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Biged
| | Posted on Monday, February 18, 2002 - 12:41 pm: | 
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My tech that I use quite often says that most radios and linears will swing backwards on a bird meter at high power.He says that a dosey meter is a very, very, very forgiving meter.It just gives you a ballpark figure,which I believe is true.The reason that he talks about reverse swing is because they sell a "POWER FORWARD KIT"that they will also install in the radio for a price.I had asked him about the NPC mod and he had never heard of it.He seems to think that it is nothing but lowering the carrier so low that it looks like a heavy swing on a meter.He also said that the ideal wattage to run my amp (Galaxy 400 with 4-1446's) would be 3-3 1/2 watts otherwise I would be under driving my amp.He said "quote" Never drive an amp at 1 1/2 watts, which is what the NPC mod usually does.Can anyone elaborate on this before I go and spend alot of money that I really dont need to. |
Bigbob
| | Posted on Monday, February 18, 2002 - 8:28 pm: |
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POWER FORWARD kit? gimme a brake,my tech has been in the business 40+ yrs. has several bird meters and 2or3 scopes .He has never said anything about reverse swing,he has said some amps on hi will over modulate,flattop and/or splatter.His remedy is an L-PAD it cuts back the drive to the input of the rf transistors much the same as turning down the output of the dead key.An L-PAD is 2 resistors 1-series,1-parallel in the shape of an L hence the name.The value of the resistors are specific to the input bias.My opinion,some amps need more than 1.5 watts dead key,but 2or3 should plenty.My tech tunes the specific radio and amp together for best modulation and reliability.Oh by the way,there's a culprit called Downward-Modulation and is a question for 307,it can be tuned out of the radio and the L-PAD will do the same for the amp,but the output needs to be viewed on a scope to determine this,that kit you first spoke of shouldn't cost you more than 50 cents. |
Warlock
| | Posted on Monday, February 18, 2002 - 9:23 pm: |
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When wondering what carrier (dead key) your radio should be for ANY amp, just remember this rule of thumb:
The amp's carrier should be 1/4 of what the peak is. Whatever is necessary for the radio to be to achieve this.
With a Palomar 225 or the like, 1.5 is usually optimum. For the Galaxy 400 or the like, 3w is usually optimum. It may be different with every radio and amp, but keep that ratio in mind and providing there are no problems in the system you will be loud and clear.
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Biged
| | Posted on Monday, February 18, 2002 - 10:19 pm: |
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Warlock and bigbob,
I thank you very much for your prompt reponses.This forum is the greatest thing since sliced bread.You can certainly learn alot from others.By the way,where could I find the NPC mod for a Galaxy 66?Thanks once again...Eddie |
bruce
| | Posted on Tuesday, February 19, 2002 - 12:14 pm: |
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the reverse swing is because the final amp is starved for drive or is not getting a well regulated voltage as for meters my byrd my drake and my copper 29 dollar antenna tunner/ swr/ power meter all read about the same and guss what wissle into the mike and all 3 go up ...like i said i only trust one tech.. ME... kidding guys |
rf tech
| | Posted on Thursday, February 21, 2002 - 12:56 am: |
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the reverse swing is 98% of the time due to too high of a carrier. if the amp is capable of 400wpep and you have more than a 100w carrier (4:1 ratio), then you WILL have reverse swing. it is called neg carrier shift in the broadcast world. it is also known as downward modulation as bigbob has stated. it is not good. you'll have a good signal but will sound weak on the rxing end. this will show as backward swing on a standard bird 43. this will also show as backsward swing on a power supply with an amperage meter. the way to fix this is to reduce carrier power until the standard bird 43 stays still or moves forward, or look for the same results while watching the amperage needle on a power supply. the more forward it moves, the more audio you'll have.
those dosy and pdc/astatics can fool you into thinking everything is good because they swing forward. not so since they will still swing forward with a high level of downward modulation. |
jyd
| | Posted on Thursday, February 21, 2002 - 11:11 am: |
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thats what i was thinking to rf tech |
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