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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.
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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
My 667 i picked up from a guy was only putting out 250 watts with a swing input of 11 watts. I opened it up and found a 1000pf cap just loose sliding around inside. After finding a schematic, I realized that there were 2 inside attached to the output transformers. 1 per output. I hooked up the loose 1, watts went up to 300 but I could smell heat. Using my heat detector gun, I checked the transistors. They were warm but not hot. In circuit testing appeared good but the combiner resistor git quite hot to touch. Im thinking that by replacing one stacked mica 1000pf cap and not having another to put onto the other set is causing this imbalance and heating up that resistor. Thoughts?
Yep. Both of the two-transistor push-pull circuits have to deliver the same power to the output combiner. Leaving off that capacitor guarantees that one half the amplifier is putting out less than the other half. That imbalance tends to totally toast the resistor inside the output combiner toroid, and eventually the wires on the combiner as well. The combiner only sees the difference in wattage between one pair and the other. If they're equal, the difference is zero and the combiner runs cool. But the bigger the difference, the hotter the combiner resistor will get.
Very first item offered on this web page is a 1000pf Semco-style metal mica cap. Has longer leads than the one Texas Star used, but will do the job.
https://www.icamanufacturing.com/shop/capacitors
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Very first item offered on this web page is a 1000pf Semco-style metal mica cap. Has longer leads than the one Texas Star used, but will do the job.
https://www.icamanufacturing.com/shop/capacitors
73
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Im checking with them to see if they ship to Canada. If not, maybe someone here has a matched set? Need a correct meter as well. I appreciate the response. 73s
I contacted them. They dont ship to Canada from ICA and it appears that they stock what I need. They do have an ebay site and on that site they do ship to Canada but dont have the part on the ebay site inventory. So,, Im in a bit if a pickle. They stopped responding. I guess that needing parts under 10 bucks that its not worth the hastle to respond anymore. I only need 1. Semco metal case 1000pf 1% 1000v capacitor. Its like pulling teeth to get anything shipped to Canada these days.
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@ Geoexplorer69:Coax question. I just put up a new Sirio monsoon base antenna with 100’ of rg8u coax testing with myMFJ antenna analyzer shows 1.4 swr at 26.965mhz and a 1.1 at 27.405 mhz . When I checked the ohms it’s steady a 30 ohms across the entire bandwidth.
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