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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.
Spivey, when you get a chance to look at the radio again. A real quick test would be to put your scopes probe (try X10) on R63 just behind L20. On ch34 key the mic and see if you are getting a signal on the scope, and also check the frequency which should read 27.345mhz. If you can't verify the freq with your scope, use your frequency counter. Then do the same and check the signal again in TX mode on ch35. You should see the signal with a frequency reading of 27.355mhz. I'm assuming that you won't see this signal which should at least partly explain why it won't transmit on 35 thru 40. This signal comes from IC3 mixer pin 9 throught L20 to TR16 buffer and to TR15 driver, per the schematic.
Ok Night Stalker, this is what I've came up with. It is transmitting on 34 and 35 but 36-40 isn't showing anything but it seems to be receiving on them cause I've heard some skip coming in. When I key it up on 34 and 35 I notice it buzzes through my IFR speaker then it goes away and seems to stabilize. It's less then a second before it clears up and you can see on the analyzer sporadic side tones or voltage spikes before it clears up and centers on frequency. I also did those checks from your previous post.
IC3 ch34
Pin 1 2.7 vdc; Pin 2 1.95vdc; Pin 3 1.2vdc; Pin 4 2.7vdc; Pin5 0vdc; Pin 7.6 vdc;
Pin7 2.2 vdc; Pin8 4.8vdc: Pin9 7.6 vdc
IC3 ch35
Pin1 2.7vdc; Pin2 2vdc; Pin3 1.2vdc: Pin4 2.7vdc: Pin5 0vdc; Pin6 7.6vdc; Pin7 2.2vdc; Pin 8 4.8vdc; Pin9 7.6vdc
IC3 Pin 1 ch34 16.6503 mhz; ch35 16.6603 mhz
Pin 4 ch34 10.24 mhz; ch35 12-13mhz
Pin 6 ch34 13.1mhz; ch35 13.1mhz7
Pin 7 ch34 27.345mhz; ch35 27.355 mhz
Pin 8 ch34 27.345mhz; ch35 .008mhz
IC2 Pin 1 ch34 2.7vdc; ch35 2.7vdc
Pin 2 ch34 2.0vdc & 16.55mhz; ch35 2.5vdc & 16.55mhz
PLL Pin22 ch34 0vdc & 27.305 mhz; ch35 0vdc & 27.355
Pin 12 and 13 0vdc on TX
All values were in TX
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Okay, so If ch36 isn't transmitting, just do the same frequency test in transmit mode on IC3. Pins 6, 7, and 8 should show 27.365mhz for ch36. I'm not sure why it is showing 13.1mhz on pin 6 when you checked it on ch34 &ch35. Pin 4 should show 10.240mhz on all the channels in receive mode. Also it looks like its showing .008mhz for ch35, that puts up a red flag for me, since it should be 27.355mhz. IC2 looks good. I probably forgot to mention that the testing of the PLL was in receive mode. But even in transmit mode pin 22 should be around 1.2mhz and voltage of 2.5v, so I'm not sure why your reading is 0v. Pins 12 & 13 should be around 2.5v even in TX mode. Maybe you were on the wrong pins?
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