TU56/TC11 restoration - VARIAC question

[Tony Duell]

> 
> > 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 wouldn't actually use a variac for either...

The Tek 'scope is prssumably a 500 series, and therefore valved. It's not 
going to do a lot if the heaters aren't up to temperature, therefore 
reducing the mains input is not going to help. Which fuse (AC input, or 
HT) does it blow, does it happen at swtich-on or when the HT relay pulls in?

You may find that you can disable supplies by pulling the appropriate 
6080 double triodes (but IIRC, the -150V supply must be kept for last, 
since the others use it as a reference), and then see if you can find out 
where the short it that way.

The VT100 has a switching supply. A variac is not particularly useful 
there either -- the series lightbulb is more use. I use a couple of 100W 
240V bulbs in series connected between the +ve side of the mains 
smoothing capacitor and the chopper transformer (I remember I had a to 
cut a track to do this). 

Most likely the chopper has failed, and taken out some low-value 
resistors with it. 

You really haven't a hope of fixing this without the schematics and a 
good understanding of what should go on. Rememebr that much of the 
circuitry -- including the chopper transistor -- is directly connected to 
the maisn, and lethal to touch (this thing generates about 340V from the 
mains by voltage-doubling the incoming 115V). Fortunately the control 
circuitry is on the isolated, output side, coupled to the chopper 
transistor by the round, black, pulse transformer, but it's still not 
pleasant to work on. I have done it, though.

-tony