cobra 148 finals
cobra 148 finals
I have a 148gtl made 1989 in panama.I was tring get a litlte more swing out of it using info from this forum.It keys 4 watts and swings to about 10 or 12 after the mods.I hear a lot about the final c1969 mod so I looked at fhe finals and it had c1969 on it.My question is did the c1969 c0me in it or has it been changed and why want it do more watts.
-
ramblingman
- Technical Helper
- Posts: 331
- Joined: Sunday 21st Jan 2007, 16:35
- Location: mississippi
-
sonoma
- Frequent Keyer
- Posts: 97
- Joined: Monday 27th Dec 2004, 19:32
- First Name: Johnie
- Location: oklahoma, United States
http://www.cbretards.com/cbpages/cobra148.htm
this sight has a copy of a npc mod.ramblingman may have a better one.
this sight has a copy of a npc mod.ramblingman may have a better one.
-
sonoma
- Frequent Keyer
- Posts: 97
- Joined: Monday 27th Dec 2004, 19:32
- First Name: Johnie
- Location: oklahoma, United States
cobra 148 uniden XL ,LT mod
this is a good mod also
your radio is stock, it is fine. want to mod it? you ready, here goes!
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.
if your radio was working well to begin with, your XL/LT/148/texas ranger 296/general sherman should now be dead keying 2-3 watts into your antenna & swinging 20 give or take on a peak meter, or 10 or 11 or so on a rms/avg meter(the infamous 'bird watt' reading). there are current bias adjustments that can be done to the driver transistor & final transistor as well, but should not be neccessary, unless you needed to replace those transistors(due to failure) or a magic screwdriver had already fiddled with them. if 1 had, you'd either have suffered that above mentioned failure already(too high) or you would have real weak dead key output(too low). if you want those adjustments, let me know & i'll post 'em.
in step 1, you COULD remove tr24 altogehter, (clipping the limiter) & the mic gain adjustment would be needed to be backed down to prevent overmodulation & a power mic will NOT be neccessary-it will be LOUD. swing will probably increase another 2-3 pep, maybe 1 rms, so it's not neccessary to remove as gain is minimal.
i have a couple of questions about your version of the mod.
on other sites i have seen the NPC mod (the 10uf cap on the audio IC) done a bit differently.
the positive side still goes to pin 9 of IC 6, but the negative leg goes to the junction of R194/D63/R228. i noticed that you put it somewhere else. any particular reason for this?
just curious.
also, you recommended replacing R131 with a 4.7k resistor. since the stock part is a 10k (pretty sure), and clipping it increases modulation, i would think that increasing the resistance of this part would be the recommendation, like maybe 100k ohms. mine is on a switch that either opens it up or adds the stock 10k.
airplane1, try this, it's simple and will give you some swing.
remove C207.
replace R206 with 33-39k. (anything within this range will work.)
add 1k resistor to middle leg of TR24.
using a plastic tuning tool (available at rad shak).
key mic on ch.19 or 20
say, "aaahhhhhh" into the mic as you SLOWLY turn L47. the s meter on your radio will move to the left. keep turning L47 until the meter is as far over to the left as it will go before it starts backing down. you must do this WHILE you are speaking into the mic.
repeat this process for L48, L46, L45, and finally L38. (L38 will look different and will need a different tuning tool. do not turn the other coil that looks like L38, it is your TVI coil and is already set with equipment you and i cant afford.)
now find VR10 and turn it down until your dead key is between 2-3 watts. no more than 3.
this is a good mod also
your radio is stock, it is fine. want to mod it? you ready, here goes!
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.
if your radio was working well to begin with, your XL/LT/148/texas ranger 296/general sherman should now be dead keying 2-3 watts into your antenna & swinging 20 give or take on a peak meter, or 10 or 11 or so on a rms/avg meter(the infamous 'bird watt' reading). there are current bias adjustments that can be done to the driver transistor & final transistor as well, but should not be neccessary, unless you needed to replace those transistors(due to failure) or a magic screwdriver had already fiddled with them. if 1 had, you'd either have suffered that above mentioned failure already(too high) or you would have real weak dead key output(too low). if you want those adjustments, let me know & i'll post 'em.
in step 1, you COULD remove tr24 altogehter, (clipping the limiter) & the mic gain adjustment would be needed to be backed down to prevent overmodulation & a power mic will NOT be neccessary-it will be LOUD. swing will probably increase another 2-3 pep, maybe 1 rms, so it's not neccessary to remove as gain is minimal.
i have a couple of questions about your version of the mod.
on other sites i have seen the NPC mod (the 10uf cap on the audio IC) done a bit differently.
the positive side still goes to pin 9 of IC 6, but the negative leg goes to the junction of R194/D63/R228. i noticed that you put it somewhere else. any particular reason for this?
just curious.
also, you recommended replacing R131 with a 4.7k resistor. since the stock part is a 10k (pretty sure), and clipping it increases modulation, i would think that increasing the resistance of this part would be the recommendation, like maybe 100k ohms. mine is on a switch that either opens it up or adds the stock 10k.
airplane1, try this, it's simple and will give you some swing.
remove C207.
replace R206 with 33-39k. (anything within this range will work.)
add 1k resistor to middle leg of TR24.
using a plastic tuning tool (available at rad shak).
key mic on ch.19 or 20
say, "aaahhhhhh" into the mic as you SLOWLY turn L47. the s meter on your radio will move to the left. keep turning L47 until the meter is as far over to the left as it will go before it starts backing down. you must do this WHILE you are speaking into the mic.
repeat this process for L48, L46, L45, and finally L38. (L38 will look different and will need a different tuning tool. do not turn the other coil that looks like L38, it is your TVI coil and is already set with equipment you and i cant afford.)
now find VR10 and turn it down until your dead key is between 2-3 watts. no more than 3.
-
sonoma
- Frequent Keyer
- Posts: 97
- Joined: Monday 27th Dec 2004, 19:32
- First Name: Johnie
- Location: oklahoma, United States
Here is another version that seems to work good.
Bills npc mod 148 2000
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 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.
Bills npc mod 148 2000
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 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.
-
perrym1962
- Frequent Keyer
- Posts: 52
- Joined: Monday 4th Sep 2006, 17:00
- Location: East View, Ky.
