If you have replaced the transformer with no effect, you might try
unhooking the transformer primary (just one side is enough) to isolate
the supply from the primary switching. In a simplified schematic I see a
couple capacitors to ground, and a step start or soft start circuit. If
the relay contacts for the soft start are stuck together the main filter
capacitor initial charging currents can be enough to blow the line fuses
and outlet breaker. The purpose of that soft start circuit is to charge
the capacitors a little slower and so keep that surge current
reasonable. Also some transformers tend to remember the last peak
magnetization of the core and if energized with opposite polarity the
core magnetization will draw a big surge current (accompanied by a
"thung" sound from the transformer).
There is a series resistor and a small fuse to protect it that isn't
shorted by K7 when it works right for a second or two. That resistor
can't be shorted out or its fuse would blow (unless that fuse has been
replaced with something large or a brass bar) if the contacts of K7 are
opening. K7 would close fast if the capacitor in its circuit was to go
open. Check out the 1000 mfd across K7's coil. Check K7's contacts.
Such a soft start circuit is used in many linears and the usual failure
is the surge limiting resistor going open or the relay coil going open,
but the contacts could have welded or that time delay capacitor could
have gone open (either a faulty capacitor or bad solder connection) to
make K7 operate way too fast.
Those line bypass capacitors are not above suspicion. And all the
terminal strips and wiring in the primary can have seen some lightning
surge voltages to have created carbon paths to ground. You have to
eliminate all of those.
The classic service technique for such a problem is not to buy a case of
fuses, you can't learn much from them, but is to connect the power
through a tungsten lamp that can act as a current limiter and as an
indicator. Then with power applied and taking care to keep the fingers
and elbows out of the high voltage you can trace the voltages to ground
to find the short and see the effects of cleaning up the short, if there
is one. With a sturdy short, you can test with 120 volts and probably
one 500 watt lamp. Testing from 240 you probably need a couple 500 watt
lamps, one for each side of the line.
73, Jerry, K0CQ
On 12/23/2010 7:58 PM, Steven Stepansky wrote:
> Hello all,
> I'm working on a Ten Tec 425 amp that blows both 20 amp line fuses and the 240
> volt outlet breaker. I've gone through the power supply in detail, inclusive
> replacing the transformer. I've noticed that it will blow both line fuses and
> the outlet breaker once the interlock switch on the back of the supply that
> feeds the orange wire to the amp is engaged. But everything I check on that
> orange line seems to check OK.
> Any advice would be greatly appreciated.
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