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Cobra 29 st transmitt
- Thread starter Coondog27
- Start date
Don't get ahead of yourself and I say that without trying to be smart, just trying to save you a bunch of headaches.
1. Get a swr meter
2. Mount your antenna and radio and coax.
3. Check and set swr the proper way, there are lots of videos on the tube. Check it on channel 1, 20, and 40. Then adjust the tip to try to get channel 1 and 40 to be near the same with a dip in the middle on 20. I don't care what anyone says this is the way to do it in a mobile.
4. If and when you add an amp, take out the swr meter and place the amp in it's place, if you try to check swr with and amp in line you will get different readings, there have been thousands of post on this subject.
If you can get it to 1.5 on each end with a dip in the middle you will be golden.
1. Get a swr meter
2. Mount your antenna and radio and coax.
3. Check and set swr the proper way, there are lots of videos on the tube. Check it on channel 1, 20, and 40. Then adjust the tip to try to get channel 1 and 40 to be near the same with a dip in the middle on 20. I don't care what anyone says this is the way to do it in a mobile.
4. If and when you add an amp, take out the swr meter and place the amp in it's place, if you try to check swr with and amp in line you will get different readings, there have been thousands of post on this subject.
If you can get it to 1.5 on each end with a dip in the middle you will be golden.
Well guys, thanks to your help, I made two contacts the other day. Both were in the 40 mile range. I made contact with my friend. He runs a base station and said it was coming through clear, just a little quiet. Running my Cobra 29 sound tracker into a Texas Star dx100 amp and out to the Wilson 1000 mag mount.
Another question to pick your guys brains. I have my Texas Star dx100 hooked up inline as it should be. Good voltage to it. Amp lights up good, I have my workman meter inline after the amp. Swr checks out good, no complaints. when I key out, my watts jump up to 250 going out. Do you think that maybe this amp has been modded?
What is the dead key out of the radio? Should be 2 watts or less and about 8 watts pep on AM, 12 watts pep on SSB if it has a relay for SSB and a switch, if it doesn't it may be choppy on SSB due to not having the relay keeps the amp keyed in between words.
If I took a picture of the board, would you be able to tell me if it has been modded?
You should be able to tell the dead key on AM with a watt meter. I shouldn't need to look at the board. With a watt meter and the amp out of line, and the radio keyed up, how much power does it key up at. 1 watt, 2 watts, or is it a stock radio that keys 3-4 watts? If so this may be your issue. You need to have your AM carrier or the dead key turned back or have the modding done to make it dead key or show a carrier of no more than 2 watts. The amp May only need .5 watts. It can take range from very little power, like .25 watts to about 2 watts for most amps. Again, this is what the radio should show on a watt meter just when you key the mic up, or talking. You then want to set the radio up to have a proper 1:4 ratio of swing. So if the radio dead keys 2 watts it should really be doing about 8 watts pep or modulated power of 8 watts pep. This will give a nice clean signal into the amp and also the proper dead key as well. I can look st the board to see if someone has added a cap and resistor, but this may require a pic of both sides of the board. Top and bottom. One side to see the capacitor and the other to see the resistor. And I still won't know how many watts it dead keys. One of the techs might be able to tell if they can see the value on the cap and know the value of the resistor by looking at it. But a simple watt meter check would tell you this. Key the radio amd look at the meter without the amp on or without it in line. Either way should give you the dead key power. Shouldn't be over 2 watts or your radio has not been modded. This is important as most small amps don't need 3-4 watts of dead key and this may be why you are seeing the high power out when you key the radio into the amp. Or there is another issue at hand. Make sure your VSWR is good and then check the dead key without amp being ON. Leave amp OFF. Hope this helps and keep us posted. You may need to send the radio to a qualified tech to have the dead key mod done or have a variable power installed.
To lower th dead key on that radio you either need to add a cap and resistor, TIP 120, or a variable power is another option as well as some others, but I don't think the cobra 29's have a pot for turning the AM power down by itself. You would have to mess with the AMC. Which you don't want to turn down. You will need some proper equipment to do the stuff right. Not as easy as just placing it on a 50ohm dummy load and a decent watt meter and turning AM power down via a pot. Mine dead key is at 3 watts on mine, but it's been modded. Pep is 12 watts on mine. Try to keep a 1:4 power ratio and you should be good. Sometimes it's just best to find a good tech and have a variable power installed where the delta tune is located. You need better parts to do it right, but then have the ability to set the dead key much more precisely. Also a good tech will make sure the modulation is set correctly and that it's aligned and working properly and making sure it's clean on a scope. Clean in equals clean out!! But seriously, modding this radio is a bit more than just turning a pot in the radio. If you choose to perform it do so with caution!!
Probably needs the AMC turned up. Radio most likely needs to be completely tuned and aligned. If memory serves me correct 3 watts is about stock for those radios. And no they won't have much swing as to keep them legal. You turn up the modulation to about 95% andyou ahoukd see some swing then. Should put you close to 10-12 watts pep from a 3 watt dead key. Like I said, I'm pretty sure you have to lower the dead key with some modding of the radio.
I agree. Radio needs a proper tune & alignment with the dead key set at 1.5-2 watts. Only way to see what has been done to your amp is look at it. Is it a 1 or 2 final (pill) amplifier and what number is on the final or finals? If that amp is really keying at 250 watts I would stop using it right away or you will burn it up.
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