[TenTec] Titan 425 Amp

[Dr. Gerald N. Johnson]

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 of
> 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.
>
> Thanks,
>
> Steve
> KC3WX
> _______________________________________________
>