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
-
You can now help support WorldwideDX when you shop on Amazon at no additional cost to you! Simply follow this Shop on Amazon link first and a portion of any purchase is sent to WorldwideDX to help with site costs.
cobra 29XLR tx/frequencys
- Thread starter ira parker
- Start date
It has receive, so we know that the output from the PLL is working. The PLL has to be locked, or you would not have receive signals. Some radios only lock out the transmitter when the PLL loses its lock on the channel frequency. This radio shuts down both transmit and receive when the PLL is out of lock. L20 adjusted for a proper 3 Volts at TP5.
TR9 is the transmit mixer. One gate should show the PLL output, the other one the 10.695 crystal output. If that crystal quits, no transmit. It's a long shot, but check the DC voltages on TR10. The odds make either the 10.695 crystal or TR10 about the same chance of failure. Not high odds, but possible.
Any way you can get your hands on a 'scope with a times ten probe? Would reveal what is or isn't feeding into TR9. Has to have both inputs to transmit.
73
TR9 is the transmit mixer. One gate should show the PLL output, the other one the 10.695 crystal output. If that crystal quits, no transmit. It's a long shot, but check the DC voltages on TR10. The odds make either the 10.695 crystal or TR10 about the same chance of failure. Not high odds, but possible.
Any way you can get your hands on a 'scope with a times ten probe? Would reveal what is or isn't feeding into TR9. Has to have both inputs to transmit.
73
Last edited:
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