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Icom 718- ALC modification for AM possible?
- Thread starter High Miler
- Start date
Actually, "duty cycle" isn't quite the right term - and I don't know what WOULD be :?
Duty cycle, in its "real" sense, means how long a transmitter, light bulb, TV set, whatever, is turned on vs. how long it's turned off, over a period of time.
FM transmissions are typically always at the same level of RF voltage, as seen on an O'scope; there's no halfway point - it's at 100% when the transmitter is on, and zero % when the transmitter is off.
A CW transmission is pretty much the same thing. The full signal level is there when the key is down, and nothing's there when the key is up.
AM (double sideband, full carrier) is a bit different. With no modulation at all, what you'd see on a 'scope is indistinguishable from a keyed CW transmitter. Add modulation and you start to see the analog effect. There ARE gradations between zero and 100%, and the signal level changes in step with them.
It's here that "duty cycle" loses some of its meaning, as far as the meaning used in physics. The percent of modulation really doesn't count, though.
Duty cycle, in its "real" sense, means how long a transmitter, light bulb, TV set, whatever, is turned on vs. how long it's turned off, over a period of time.
FM transmissions are typically always at the same level of RF voltage, as seen on an O'scope; there's no halfway point - it's at 100% when the transmitter is on, and zero % when the transmitter is off.
A CW transmission is pretty much the same thing. The full signal level is there when the key is down, and nothing's there when the key is up.
AM (double sideband, full carrier) is a bit different. With no modulation at all, what you'd see on a 'scope is indistinguishable from a keyed CW transmitter. Add modulation and you start to see the analog effect. There ARE gradations between zero and 100%, and the signal level changes in step with them.
It's here that "duty cycle" loses some of its meaning, as far as the meaning used in physics. The percent of modulation really doesn't count, though.
Toll_Free
Well-Known Member
High Miler said:
Ahh yes, the ALC mod / don't mod debate.
Amateur transcievers act differenter than CBs in that they can't be cranked up just by turning a knob, like AMC.
The ALC causes distortion to the transmitted product. It doesn't show up as distortion on the output... It's designed NOT to.. It shows up as distortion on the transmitted audio signal... IE, it "squishes" the audio / pinches off the transmitted signal.. Causing that "clipped" sound we are SO unfond of with amateur transceivers.
Herein is the problem / cure / whatever. If you crank the ALC up, you can run bigger numbers on AM. Without any type of processing, most ham rigs will run between 15 and 25 watts of carrier, and peaks up to 100 to 110 watts. You can crank the carrier DOWN, and get them a LITTLE louder, but they start to flat top and SEVERELY pinch / clip the transmitted signal. That's the <<sound>> we hate. Welcome to low level modulation. Not a LOT we can do about it.
BUT, we CAN take the ALC and crank it up a bit. Put the radio on FM, and set your ALC so that you can get about 150 watts of carrier power on TX. This will allow you to run up to 150 pep on AM as well... And allow you to hit that 150 BEFORE the ALC starts severly acting to clip your signal into shitty sound.
IE, you can now run about 35 watts of carrier. This is about a 50 percent increase. I hope your power supply can handle this.
If you run a processor, and clip the negative going modulation product, you can run > 100 percent modulation, as long as you keep the negative less than 100 percent. That's the key in low level audio, keeping the negative peaks at or below 100 percent. You can go > 100 on positive peaks, if you set up the radio correctly, but...
Lastly, I would also say that doing this will effect your SSB transmitted signal. If you want to keep your radio within spec, as far as filth on the output, you now NEED to run the compressor, so you can keep positive peaks at 100 to 110 watts PEP. Your ALC doesn't work until you hit 150 now, remember
It's either a real bitch, or a small price to pay.. You make that call. (You can also go farther by playing with bias and other things, but I've gone farther than I believe your capabilities can cover based upon your question... Not knocking you, but..).And anyone that doubts me on using a transciever on AM, go ask anyone on channel 6. My TS440 was a wanted radio.. Guys are snappin them up now and sendin them to me to get them hotrodded.
--Toll_Free
Ummm....How about using it for AM on 10 meters???The 718 ain't no CB!! Just what were you thinking of "clipping/removing"??? Don't forget to add a power mic and echo. If you hang the 718 from a celing fan at home or if mobile from the roof with 18" bungee cords and shake the steering wheel back and forth you can get lots of SWIIIIING
Shadetree Mechanic
Delaware Base Station 808
My icom 718 did great on am. Carrier set at 15w, mic gain to proper level as not to buck the alc. It had the full 100w swing with about 85%-90% audio. With some eq it sounded very nice for a rice box. You just have to set things correctly
I would expect a ham snob answer like this on one of the ham snob forums ,but not on this site.sad!The 718 ain't no CB!! Just what were you thinking of "clipping/removing"??? Don't forget to add a power mic and echo. If you hang the 718 from a celing fan at home or if mobile from the roof with 18" bungee cords and shake the steering wheel back and forth you can get lots of SWIIIIING
Always make sure you post in full details next time,saying mod for am use,bigger swing wakes up the old hams that never talk on their radios but set back like they own the bands.
Speaking of that you all got nice radios but never speak or use them, why?
Speaking of that you all got nice radios but never speak or use them, why?
Some ham rigs are hard to get decent am audio and some are not.
Kenwood TS-570 or TS-480 are fairly good on am.
I had both and with 35% mic gain & 10db compression using the stock
hand mic I got very good audio reports on am.
As long as you don't eat the mic or yell into it you will sound good.
The 718 is a different animal.I set mic gain at 15% with a 10 watt dead key
processor on & hold the mic about 3" away from my mouth & speak in a
normal volume as if the person were in the room with me.
Don't shout or it will clip & don't eat the mic!
Kenwood TS-570 or TS-480 are fairly good on am.
I had both and with 35% mic gain & 10db compression using the stock
hand mic I got very good audio reports on am.
As long as you don't eat the mic or yell into it you will sound good.
The 718 is a different animal.I set mic gain at 15% with a 10 watt dead key
processor on & hold the mic about 3" away from my mouth & speak in a
normal volume as if the person were in the room with me.
Don't shout or it will clip & don't eat the mic!
My 718 Kicks on AM. It is the SSB that is a bit low but all and all a good radio. It really was not made to be a Good AM Radio. Buy a CB if you want good AM.
It isn’t not ain’t.The 718 ain't no CB!! Just what were you thinking of "clipping/removing"??? Don't forget to add a power mic and echo. If you hang the 718 from a celing fan at home or if mobile from the roof with 18" bungee cords and shake the steering wheel back and forth you can get lots of SWIIIIING
AM has a lower powered carrier than AM so explain to me your statement because you are wrong. Lock down a radio on FM & see how long it last. The 718 has no FM mode.