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Cobra 29ltd no transmit 35-40
- Thread starter Spivey
- Start date
Sounds like it just needs an alignment. Without the proper tools, you should not attempt. You didn't say, what tools do you have?
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.