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
