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repairing a texas star 667
- Thread starter bigred222
- Start date
Thanks for all your help nomad, I wanted to tell you that i that I got the amp from a friend that he said it just quit working.. But he did tell that he removed the remote switch wire..I ran a new wire from power switch to relay and it just chatters with no power out put except from radio.. would not having a choke on the wire cause this? thanks for all your help God bless
Well , another thing that will cause "chatter" is having the goesinta and the goesouta reversed...
If it still chatters the relay, this suggests that the amplifier's input impedance is way too low, or maybe the power supply voltage is dropping when the relay is activated. Is there a meter showing the power supply's output voltage? If you see it drop when the relay chatters, you may have a power supply that's too small, or has a problem delivering the current it's supposed to.
More than one way for that to come about, including bad RF-power transistors.
73
More than one way for that to come about, including bad RF-power transistors.
73
Hey nomad I checked the power supply voltage and it is ok.. But wouldn't there be 10 ohm resisters blowed ou burnt?I have replaced a keying circuit, but I know it works because I took it out of a working xforce that's exactly the same had this box.. If I pull the transistors will they be around 600 ohms? Thanks and God bless
Zero.
To the nearest whole percent.
Those parts overheat as a result of transistor failure. The parts that actually look toasted are never the cause of the trouble, only a result of it. The parts that caused the episode are blown inside, but look absolutely PERFECT on the outside.
This amplifier is built from a basic two-transistor circuit called "push-pull". Means that when one of the two transistors is on, pulling current the other one is shut off.
This amplifier has two of those two-transistor push-pull circuits. One in the rear, built on a cut-down portion of the same circuit board mounted in front with the relay push-button switches and the other two transistors on it.
So long as both pairs deliver the same power, the resistor inside the toroid combiner coil stays cool. If both deliver the same power, the difference is zero. It only receives the difference in power between one pair and the other pair. Once this out-of-balance power gets up to a few Watts, that resistor gets hot.
And when one pair shuts down entirely, that 2-Watt resistor gets an overload factor of 50 or 100 to one.
Gets it right toasty. Along with the toroid combiner core and wires.
Bear in mind that if another portion of one push-pull pair fails, you will get this out-of-balance problem, and a hot combiner resistor. The metal capacitor soldered across each of the two output transformers seldom goes bad, but if it does you'll get a toasted combiner resistor.
Even if the transistors are still good.
That capacitor almost never goes bad. But the low-bidder RF transistors that Texas Star uses these days are very unforgiving. Keying it into a badly-soldered coax connector will pop a transistor. Using a radio that's "just a little" too large on wattage does this, as well.
Yes, there is a less than one percent chance something else toasted those resistors. But if so, one or more damaged RF transistors is almost certainly part of the picture, even if other stuff is also blown.
Pro tip: Always make sure the bias resistors are not damaged before you key up a Texas Star after replacing those expensive transistors. If they are, those transistors might deliver a total service life measured in one or at most two keys of the mike. Blown bias resistors cause the RF transistors to draw all the current the power supply has to offer.
Until a tenth of a second or so goes by and the transistor is roasted forever.
73
