From: george@wixer.cactus.org (George Wenzel)
Subject: Re: Where can I find this capacitor for Mac?
Keywords: 512K Mac
Organization: Real/Time Communications
Date: Fri, 20 Nov 92 23:56:57 GMT

In article <1992Nov19.182542.458@mcc.com> wshen@nanook.mcc.com (Wei-Min Shen) writes:
>Hi, 
>
>I am fixing a 512K Mac, and need to buy a 3.9uF/25v capacitor with temperature
>tolerance 85c degree or higher. I checked serveral electronics stores in town and
>they don't have such capacitors with such high temperature.
>
>Does anybody know where I can find such capacitors?  Please reply to
>wshen@mcc.com. Thanks.
>
>Weimin Shen
>MCC
>338-3295

Ahhh, yes, the ol' horizontal output cap.  I used to fix mac analog boards
back in the old days for $25 a pop.... those caps were always difficult to
get.

What they are is a non-polar electolytic cap used to couple the horizontal
saw-tooth to the yolk coil of the crt.  I have found these caps in almost
every crt circuit from tv's to dumb terminals.  The problem is the value
3.9uF is not a common value.  For a while I was able to get them from
Shreive Systems in Shreiveport La., but they stopped selling them when they
found they could make more money if they repaired the analog board for
you... same for the flyback transformer.

The guy I used to repair the boards for runs Macintosh Support Services,
you might call him and see if he will sell you one... he may, or may not...
451-7575, ask for Dick.

Failing that, all you really need is a cap that can withstand the
particular application.  For a while I was using 4uF 100v polyester film
capacitors that I got from my uncle.  These are very high quality caps, and
expensive too.  He uses them for something he was manufacturing at the
time, and the application required mill-spec components.  You should locate
a high-quality film cap in the 3.5 to 4.5uF range, with a 100v value...
This will give you enough operating headroom, and reliable preformance.  In
fact, a good quality polyester film cap will outlast the original cheap
tin-can cap that apple put in the monitors.  The original cap cost apple
about $.05, and the polyester cap would cost them $1, so it is easy to see
what they used what they did.

As for the value range, you actually have a good amount of leeway.  The
cap's value will alter the horizontal measurment of the display, so as long
as the value you choose gives preformance that can be adjusted by the
horizontal width adjustment, you will be in good shape.  You should be able
to stray outside of the values I listed above and not harm the unit, so
feel free to try what values you can find.  Don't try polarized
electrolytics, they will explode.... they won't damage anything if you do
try them, but the pop will scare you.  The circuit is pretty robust, so
trying alternate parts won't damage the board any, although a low-grade
part will likely fail in a smelly fashon.

I assume you are aware of the high-voltages that are associated with these
boards and the anode cap of the CRT.  If you don't know how to safely
discharge the 15,000 volts present at the crt anode, you shouldn't be
working on this board.  Unsafe practices could kill you (or more likely
wake you up real good!)

George
-- 
George Wenzel                    |  Real/Time Communications
george@wixer.cactus.org          |  "faster, smarter and costs less..."
cactus.org!wixer!george          |  "Hardware solutions for the real world."
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