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Cobra 2000 GTL Question

S

Sickpuppy22

Member
Nov 24, 2008
85
2
18
Montreal,Quebec.Canada
I am somewhat confused, i had to lower my carrier with VR10 for my 4x1446 ab box which was not a problem.

Carrier at 2.5 watts has maybe 4 watts pep of foward swing with 30% modulation..... though at stock factory settings it will key 5 watts and swing 10 pep with 100% modulation.

Now why would there be such a huge difference in modulation when i lowered the carrier? even with vr12 turned all the way up and mike gain all the way to the right i still cant get 100% modulation at 2,5 watts carrier.

Limiter is not cliped and i would rather not clip it either. What options do i have to get at least 10 watts pep so i can drive my amp better.
 

You may have to retune it or put an audio kit in it Ive seen this before with stock radios like the 148 2000 ETC There are also mods with part swapd you can do Ill try digging through my piles of books and see if I can find some of this info unless someone else remembers off the top of there head what the resistor and part swaps are.
 
are you going by the modulation meter in the radio?

if so, that meter has been calibrated to show full modulation at a 4 watt carrier.

turn the carrier down and the modulation meter will show less modulation.

you are not actually at 30% modulation, the meter is just reading wrong.

your modulation is roughly doubling your carrier.
at 5 watts you are getting 10 PEP.
at 2.5 watts you are getting 4 PEP.
add in the margin of error in the meter readings and you can see that it is about the same relationship of mod to carrier at either level.

if the lack of meter swing at the 2.5 watt carrier level is bothering you, you can adjust the modulation meter with VR7. this has no effect on actual modulation, just what the meter shows.
be sure to make a mark on VR7 of the original setting so you can put it back.

get a radio check, i bet they say you sound about the same.
LC
 
Thanks LC the cobra meters shows 30% and my dosy shows 50%

even the pep is low, 2.5 dk with 4 watts pep. there was one guy on the cb tricks forum that tried a mod with some good result nothing to high on the pep side lower carrier and decent avg power,

cobra 2000gtl

Longhaireddwb i may give that a shot and hopefully it doesnt do too much distortion.
 
lowering the deadkey using the AM POWER adjustment (VR10) should not affect the modulation percentage.
turning the carrier down should make it easier for the radio to hit 100% mod.

you say your 2000 is not modded and the AMC is intact.
judging by your PEP readings, that sounds like the case.

if you want to try to get the max PEP out of the radio without disabling the modulation limiter; try this:

first, turn VR12 all the way up.
now, using a plastic or wooden tuning tool, tune L47, L48, L46, L45 and L38 for max PEP reading on an external wattmeter while saying "ahhh" into the mic.

do not try to adjust these using just a deadkey, as you will leave no room for modulation and end up with results similar to what you are experiencing now.

do not mess with L36. it is the TVI control and has nothing to do with power output.

now set your deadkey with VR10 to wherever you want it.

at a 2 watt deadkey, with VR12 turned up, and the dynamike control on the front panel all the way up; you should be able to hit 8-10 watts PEP with the limiter intact.

if this does not work, you might have a weak TR41, but a low power mic battery is much more likely.

also make sure you are re-calibrating your dosy modulation meter at the lower carrier level so the reading is accurate. well, as accurate as a CB style modulation meter can be. they are notoriously inaccurate.
LC
 
thanks LC i will give that a try, this evening i by passed vr10 and hooked it up to the tone control of the radio, as i dont use it at all.

I know what you mean about the dosey there pretty inacurate but at least it gives me a base line to see what is going on. I'm calling it a night as this is just agravating me lol so i'll tinker with it tomorrow with a fresh start and a clear mind.

oh before i forget the driver is a 1306 final is a 1969 and the pll is the mb8734 don't know if the different pll has anything to do with construction/ capabilities.

Thanks for you time LC much appreciated
 
lol mike, it is always better to have more than one suggestion than have no clues as to what to do it nay lead me around in circles but at least it is forcing me to understand this hobby better.

if i figure something out be sure i will let you know mike.
 
none taken at all Mike, I'm just trying to understand this radio a little more so i can avoid going to "coil spreader tech's" and looking over the schematic for this thing , almost seems like algebra. going to take my time on this and do alot more reading.

Xtra large Double double youre my savior!:lol:
 
yes, the MB8719 and the MB8734 are exactly the same chip minus the pin 10 function, which means that the 8734 cant be modified for extra channels the way the 8719 can.

the 2000's came with the 8734 chip and i dont know why.
some sort of cruel joke i guess. LOL
the 142's, 148's, washingtons and madisons all had the 8719 chip.
LC
 

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