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Monster
| | Posted on Thursday, December 25, 2003 - 8:45 pm: | 
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The stock 2SC2078 has been replaced with the 2SC1969. the necessary accomodations have been made to the input/output coupling of the new device. the 45 watt figure results from 50 - 100 mv injected into the mic input. with a microphone in a normal voice it's closer to 40 watts pep. the amc circuit remains unaltered and as a matter of fact it's only 2/3 of the way open. all measurements are made at 13.8 volts on the bench and as to your comment about volting, not in the radio but, when it's in a mobile environment with an operating charging system at 14.0 - 14.4 VDC it produces every bit of the 40 watts and then some. Is this even possible? |
2600
| | Posted on Friday, December 26, 2003 - 1:55 am: |
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A couple of things make this possible, even aside from harmonic power produced by modulation over 100%. Some watt meters are extra-sensitive to frequencies higher than the 27 MHz channel frequency. This can cause a power reading to seem unnaturally high from an overmodulated transmitter.
The Cobra 29 has something that SSB CB radios designed since 1980 or so just don't: a modulation transformer. ALL the legal 40-channel SSB radios put onto the market since then have used a transistor in line with the 13.8 Volt battery power to deliver "high-level modulation" to the final and driver transistors. The peak audio waveform that gets pumped into them can't get any higher than the battery voltage that runs it. Simple (legal 40-Channel) AM radios typically still use a step-up transformer for this. The stock final on a 29 will seldom peak beyond 20 or 25 Watts. A bigger final like the 2SC1969 will pull a little more current, and show 30 to 35 Watts PEP. Changing the base resistor on the final to a lower value can get it to 40 Watts. More than that, and I wonder if 5 or so out of those 45 Watts isn't suspect.
Some wattmeters will be more accurate on a dummy load than on an antenna. Expecting a RF wattmeter to be more accurate than about 10% is a long shot unless the load has a perfect 1 to 1 SWR like a dummy load. The difference between my upper limit of 40 and your reading of 45 is around 10%, so that puts it within the range of uncertainty, anyway.
The 29, and a lot of pre-1980 SSB/AM CB radios place a step-up transformer between the audio power amp in the radio and the drive/final. The peak supply voltage that reaches them is roughly twice the steady power supply voltage that is getting the audio power added to it. This higher peak audio voltage on the driver and final collectors will permit a higher peak RF power level on the modulation peaks.
Bear in mind that there is no free lunch here. This places a larger heat load onto the audio power chip, and tends to make them fail more quickly. Simply making sure that the heat-sink tab on that part has a decent layer of silicone heat-sink compound under it is a good safety measure to take any time one of these radios gets maxed out. Kinda like checking the oil in a car. It's a lot more critical for a racing engine than for the daily driver.
73 |
Monster
| | Posted on Friday, December 26, 2003 - 10:22 am: |
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Mr. 2600:
Thanx for the reply. I asked this because a truck stop CB shop offer to sell me a Cobra 29 that would swing 45watts. I imediately told him no thanks, because I've heard this debate before. I respectfully informed him that if I wanted 45 watts, that I'd be better off just buying a Texas Star Modulator. No sense hacking a radio to the brink of oblivion when you can add an amp. Anywho this (alleged) tech's explaination was "it has the bigger Galaxy final in it". Obviously this was a ploy to get in my wallet, while unloading a radio that maybe has 2 more hours to live. Of course he would not allow me to buy the radio, have it checked on another bench, and get my money back if it didn't do what he claimed. Thanx to my fellow hobbyist on this forum!!! 73's and Good DXing |
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