| Author |
Message |
    
CM 3885
| | Posted on Tuesday, May 20, 2003 - 1:58 pm: | 
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Anyone know of the 12 volt to final mod and a good NPC/RC mod for the Grant XL? Also the clarifier in the radio is stock and id like to open it up. |
Adshar64
| | Posted on Tuesday, May 20, 2003 - 5:37 pm: |
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148/Grant/2000/Madison etc. (MB8719 single final, dual conversion)
transmitter modification package including NPC and final to linear
mods.
Note: When I do this I see the radio as having two sides; the
solder side of the main board, and the parts side of the main
board. The most efficient way to get this done is to do all
the solder side work first, then do the parts side work. The
following steps are presented in that manner.
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. |
CM 3885
| | Posted on Wednesday, May 21, 2003 - 12:38 am: |
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A OK TY. Seems i did this mod to my 142GTL once but i forgot to write down the docs of the mod.. |
CM 3885
| | Posted on Wednesday, May 21, 2003 - 1:47 am: |
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Another thing is do the mods that are in the CB tune up manual for the Cobra 148GTL work you know, the mod were you change the resistors to diff values(and i think change the final too as another mod) to get more audio out? I tried them in my 142GTL and they work but ive never tried them in a Grant... |
Xlaxx
| | Posted on Friday, May 23, 2003 - 9:35 pm: |
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This is a good mod and worked on my Madison too. Thanks Adshar64 and Haymaker 150 VA.
XLAXX |
Haymaker
| | Posted on Saturday, May 24, 2003 - 12:23 am: |
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XLAXX ,YOUR WELCOME BUD! ENJOY THE MADISON. ILL SEE YOU LATER.. |
CM 3885
| | Posted on Saturday, May 24, 2003 - 2:01 pm: |
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One thing though is the mod dont take to well to the Cobra 2000 GTL. I did it ot the 2000 when i had one and only gained about 2 watts more audio swing.. But every other 8719 equpieed radio ive had took it very well. This grant XL being the best to take it so far aside from the 142GTL i have.
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Adshar64
| | Posted on Saturday, May 24, 2003 - 9:37 pm: |
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Which mod was that CM 3885?
cheers Adrian |
CM 3885
| | Posted on Sunday, May 25, 2003 - 12:23 am: |
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The 12 volt to final mod.. |
Adshar64
| | Posted on Sunday, May 25, 2003 - 3:03 am: |
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Thats unusual, however the cobra 2000 runs best at 15v . Some upgrading of external power lead in and internal power supply wires help a lot with volt drop to finals.Internal power supply needs a zener change, eg ~14.5v as not adjustable.I run mine from a 20a external supply with front panel adj volts (got other stuff on same supply, 8amp rating is plenty). On a cobra 2000 the final mod to d55 connection which converts it to a linear stage with the driver left high level modulated (as part of npc mod) usually gives a 5 -6 watt increase. With a volt change from 13.8v to 15v typically a overall increase of 10 watts is achieved, with 28watt swing quite achievable with clean modulation. Over 15v is not recommended and gets hard on regulator and bulbs and a accurate voltmeter should be used
ie: multimeter etc. However the finals really like that 1.2 v increase. |
CM 3885
| | Posted on Sunday, May 25, 2003 - 3:42 pm: |
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The 2000 and the late model madison do NOT have a varable VR for the power supplier that the 142, Washington and a few others have that allow you to peak up the power supllier a tad to get more out of the radio.. Im sure there are a few naysayers that will chime in here and say "OH no no no that will blow up the radio!" but i will say this, i have a 142GTL and i sold off a teaberry stalker base last year that i did this mod to and they are still working just as good as they did when i had them.. |
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