Re: Working on a //+

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

Stephen Shaw (>) wrote:

>On 09 Sep 2001 09:12:37 GMT, Michael J. Mahon (>>) wrote:
>
>> Ralph Glatt (>>>) replied:
>>
<snip>
>>> ... With my luck, a capacitator WILL be out, and I won't be able to
>>>tell which one!
>>
>> You won't, in general, be able to judge the health of a capacitor by
>> sight.  It can look perfect and be dried-out.  The easiest way to
>> "test" a capacitor for low capacitance is to "bridge" it with a known
>> good capacitor of similar or better voltage and capacitance ratings.
>> In the power supply, this should be done carefully, and with clip
>> leads.  If the supply works with a cap bridged, that cap needs to
>> be replaced.
>
>PLEASE!!! When you do this "bridge" test 
>a) Only do it with one hand - keep the other in your pocket to prevent an
>electric current path across your chest which may make your heart stop or go
>into fibrillation.
>b) Try and use an isolation transformer to connect the power supply to your
>house supply - again to minimise chances of shock
>c) Remember there are HIGH VOLTAGES in a power supply. They are AC and DC. AC
>"tickles" you. DC grabs you (causes muscle lock up) and (to my mind) is even
>more dangerous than AC.
>
>TREAT ELECTRICITY WITH RESPECT! 
>I still want you around to collect more Apple IIs :-)))

Of course care is appropriate.  But, as I said in another (earlier?) post
in this thread, only someone with electronic experience should be
working on a power supply in any case.  And I explicitly recommended
using clip leads, so this is a NO HANDS bridging activity.  (Admittedly,
I don't always do it this way, but I wouldn't recommend my approach
without some "hands on" tutorial.  ;-)  I also assumed that the reader
knows that the bridging capacitor should be discharged (by carefully
shorting it after disconnection), lest inadvertent shock or "welding"
experiments occur.

On the subject of DC shocks, I once picked up an open frame 450 volt
power supply that did not have a "bleeder" resistor, curled my fingers
under the chassis, and involuntarily hurled the supply thirty feet across
the basement, striking the opposite wall!  I immediately installed a
bleeder resistor and considered briefly taking up the shot put.  ;-)

>You mean 15" sparks are supposed to come out of this thing?

With this .sig, I can see why you're concerned about voltage...

-michael

 Email:  mjmahon@aol.com
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