[sdiy] isolating variac unit

[Roy J. Tellason]

On Saturday 03 November 2007 00:15, KA4HJH wrote:
> >> 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.

Yeah,  it does get easier when you're doing that...

> Even so it was really helpful while repairing an unfamiliar supply.
>
> Contemporary switchers are way too complicated for me.

Too complicated in a lot of instances.  The last one I fixed, some years back,  
was in a VCR.  It had been working fine,  got unplugged and when it was 
plugged back in wouldn't power up.  The fuse was blown and so was the single 
power transistor in there.  Replacing both of these got me the same result 
again.  I found a "rebuild kit" at a place for under $20 that involved 
replacing a whole mess of different parts,  diodes and zeners and caps and 
whatnot,  and it's been fine ever since.

> >> A set of loads is also good to have for PS work. Roy's ballast
> >> bulb sounds like a good idea.

(Snip)

> >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.

What sorts of values have you found most useful?

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