| Author |
Message |
    
harvester
| | Posted on Tuesday, December 17, 2002 - 10:40 pm: | 
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well i think it's just a junk mic
the radio is new to me.. but i did make a dx contact or two with it |
Marconi
| | Posted on Tuesday, December 17, 2002 - 11:27 pm: |
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I would guess right off on that radio that the dead key is probably set pretty high. Just a guess.
Marconi |
Taz
| | Posted on Wednesday, December 18, 2002 - 12:32 am: |
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yeah, my guess too. |
307
| | Posted on Wednesday, December 18, 2002 - 6:07 am: |
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Wrong size jumper...Use a longer one...
307 |
Radiodude
| | Posted on Wednesday, December 18, 2002 - 6:54 am: |
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Ck the SWR |
Znut
| | Posted on Wednesday, December 18, 2002 - 6:18 pm: |
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With or without amp? |
Tech833
| | Posted on Wednesday, December 18, 2002 - 12:27 pm: |
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Have the radio retuned as per the factory specs. If your 148 has not been modified, Cobra can do this for you, however, I think 307 could do this for you for less money and probably even do a better job of it. Copper does repairs!
Not because I am a commercial engineer, but because I am an electronics engineer, my advice- Don't try and 'peak' your 148. Your radio will last longer and work better if you have it tuned to original specs. |
Skilletlicker
| | Posted on Wednesday, December 18, 2002 - 12:16 pm: |
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lou franklin in his book understanding and reparing cb radios page 168 ist paragraph gives
7 reason for downward modulation, you might want to check it out |
harvester
| | Posted on Wednesday, December 18, 2002 - 11:20 am: |
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swr is about 2 or higher 102 ant....rg8 ...what is this jumper you speak of ?could it be not a large enough power supply wire? |
Kirk
| | Posted on Wednesday, December 18, 2002 - 9:19 am: |
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ALC possibly? |
Dx431
| | Posted on Wednesday, December 18, 2002 - 7:48 pm: |
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First thing i would do is install new mini 8 coax and get those swrs down below 1.5.Then turn the carrier down on the 148 to around 3w and let it swing.
JMO
431 |
harvester
| | Posted on Wednesday, December 18, 2002 - 10:29 pm: |
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i got the swr down to 1.3 loose coax fitting .
it swings forward w/ no amp. down with amp in-line.. i dont know how to adjust anything inside.
thanks all |
ss8541
| | Posted on Thursday, December 19, 2002 - 12:03 am: |
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no one mentioned tr41, the voltage reg/am modulation amp and tr42 the vr driver/am mod driver. if one or both go bad you will have backward swing/downward modulation/negative carrier shift. a weak driver or final can also give the same results.
with the swr you listed(2.0:1), a cheap wattmeter may also be at blame for what you are calling backward swing. you really need to have this checked while txing into a dummyload and while using an average reading meter(like the bird 43) or while observing a scope. like 833 said, you may want 307 to do this properly for you, since misadjustment of the tx section can give the same symptoms.
the 1st thing i would do is fix the antenna problem. the 102" whip should have no higher and slightly lower than a 1.5:1 swr when installed PROPERLY (1/4 wave impedance is about 35ohms). |
307
| | Posted on Thursday, December 19, 2002 - 12:29 pm: |
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Again..Try a different LONGER jumper to the meter ,pending you are using a meter. If you are just using the rig and the antenna , run 19' of coax and try it...
307 |
ss8541
| | Posted on Thursday, December 19, 2002 - 1:53 pm: |
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harvester, either i didn't see your 1029pm post, or it wasn't up yet when i was making my 1203am post. i was using info from your previous posts, but with the info from your 1029pm post i would definitely say go with 307's advice.
the amp is most likely showing a mismatch to the radio. the outgoing and reflected power is most likely in a phase relation that is 'showing' downward modulation on a meter, when this most likely is not really happening.
adding a longer jumper will change the impedance that the radio 'sees' from the amp, and will most likely fix your problem. |
Znut
| | Posted on Thursday, December 19, 2002 - 3:16 pm: |
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What about the deadkey of the radio? If I key my radio with more than a 3 watt dead key with my two pill (SD 1446's) on high, it doesn't swing. If it does, it's backwards. That'll get the ole shooting star to about 175 watts swing on an average reading meter. That's really a little too much for those pill's longevity.
Try with the amp on it's lowest power after you make sure your jumpers are all tight. |
Czar
| | Posted on Thursday, December 19, 2002 - 7:23 pm: |
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"backward" modulation is usually a sign of mis-tuning of the tx stages isn't it??? I know when you improperly "peak" the colis and rf stages that is what usually causes "backwards' modulation,or am I wrong??? {I have been known to be.......once I think? HI,HI,} |
de
| | Posted on Thursday, December 19, 2002 - 9:28 pm: |
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I agree with tech 833: "Not because I am a commercial engineer, but because I am an electronics engineer, my advice- Don't try and 'peak' your 148. Your radio will last longer and work better if you have it tuned to original specs."
If you drive an amplifier circut too hard one of two things will happen. And perhaps both. One you will shorten the length of life of the equipment. A circut pushed beyond its design specs has a very much shorter life span than one that is tuned properly in accord with the manufacturers directions. And secondly, you probably will distort your signal rendering any extra power gained useless.
I agree with 833. Read and follow the manufacturers instructions and your equipment will last much much longer. |
Czar
| | Posted on Friday, December 20, 2002 - 12:46 am: |
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de, you are right !!!! That is what I am trying to stay,"peaking" and "tweaking" the WRONG way ends up with backwards modyulation.There is a "peak" on some of the circuits{coils}and if they are not tuned the "right"
side,you get "backwards" modulation!!!! Am I right???!!!! I have seen this about 1 million times over the years!!!!!! |
harvester
| | Posted on Friday, December 20, 2002 - 12:05 pm: |
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ok thanks
everything appears to be working now ..
i only use the meter on the radio ..
the needle swings up in to the red and, just at the end of the green dead key..
with the amp in-line[ on or off] the needle acts the same just lower on the scale. IS THIS NORMAL?
backward swing of the needle appears to have stoped ..
harvester |
307
| | Posted on Friday, December 20, 2002 - 11:08 pm: |
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What did you do? Take the meter out of line which corrected the miss-match? or added a longer jumper (feed line)?
307 |
CM 3885
| | Posted on Sunday, December 22, 2002 - 7:05 am: |
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I might add here.
I usedf to own a 148 GTL that i tuned myself and i never had any reverse modulation but then again i never really messed too much with the coils. Some of those coils are TVI traps so i leave them alone. I pulled the AMC and then later reinstalled it because i noticed nothing changed in regards to swing from the radio...
Just my.002 cents from a average Joe CBer that likes to work on his own stuff.... |
harvester
| | Posted on Saturday, January 04, 2003 - 10:16 pm: |
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it must have been a bad conection at the antenna.
it works fine now |
