davisson
Westwind Computing	(206) 632-8141
4518 University Way NE, Suite 313, Seattle WA 98105