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Cobra 29ltd no transmit 35-40
- Thread starter Spivey
- Start date
Thanks cable guy. I actually did the alignment twice now just to check myself. Unless I’m doing something wrong according to the service manual. I have an IFR 1200s, tektronix 475 scope, freq. counter, and vtvm.
That's an interesting problem. Have you checked the voltages and frequencies in the IC3 TA7310p TX mixer? Pin 1 gets its 16-17 mhz reference signal from TR20 (osc) which should be 16.649mhz while on ch 34, and 16.659mhz on ch 35 in RX mode. Notice the 10khz difference in freq, are they changing with the channel selector? If they are thats good. In TX mode does the frequencies show a small change when you change the channel from ch 34 to ch 35? If no, there maybe a problem with IC3 or its circuit. Also check pin 4 (Mixer Input), it should have 10.240mhz in RX on pin 4 and shows a higher value in TX mode. Now when you test pin 6 (Mixer Output) in TX mode, ch 34 should be close to 27.345, and ch 35 should be 27.355mhz. The same readings on pin 7 and 8 using freq counter or scope. They should show the frequencies per the channel the radio is on. While on the IC3 TX mixer, what are the voltages in TX mode? They should be something like this:
pin1 = 2.7v, pin 2&3 = NC, pin 4 = 2.74v, pin 5 = GND, pin 6 = 7.6v, pin 7 = 2.2v, pin 8 = 5.1 v (Vcc), pin 9 = 7.6v.
It might not hurt to check IC2 VCO too. Pin 1 ch34 should be 16.649mhz (around 2.7v) in RX mode, and pin 2 should be the same 16.649 mhz, around 2.0v. On ch 35 the freq for pin 1 should be 16.659 mhz (2.7v) and pin 2 is 16.659mhz at 2.0v also.
The PLL should be around 1.3mhz on pin 22 and around 2.6v according to the o'scope. And of course can't forget the 10.240mhz signal on pins 12&13. Not sure if this will help, but I think its a good place to start.
pin1 = 2.7v, pin 2&3 = NC, pin 4 = 2.74v, pin 5 = GND, pin 6 = 7.6v, pin 7 = 2.2v, pin 8 = 5.1 v (Vcc), pin 9 = 7.6v.
It might not hurt to check IC2 VCO too. Pin 1 ch34 should be 16.649mhz (around 2.7v) in RX mode, and pin 2 should be the same 16.649 mhz, around 2.0v. On ch 35 the freq for pin 1 should be 16.659 mhz (2.7v) and pin 2 is 16.659mhz at 2.0v also.
The PLL should be around 1.3mhz on pin 22 and around 2.6v according to the o'scope. And of course can't forget the 10.240mhz signal on pins 12&13. Not sure if this will help, but I think its a good place to start.
Trying to wrap my head around this one. You've done the alignment twice, so that's a clue that the PLL, VCO, etc, should be working or you couldn't get through the PLL portion of the alignment procedure. You also said no output power on 35-40, indicating that receive is working on those channels, which would also (mostly) exonerate the PLL, VCO, and mixer(s).
Yet this is a pretty simple radio, with not a hell of a lot to break. So, even though I've never worked on a 29LTD, here's my ideas.
Look at the inputs to the TX mixer on channel 40. Should be 10.240 and 17.165 coming in, 27.405 coming out. If that's not happening then you know it's in the PLL, VCO, or mixer and you can troubleshoot from there. If it's good, then follow the signal down the TX chain to see where it disappears. Then check the voltages for that component on CH1 vs CH40. If something is going to low (or high) you can chase it down.
To my untrained eye this looks like a TX inhibit signal is getting switched in that probably shouldn't be. Something like the PLL's out of lock signal. But, I've been wrong before.
Now, if I misinterpreted what you said and it there's no receive on 35-40 either, then that indicts the PLL, VCO, mixer area. I'd start at the PLL programming pins to make sure they're right and go from there.
Or just ignore me and listen to the smart people that are weighing in. They may have actually fixed a few of these in the past.
Yet this is a pretty simple radio, with not a hell of a lot to break. So, even though I've never worked on a 29LTD, here's my ideas.
Look at the inputs to the TX mixer on channel 40. Should be 10.240 and 17.165 coming in, 27.405 coming out. If that's not happening then you know it's in the PLL, VCO, or mixer and you can troubleshoot from there. If it's good, then follow the signal down the TX chain to see where it disappears. Then check the voltages for that component on CH1 vs CH40. If something is going to low (or high) you can chase it down.
To my untrained eye this looks like a TX inhibit signal is getting switched in that probably shouldn't be. Something like the PLL's out of lock signal. But, I've been wrong before.
Now, if I misinterpreted what you said and it there's no receive on 35-40 either, then that indicts the PLL, VCO, mixer area. I'd start at the PLL programming pins to make sure they're right and go from there.
Or just ignore me and listen to the smart people that are weighing in. They may have actually fixed a few of these in the past.
I would be curious to know (as a quick check) whether the PLL is locked or unlocked on those channels with no TX. I would think it would kill receive as well if it WERE unlocked but I am not sure that you explicitly said whether RCV was working or not...... only that it did not have TX.
Hope you get it figured out!
Hope you get it figured out!
I assumed he had receive on channels 35 - 40, just no transmit. That's why I thought it would be good to check things out with the IC3 TX mixer, PLL and VCO first. If everything checks out okay in those circuits, then the next thing that might could cause this problem would probably be something wrong with the channel selector circuit like Brandons post suggest. Usually this kind of problem is more common in the old multiple crystal 23 channel radios.
I posted that for reference only. The binary shows no inputs common only to the problem channels so I do not think its a binary or channel selector issue.