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Cobra 29XLR tx/frequency issue.

ira parker

Member
Apr 18, 2015
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cobra 29xlr has receive, audio from pa, no tx output. Whats been done, aligned pll/vco per manual L5 peaked , 10.240 good stable, L20 3v stable, L21 peaked, L24 peaked at 37.878 (42-96) last 2 digits are oscillating? checked 36.570 at gate of tr18 spot on and stable, I think there should be 2 freqs coming into tr18 to mix but I could only find the 36.570? Moved on to tx alignment scope on tp3 base of tr8 no signal or adjustment from L18 or L17. checked tr6,7,8 voltage switching and they tested good on the atlas dca75. Here's where i need some help moved back to tr10 tested it checked good voltages switching, no frequency on it in rx, tx I get 34.016 (last 2 digits are oscillating up and down) on the base nothing on the collector and 3.20 (oscillating) on the emitter. Checked the legs on the 10.695 crystal they're good but I can't find 10.695 freq anywhere and no freqs on tr9 at all in either mode. Any help/suggestions, what am i missing or doing wrong here. Thanks Ira
 

cobra 29xlr has receive, audio from pa, no tx output. Whats been done, aligned pll/vco per manual L5 peaked , 10.240 good stable, L20 3v stable, L21 peaked, L24 peaked at 37.878 (42-96) last 2 digits are oscillating? checked 36.570 at gate of tr18 spot on and stable, I think there should be 2 freqs coming into tr18 to mix but I could only find the 36.570? Moved on to tx alignment scope on tp3 base of tr8 no signal or adjustment from L18 or L17. checked tr6,7,8 voltage switching and they tested good on the atlas dca75. Here's where i need some help moved back to tr10 tested it checked good voltages switching, no frequency on it in rx, tx I get 34.016 (last 2 digits are oscillating up and down) on the base nothing on the collector and 3.20 (oscillating) on the emitter. Checked the legs on the 10.695 crystal they're good but I can't find 10.695 freq anywhere and no freqs on tr9 at all in either mode. Any help/suggestions, what am i missing or doing wrong here. Thanks Ira
P.S. I'm thinking IC5 going to check voltages on it.
 
Nope, you've already narrowed it down to TR10 or the 10.695 crystal. Two ways to test a crystal. Connect it to a crystal tester, just an oscillator circuit with a counter on it. Or substitute a working spare crystal for the one that's suspected of failure. If the circuit runs with the new crystal, the old one's bad.

73
 
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Nope, you've already narrowed it down to TR10 or the 10.695 crystal. Two ways to test a crystal. Connect it to a crystal tester, just an oscillator circuit with a counter on it. Or substitute a working spare crystal for the one that's suspected of failure. If the circuit runs with the new crystal, the old one's bad.

73
Thanks Nomad, That's what I started with sure enough the crystal was bad was getting 23 mhz on the b and e of tr10 changed it out got 10.695 (odd see a lot of the legs ate up but rarely one that's gone high). Still had nothing to l18 and tr9 all the freqs were there just not getting through l18, tr9, and l17. Pulled tr9 and the caps associated with it tested them all, they were good further testing showed low voltage coming from l17 to tr9 sure enough l17 was bad changed it out got rf aligned it and everything is good. Once again thanks for the help. Ira
 

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    So I have a cobra 138xlr and the meter dose not work... It dose not key up and signal dose not work.... It is not stuck it the meter moves freely. now wires are resoldered on both ends...So now that do I look at. Could it be a diode or a transistor?
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