| Author |
Message |
    
Sintronic_Cpu
| | Posted on Thursday, April 04, 2002 - 4:11 pm: | 
|
Is there any NPC-RC Mods available for a Cobra 2000GTL? I get 22 watts or more out of SSB, but never more than 15 on AM, never more than 80% modulation unless I'm yelling into a power mike turned all the way up!!! I know when you do the NPC-RC Mod on Melaka's and other Galaxy's, it lets you swing to your full peak no matter what you key... any ideas on a Cobra 2000? |
Adshar64
| | Posted on Sunday, May 18, 2003 - 3:42 pm: |
|
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 at 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).
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.
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. |
Xlaxx
| | Posted on Friday, May 23, 2003 - 11:58 am: |
|
This mod works. Worked on my Madison,... dk's 1.5 -2 watts swing 25 as Adshar64 claims. No heat build up either. Good reports.
XLAXX |
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