TU56/TC11 restoration - VARIAC question

[John Lawson]

On Thu, 17 Mar 2005, John Allain wrote:

>> 20amps, wow. Someone give me this for christmas!
>
>
> I now have Two nice machines (Tek scope, VT100) that blow
> fuses and have been dancing aound the issue to get one for a
> few weeks now.  I'm not really an electronics guy but I've been
> led to believe that a variac is the single best thing to get to help
> remedy fuse blows.  Any strong yes or no votes towards this
> decision?


  I have two or three of various sizes around the shop - they're very handy 
for a number of things apart from slow-starting of sleepy linear power 
supplies.

   Put 'variac' into eBay's search field - you'll see a good selection of 
them to choose from.  One dealer has what looks like Taiwanese units of 
20A capacity, and a meter and circuit breaker, for $110 but-it-now from 
Canada....  but there are a bunch of others just now.


   Also - fuses blowing in analog gear with linear (as opposed to 
'switching' type power supplies can be caused by shorted rectifiers, 
couipling capacitors gone Horribly Wrong, a pinched wire that finally 
shorts - any number of things.  One clue is how fast the fuse blows, and 
is it violent - is the inside of the fuse cartridge discolored and 
melted-looking, or is it just seperated?  In other words, is it a hard, 
catastrophic short, or is an abnormally high load on the PS?

   To trouble shoot, you should be able to isolate the sections of the 
failing device from the power supply lines - as in basic triage for 
computers, add things back until the symptoms appear, et voila! there you 
are.


   Cheers

John