Cobra 148 GTL mod confusion...

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Shadow
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Cobra 148 GTL mod confusion...

Post by Shadow » Monday 19th Jan 2009, 11:44

Every since I acquired my first 148 GTL I have been afraid to do anything but a simple peak on it because there are millions of different versions of mods out there for it (just like every other radio). The problem is I like my 148 too much to experiment on it so I am asking if anyone knows of the best way to mod this radio. I have seen several different versions, 4 of which I have quoted below. Has anyone done any of these version with successful results? Does anyone know a better way to boost the modulation and get the radio to sound better? Should I just leave it the heck alone and be happy with a simple peak? Do each of these versions produce different results and if so, what is the difference?

I know, lots of questions, but i'm sure this will clear things up for many people who are in the same predicament. Thanks in advance!

change R130 (a 1.5KOhm for a 500 Ohm, it may be in a diode location close to TR26)
1. add a 1K resistor in series on the the middle leg of TR24, to do this just cut the track for the leg and put the resistor across the cut.
2. Change R126 (10k) to a 4.7K
3. Change R124 (10k) to a 4.7K
4. Re-tune L37 and L38 for maximum AM forward power.
1- cut trace to leg & add 1.5k resistor to middle leg of tr24.

2- replace r126, r124, & r131 with 4.7k resistors

3- adjust vr10 for 2-3 watts(or wire to variable)

4- solder positive side of 10uf 35 volt cap to pin 9 of ic6

5- solder negative leg of that cap to junction of r271-d63-r228-put tubing/insulator over legs to prevent shorting.

6- turn vr10 wattage down as low as it will go

7- clip TP8 & tape ends.

8- on trace side of board, solder heavy wire from junction c148/L39(on older boards, it's c146/L39), to 13.8 volts at junction of c188/d55/red wire/input coil

9- jumper r196

10- readjust vr10 for 2-3 watts, unless you put it on external variable, then adjust it there for 2-3.
1. Remove TR24

This deactivates the modulation limiters in all modes. Use the front panel mic gain (aka dynamic) control to set the modulation percentage.

2. Add a solder bridge to the solder side of the board that effectively jumpers out R196. This is a quick way of replacing R196 with a jumper (reducing its value to zero ohms).

This increases the range of VR10 (AM dead-key power) so that the dead-key can be set to 1.5 to 2 watts later on.

3. Add a 10 uF 25 or higher volt electrolytic cap to these points: the positive leg goes to the trace that connects to pin 9 of the IC6 (the audio IC), and the negative leg goes to the R194/D63/R228 junction.

This is the mod that compresses the negative modulation peaks and allows the average power to increase based on the modulation percentage (aka the NPC mod).

(This is the end of the solder side work. The rest of the work is done on the parts side of the board.)

4. Set the driver bias to 50 mA. (Power up the radio, put it in LSB or USB, set the mic gain to minimum, remove the wire from test point 8, insert a milliamp meter in series between the test point (which is positive) and the wire, key the mic, and adjust VR9 until the meter reads 50 mA.)

5. Set the final bias to 100 mA. (Same instructions as in step 4 except the test point is test point 7, and the adjustment is VR8).

NOTE: On some of the newer radios the final bias can't be set higher than about 50 mA. The reason is that the value of R179 has been increased in order to decrease the effective range of VR8. To solve the problem, replace R179 with a 500 to 1000 ohm resistor.

6. Once the final bias has been set, unplug the DC power cord, put the final bias wire back on the test point, cut the final bias wire 1/4 inch above the connector, strip and tin 1/8 inch of the wire, tin the cathode (banded) leg of D55 (the reverse polarity diode), and solder the wire to D55. This assumes the test point connector is at the end of the wire that is furthest from the final transistor. On some of the newer models the test point connector is at the end of the wire closest to the final transistor. On those models, completely unsolder the wire at the end opposite the test point connector and solder it to D55.

This is the mod that converts the RF final stage to linear in all modes.

7. Power up the radio, put it in the AM mode, key the mic, and set VR10 (AM dead-key power adjustment) for about 1.5 watts.

8. Tune the RF chain coils (L38 and L45 through L48) for maximum peak (modulated) output power in the center of the band (that would be Channel 19 on a stock radio and Channel 40 on one that has the popular expanded frequency range of 26.815 to 28.045).

If you have a favorite channel that is more than 30 channels from 19 or 40, do your tuning on that channel.

9. Double check the dead-key power. It should be around 2 watts. If it is higher than 2 watts, use VR10 to cut it back to between 1.5 and 2 watts. Don't overdo it. Keep in mind that the carrier (aka dead-key) power increases up to 10+ watts with modulation, so there's absolutely no point in having the dead-key power any higher than is required to reliably key an amplifier. Most amps will key reliably with as little as 1/2 watt of dead-key power. If you overdo the dead-key, the transmitted audio will sound weak because the negative modulation peaks will not reach 100%.

The following numbers are what you should expect. However, since there is a lot of variation in CB test equipment setups, don't be alarmed if you don't see these exact numbers. These numbers are provided as a guideline to make sure you did the mods properly.

The dead-key wattage should be 1.5 to 2 watts. The maximum average power should be 10 to 12 watts. And the maximum peak power should be around 25 watts.

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Shadow
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Cobra 148 GTL mod confusion...

Post by Shadow » Saturday 24th Jan 2009, 11:09

hahaha looks like i'm not the only one confused about this. :D

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Shadow
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Cobra 148 GTL mod confusion...

Post by Shadow » Saturday 7th Feb 2009, 11:40

bump! anybody? anybody? Bueller?

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Re: Cobra 148 GTL mod confusion...

Post by frogman296 » Saturday 14th Feb 2009, 8:10

which mod do you recommend rick??

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Re: Cobra 148 GTL mod confusion...

Post by 1096 » Thursday 26th Feb 2009, 6:56

Do not remove TR24!!!! Your radio will sound like garbage in all modes.... If you must remove a limiter then remove TR26, this only disables it in AM mode.... If your going to do any of the mods then do the NPC-RC mod but revise it so that you remove TR26 instead of TR24.... I have done this NPC-RC mod before and it does work well for this radio....

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Re: Cobra 148 GTL mod confusion...

Post by The Defpom » Thursday 26th Feb 2009, 7:20

Personally I put a 1K series resistor on the middle leg of TR24, and change R124 and R126 to 4.7K, that increases your mod level on all modes with no ill effects.

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Re: Cobra 148 GTL mod confusion...

Post by frogman296 » Thursday 26th Feb 2009, 9:10

The Defpom wrote:Personally I put a 1K series resistor on the middle leg of TR24, and change R124 and R126 to 4.7K, that increases your mod level on all modes with no ill effects.
tried this mod with great success

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Re: Cobra 148 GTL mod confusion...

Post by 15minigrass » Friday 25th Feb 2011, 8:39

Does this drop the Dead key to 1 watts too???? Aloha, Jim.

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