TU56/TC11 restoration - VARIAC question

[John Lawson]

On Thu, 17 Mar 2005, Ashley Carder wrote:

> I do not own a variac, so I will need to get one of these to
> properly reform the TU56 capacitors.  I see several different varieties
> on eBay.  What would I need exactly?


  I am assuming here that you have removed the loads - all the other Stuff 
- and it's just the power supplies connected.

   There will be an overall power rating (usually in watts) for the whole 
unit, but the unloaded power supply will draw only a fraction of this.

   Variacs (powerstats, vari-trans, etc) are a class of transformer called 
'autotransformers' - they are a single coil of wire with an adjustable 
'tap' in the form of a moveable slider. The AC line goes from one end of 
the coil to the other, and the position of the tap determines how much 
power/voltage goes to the load.  Variacs are wound for a certain maximum 
power level, and it is customary to specify them in amps.

   So in order to 'bring up' the power supplies, you need a Variac that 
will have the capacity to handle the current that the supply *itself* 
draws. And that is usually an amp or two for the bigger devices.  There 
will be a bit of a surge, but you eliminate that by slowly turning up 
the voltage.

   There is the issue of how to know something is getting cranky during the 
turn-up process - the simplest method is probably to wire a common lamp of 
about 100 watts in *series* with one of the wires to the unit. If the lamp 
glows brightly right away, probably there's a short and you should 
investigate further. If the supply comes up with no problems (and is not 
loaded) then the 100W lamp should not glow very brightly, if at all.

   NOW- all that being said - if you get yourself a 10-amp Variac, it will 
serve you well in the future for Stuff like this.  There are several 
designs out there that are 'packaged' - they have the Variac itself 
mounted in some kind of box with a nice cord and usually outlets to 
connect your load device to it.  Some also have voltage and current meters 
- these are *very* handy and if you can find one that would be the best 
for this kind of work, IMHO.   Here is the eBay listing of such a unit:

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=50968&item=3880906377&rd=1

   and right now it's at a decent price, too!!


Cheers

John