CB tips and Tricks
(Always Under Construction)
CLICK HERE if this site has been useful to you
Most of these will be categorized by radio brand and model, plus a category for miscellaneous tips. Watch for new content as I get time to update this page. Note: These are NOT copied from any other source. They are the result of my own experience and research.
Note: Some tips mention jumpering a diode with a piece of wire. This does increase the output by a watt or two PEP, but it also causes a risk of extreme receive distortion when receiving high signal levels at high volumes . If you experience this problem, remove the jumper.
Brands:
Cobra
Cobra 148 GTL and F-GTL
Extra Channels: Locate the MB8719 chip (usually right behind the channel selector). Wire a single-pole, single-throw switch from pin 10 to ground (SW-2 on chart), and another from pin 11 to pin 12 (SW-1 on chart). Click Here for a channel chart.
Supertune: Many of you have asked about supertunes for this radio. I do not recommend modifying the 148 very much, due to the type of modulation circuit used. I have found that the best over-all sound and performance comes with nothing more than a standard tune-up, with the mod level set for 100%. However, there are a few mods that will yield better over-all PEP and still keep decent quality sound. Change C109 to a 3.3uf or 4.7uf. RemoveC174, and run a 1000uf cap from negative side of C174's position to the base of TR41, with the positive side of the 1000uf cap going to the base of TR41. Reset the AMC level for 100% modulation.
Cobra 140/142 GTL
Extra Channels: Replace the 11.1125 mhz crystal with a 11.3258 crystal. Locate the MB8719 chip (usually right behind the channel selector). Isolate pin 10 from ground (Cut the trace). Wire a switch from Pin 10 to ground(SW-2 on the chart), and another from pin 11 to pin 12 (SW-1 on chart). You may need to re-align the frequencies a bit. Click Here for a channel chart.
Supertune: Many of you have asked about supertunes for these radio. I do not recommend extensively modifying the 140/142, due to the type of modulation circuit used. I have found that the best over-all sound and performance comes with nothing more than a standard tune-up, with the AMC circuit left intact. However, there are a few mods that will yield better over-all PEP and still keep decent quality sound. Change C90 to a 3.3uf or 4.7uf. RemoveC119, and run a 1000uf cap from negative side of C119's position to the base of TR34, with the positive side of the 1000uf cap going to the base of TR34. Remove R186.
Cobra 29 LTD:
Receive Boost: Locate pin 14 of the PLL chip, and the Positive (+) side of C-12. Install a 22k-220k ohm resistor between these two points. The smaller the value, the more the receive is boosted. Use CARE when soldering to the PLL chip! If desired, this mod may be wired through a switch.
Supertune: Replace the stock final with a 2SC1969 unit. Solder a jumper across D8 (this may cause receive distortion at high volumes, remove jumper if it does). Solder a 68pf cap across C62. Solder a 68pf cap across C59 if there is already a 33pf across it. If there isn't, use 100pf instead. Replace R 55 with a 10-15k resistor, and solder a 33-55 pf cap across that resistor. Replace R58 with a 4.7 ohm resistor. Set VR4 just a bit to the right of half-way open. If D11 or C3 have been removed, replace them. D11 is a simple small-signal swithcing diode like a 1n147, and C3 is just a 1uf electrolytic. If TR18 has been removed or cut, replace with a 2SA733. Spread Open L12 a tiny bit, and adjust L16 for best swing, using a loud, steady "aaaaaaah" sound into the mic (or if you have a function generator, set it up for 1khz at about 16mv output, and use that. MAKE SURE to use heat-sink compound (a very light coating is sufficient) when mounting the new final, and MAKE SURE the final mount screw is tight!
MOSFET Supertune (29LTD classic and newer): Replace stock final with IRF520 MOSFET transistor (pin-for-pin match when installing). Remove R56 and in its place install a 470-ohm resistor in series with a common switching diode (1n914 or similar), with the banded end of the diode towards the front of the radio. Remove C53 and solder it across C59. In C53's place solder a 3.3k resistor. Remove C 61 and solder it across C62. Remove C126. Remove R55 amd solder a 22pf cap in its place. Remove the slug from L14. Replace C71 with a 33-47uf cap. Turn VR4 (AMC) counterclockwise fully. Gently spread L13 and L12 and tweak the spreads for best PEP when modulated. Tweak L20 and L17 for best PEP when modulated. After tuning is finished, set VR4 to half-way.
Note: Older 29's (LTD and GTL) use the same basic procedure, but instead of re-using the caps, place a 100pf across C62, a 68pf across C59, and 22pf across the spot where you removed R55.
Channel 9 switch to Channel 19: Cut D32. Hard, huh?
Cobra 25 LTD:
Supertune: Replace the stock final with a 2SC1969 unit. Solder a jumper across D10 (this may cause receive distortion at high volumes, remove jumper if it does). Change R43 to 4.7 ohms. Remove C149. Place 82-150pf cap across C 46. Place 33-68pf cap across C42 (select for best output). Set VR5 half-way open. If D9 or C6 have been removed, replace them with the same values specified in the 29 procedure above. If TR14 is cut or missing, replace it with a 2SA733 unit. Spread open L8 a bit, and adjust L10 for best swing, using a loud, steady "aaaaaaah" sound into the mic. MAKE SURE to use heat-sink compound when mounting the new final, and MAKE SURE the final mount screw is tight! Optional: For more mic amplification, replace R76 with 1K ohm.
Cobra 19 Plus, 19 DX, 19 Ultra:
Supertune: Replace the final with a 2SC1969 unit. Solder a jumper across the diode going from the Audio Transformer to the Driver and Final (this may cause receive distortion at high volumes, remove jumper if it does). Solder a jumper across the resistor going from the base of the Final to the Driver. If a diode has been cut for modulation, solder it back together, or replace it. Adjust the modulation control POT (VR) for 100% modulation (usually between 1/4 to 3/4 way open). Tune output coils for best swing, using a loud, steady "aaaaah" sound into the mic. Note: These radios are superb for clarity of modulation, even on the stock mic.
Cobra 200 GTL-DX
Echo mod:
NOTE: Previously there was a part number error in this mod. I originally listed R39 as the part number. That was a typo. The mod is now correct. If you changed R39, please change it back to 100K to maintain receiver integrity.
R550 controls the delay of the echo (speed of the echo). By changing it to a lower value, the delay is decreased. Higher value (obviously) increases the delay. Values below 1k would be inadvisable. I've not experimented with a max value yet, but a couple hundred K would likely be the practical limit. If you want, you can put a 100K pot in place of R550, making sure to use a bit of hot glue to keep the wires from moving around too much. Keep a 1k fixed resistor in series with the POT.
Mic Preamp Mods:
1. R181 is the feedback resistor on the 4558 mic preamp IC. It is 150K stock. Change it to anything from 470k to 1M. This will allow much more effective use of a good noise cancelling mic on the radio. As with the talkback mods and echo mods, use hot glue on the parts you install to make sure they do not flex. Values above 1M will be problematic from a noise standpoint, and 470K is likely going to give more than enough added gain for practical purposes.
2. C158 controls the speed at which the AMC circuit responds. To speed up this circuit (and thereby give more even modulation at max levels), change this to around 10-22uf. If you go much lower than 10uf, you may start to get a choppy sound.
Galaxy
Frequency Charts:
- Galaxy 33, 55 and 77, in .DOC format
- Galaxy 44-99, in .DOC format
Galaxy 33-88, Connex 3300, and similar units:
Supertune: Change R174 and 187 from 10k units to 2.2k units. Peak L31, L33, L42, L51, and L52 for best swing, using a loud, steady "aaaah" sound in the mic. Set carrier control pots as normal for high and low power levels. For louder, cleaner talkback, solder a .1uf capacitor in series with a resistor (5k on up) from pin 4 of the audio chip to J25 (small wire jumper in front of TR32 on parts side of board, near the mic socket). If you use a resistor higher than 10k, you can skip the capacitor, unless you desire a higher tone for the talkback. Experiment a bit for best resuslts. If desired, you can add a rectifier diode inline with R238, with its banded end pointed towards the side of the radio. NOTE: this will increase the possibility of feedback in hot operating conditions, but adds alot of room in the modulation for lower-output mics like the Astatic 636L.
AMC Mod: Change C133 (center of the board, about 2-2.5 inches from back edge, next to TR 34) from 10uf to 4.7 uf. This makes the AMC action much snappier, and gives better average loudness.
Galaxy 88 (old)
Frequency Counter Noise: (applies to all older model galaxies) solder a .01uf capacitor from pin 8 of the TC5032 IC to the nearest ground point. Should eliminate the problem.
95T
RF Gain Mod: Locate R72 just behind L5 (first receive coil, the one without a metal can on it), and run a wire from the right end of it through a 1k-5k pot to ground. Its that simple. For the RF gain to work, the radio's 40-db pad must be engaged. If you use a 5k pot, I recommend running the POT in parallel with a 2.5k resistor to keep the gain from dropping too fast with a small pot movement. If you have a 2k or 2.5k pot, that works even better.
Midland
Midland Power Max (old version), Supertune: Replace final with 2SC1969, short D205, short R314 and R316, and add 68pf across C318 and C320. Retune for max power. DO NOT cut mod limiter, use Modulation Control Pot to set mod level. NOTE: The new Power Max version doesnt supertune well. Procedure NOT RECOMMENDED for the new version!
President
Washington Base - newer model
Extra Channels: Replace the 11.1125 mhz crystal with a 11.3258 crystal. Locate the MB8719 chip (usually right behind the channel selector). Isolate pin 10 from ground (Cut the trace). Wire a switch from Pin 10 to ground(SW-2 on the chart), and another from pin 11 to pin 12 (SW-1 on chart). You may need to re-align the frequencies a bit. Click Here for a channel chart.
Supertune: Many of you have asked about supertunes for these radio. I do not recommend extensively modifying the 140/142, due to the type of modulation circuit used. I have found that the best over-all sound and performance comes with nothing more than a standard tune-up, with the AMC circuit left intact. However, there are a few mods that will yield better over-all PEP and still keep decent quality sound. Change C90 to a 3.3uf or 4.7uf. RemoveC119, and run a 1000uf cap from negative side of C119's position to the base of TR34, with the positive side of the 1000uf cap going to the base of TR34. Remove R186.
RCI
2950
Supertune: add 330pf caps across C261, 262, and 270, and retune for best output. Simple, Huh!
2970
Make Amp Switchable: Locate the black and brown wires going to a plug next to the upright circuit board in front of the final and driver. Cut the brown wire and insert a switch.
2950/70
Close-Range Receive Improvement: Change R49 (near IC 1) from 100k to 33k. This will keep very close signals from overdriving the front end of the radio. If you still have some problems after this modification, Change R78 from 2.2k to 6.8k. This will reduce the sensitivity of the receiver, and may also help cut back some of the noise level.
Talkback: Cut one end of D78. To make talkback clearer-sounding, clip D115 (eliminates the PA option, however). You may also replace D115 with a .001 capacitor (some degradation of PA system output).
Uniden
-
Refer to Cobra tips for PC 76/78 and 66/68 (Cobra 29 and 25 respectively), with one exception: DO NOT remove any capacitors in 76/78 as is done with the Cobra models!
Miscellaneous Tips
Supertunes: There are a couple general items that you can do to get extra power out of almost any radio. First, replace the stock final with a 2SC1969 transistor. Second, locate the resistor bringing the driver signal to the bas of the final, and solder a jumper across it. Third, locate the diode running from the Audio output transformer to the final/driver supply chokes, and solder a jumper across it. These mods generally do not apply to sideband radios, most of which already use a 1969 unit. Some readjustment will be needed, and some radios may be able to be further increased by changing various capacitor values in the final and driver circuits. Experiment CAREFULLY!
Variable Power: This modification can be made to work in almost any radio. Click Here to see the diagram of the add-on circuit.Clean Modulation: All radios come from the factory with a "modulation limiter" or "amc" circuit. The purpose of this circuit is to keep the radio from over-modulating. which causes bleedover, and gives the radio a "blaring" sound. Many shops remove this limiter to obtain a high "swing" on a power meter. If this circuit has been disabled (usually by cutting a diode or resistor), you can regain crystal clear transmit audio by re-enabling it. Once enabled, adjust the modulation control trimmer for just over 100% modulation. Use a good oscilloscope to judge this, or if none is available, use a good modulation meter. If the radio does not have a modulation control trimmer on the board, you can insert a resistor inline with the diode (or other resistor) that was cut. Select a value that yields just over 100% modulation.
Excess noise in trucks with electric/electronic fuel injectors: Purchse Radio Shack part number 273-105, and put one on each injector wire, both hot and ground. This trick will also work with many other noise sources in the vehicle, such as blower motors, wiper motors, etc. Examples of this type of injector noise problem occur very often in trucks with the M11/M14 Cummins motors.
R-Model Mack Trucks: Ground problems with antenna systems. Use the Wilson "Silver Load" antenna, 4-footer, with 18 feet of RG-8X coax, and tie the antenna's false-ground lead to one of the antenna mount bolts. You'll need to crawl up and down a ladder (or the truck itself) to adjust the antenna, but the results are worth the hassle.
New-Model Freightliners and Volvos (With antennas located in the plastic mirror housings): Setting up a reliable antenna system is fairly simple with these. You can set the truck up for either single or dual antennas. Remove the front mirror housing by removing the two retaining screws. Locate the existing antenna mount and coax. Unscrew the factory coax. Screw in your own 18' coax into the factory mounting stud. Screw in your choice of antenna into the top of the stud, finger-tght only, and set the swr's as you normally would. Once that is done, remove the antenna, route the new coax down to the bottom of the mirror housing, and replace the housing cover. There is a small notch at the bottom of the cover that will allow the coax to fit through. Once the mirror is back together, re-install the antenna, and tighten as normal with 2 wrenches (one to turn the antenna, one to hold the top of the mount stud from turning). Repeat the process for two antennas. The coax may be routed between the door and door frame, or you may route it anywhere you wish as long as it is not severely pinched, coiled, or tightly bundled.Mounting an Antenna Stud: Click Here for a basic diagram of how to install an antenna stud with a standard 239 coax fitting.
Go to http://www.copyright.gov/circs/circ1.html#wwp for info on copyright protection