148 GTL AM and SSB limiter???

[King Mudduck]

Some one told me that there are two limiters, one for AM one for SSB. The radio is a new Cobra 148 GTL and although SSB performance is fine, AM leaves a lot to be desired.....even with a power mike! I know it's been asked a thousand times but.....what is the AM only ( if there is one ) modulation limiter location? I did a search here and on the net as well and found very little info on this, just want to make sure i remove the correct one. Thanks!

 

[freecell]

a lot to be desired doesn't tell us anything about the modulation index. if the transmitter is able to develop peak envelope power output that is just at or under 6db. above the resting, unmodulated carrier then the modulation index is 1, or 100%. the new 148 still accomplishes that quite nicely without butchering the AMC circuit.

 

[Switch Kit]

KM , IM not up on the newer 148s , but it should have the AMC VR-7 (at least thats what it was in the older radios) you can just turn that up and that should be all you need ,(if radio checks say your not loud ? there lying to you (it's about there kind of loud)) if you clip R-131 (if it's the same as the older radios ?) it will screw up SSB audio badly.

I agree with Freecell to a degree here KM , but if you want what you want ? You can have it . Another way you can do this without messing up SSB audio would be with a diode , very simlar to NPC mod (call it whatever you want) http://www.geocities.com/y40002000/2008.jpg if you were to do that ? you could then just control the power by VR10 and still have the swing if you were driving a box with your set up , louder audio is a given then without any other clippings.

 

[King Mudduck]

I mean i understand completely with both of you, i dont want to screw up the SSB because that's mostly were i stay any more, i'm over all of the CB bullshat! It's just that the radio will not come close to 100% modulation on AM even after turning up the VR adjustment for modulation to the max. The side band works just fine.....as it should....and i dont want one of those big loud truck driver radios, i really dont but i do use this radio in the big truck from time to time so with the weak modulation its kinda hard to talk to most people unless they are close.

The reason i asked about the limiter is this. If i set the key at around 4 watts it will only peak out about 6 watts. If i turn the key up higher to around 8 watts it will talk to just over 12 watts but still the modulation is very week, around 75% if not less and the on air reports are not that great, on SSB i get outstanding reports. Some say it sounds just as good as my FT101-EE so i must have done a really good job on the tune. The radio is working just as it should but i would like just a little stronger modulation on AM but if doing so will screw up the sound quality on SSB i will not touch it!

 

[freecell]

exactly what make and model wattmeter are you using to measure the power output and on what scale/s?

 

[King Mudduck]

Dosy 4002 on the 20 watt peak scale....yeah i know but i cant afford a bird meter.

 

[freecell]

measure carrier on the 20W scale and measure pep (am & ssb) on the 200W scale, then let me know what you see.

 

[King Mudduck]

measure carrier on the 20W scale and measure pep (am & ssb) on the 200W scale, then let me know what you see.

AM power.
6 watt key 11 watts peak. 200 watt scale shows 11 watts peak. 75% modulation.
RMS 6 watt key, swings back to 5 watts. To much key?

SSB power.
No key of course pegs out meter on 20 watt scale.
200 watt scale. 22 watts peak. Just over 100% modulation.

 

[74IN]

Clip one leg of R131 and add a switch between the resistor leg and the board where it was connected.

Open switch on AM, regulate mod with the mic gain.
Close switch for AMC in SSB.

 

[loosecannon]

KM,

maybe the TX section is not aligned properly.
if that is the case, turning up VR7 still wouldnt give you 100% modulation.

try this:
set the radio to channel 19 or 20.
turn the mic gain all the way up on the radio.

using a plastic or wooden tuning tool, tune L48, L47, L46, L45, and L38 for maximum PEP reading on your wattmeter, while saying "ahhh" into the mic.

if, after doing this, VR7 still does not give you the modulation level you want, you can remove R131 which will only affect AM mode.

do not mess with TR24 at all, as that will affect the SSB modes.


you may find after doing this, that you dont need to run the mic gain all the way open anymore.

later,
LC

 

[freecell]

"....you can remove R131 which will only affect AM mode."

that's correct.

 

[Switch Kit]

I stand corrected that R131 WILL NOT effect SSB that it's TR24 that you leave alone.
Even Still , these radios don't need much more then 100% modulation to sound good.
Now a clip is a clip anyway I look at it ? or am I wrong about that ?
Is somebody saying that if you clip R131 that it can be controlled my the mic gain to somehow be closer to 100% modulation ?
I didn't think it worked that way , (It was explained to me once upon a time ago)
I personally would be more concerned about the SSB side of the radio anyway , but why would it be nessasary to clip R131 ?
I've read that you can replace the resistor in front of TR26 to 270 ohm still be in the ballpark without going over 100% modulation.
I suppose my question here is , I thought it wasn't possible to obtain 100% modulation by clipping any limiter in this radio that it would go over or distort the audio weather you used the mic gain or not after the clip ?
Am I wrong about that thought ?

 

[psycho]

Clip one leg of R131 and add a switch between the resistor leg and the board where it was connected.

Open switch on AM, regulate mod with the mic gain.
Close switch for AMC in SSB.

What the hell?

 

[freecell]

if the transmitter is properly aligned it will produce 4:1 peak-to-carrier levels on am all the way up to 4.5W of carrier, 4.5W carrier and 18W pep without any modifications. some will even go as high as 5 & 20. you can ignore all modulation percentage measurements taken with a wattmeter similiarly equipped as they are completely inaccurate.