[sdiy] isolating variac unit

[KA4HJH]

>> Meters are most useful if you're working on power supplies. As you dial it
>> up a sudden drop in voltage/increase in current means something just
>> shorted.
>
>Maybe.  Or maybe something just hit a threshold of activity.  I notice that
>switching supplies don't like variac-controlled power input,  for example,
>and don't seem to do much of anything until you get up around 50-60% of full
>output.  :-)

My first job was fixing lots of cable TV equipment including primitive
switching supplies (variable frequency). We had an AC ammeter as well--you
have to watch both meters at the same time. I got the point where I could
tell you what part was bad simply by the signature of the meters. But of
course this is from repairing lots of the same device. Even so it was
really helpful while repairing an unfamiliar supply.

Contemporary switchers are way too complicated for me.

>> A set of loads is also good to have for PS work. Roy's ballast
>> bulb sounds like a good idea.
>>
>> I've been trying to get one of these put together for myself but never
>> found the right enclosure.
>
>The bulb I'm using is too darn small.  It's a small-based bulb screwed into a
>standard socket with an adapter,  and it's only 40W,  but I can connect a
>larger bulb in parallel with it using the binding posts if necessary.  For
>much of what I work on it suffices,  though.
>
>Loads?   That can be all over the place depending on what sort of gear you're
>working on.

Of course. We used a set of switched loads over the range of the supplies
we were repairing. Sometimes one would work fine on say 75 ohms but die on
25 ohms. This is why I've been collecting a variety of high wattage
resistors for years.

-- 

Terry Bowman, KA4HJH
"The Mac Doctor"