Click below for online hookup instructions:
CG for 7.8 MHz (Older 40-Ch. Cobra, Uniden, Toshiba, & NDI SSB-PLL types, most 23-Ch. SSB crystal types.)
CG for 10.695 MHz (All PLL02A & µPD2824 40-Ch. SSB-PLL, all Galaxy, RCI, Superstar, & other Uniden “clones” & "10-Meter Amateur" radios.)

INTRODUCTION
The CHANNEL GUARD filters are a very effective way to stop one of the biggest problems with CB radios: splash, splatter, or “bleedover” from other channels. Our HF versions use two special high-Q crystals (7.8 MHz or 10.695 MHz) in series on a small PC board, which is installed at the proper circuit point in the receiver’s IF stages. They work by narrowing the “skirt” selectivity around the center of the radio’s IF passband. The filter is generally installed in place of an existing IF coupling capacitor, typically a 2-5pF ceramic disk type.

Interrupting the IF signal path by inserting any extra device can also attenuate the desired RF signal substantially, sometimes as much as 6 dB per filter element. Therefore a single transistor amplifier stage is included on the PC board to compensate for this signal loss. The end result is a filter with zero gain and zero loss, but vastly improved selectivity. There’s no loss in RX sensitivity, and in fact the extra amplification may even improve the S/N ratio. There is also no noticeable effect on the received audio quality of typical AM or SSB transmissions.

NOTE: The CHANNEL GUARD filters may be too sharp for FM-CB reception, if the FM deviation exceeds ±2.5 KHz. If received signals appear to be breaking up, the other station may be overdeviating.

All CBs have tuned transformers between the IF amplifier stages to help produce the receiver’s overall selectivity. However the “Q” (the Selectivity Factor) of such transformers isn’t nearly as high as that of a crystal. The graph of Figure 1 shows this difference. Both curves at the insertion point are about 8 KHz wide. But the typical IF transformer is much more broadbanded. Even at ±8 KHz away from center, the attenuation is still minimum, about -18 dB. Strong adjacent channel signals can easily cause bleedover interference to the desired signal. Compare this broad curve to that of the crystal filter. The skirts are very narrow, and at ±8 KHz, the attenuation can be -60 dB or better. The result is a dramatic drop in adjacent-channel interference.

HF FILTER PREFERRED LOCATIONS
The exact placement of any filter in the receiver IF chain is very important, and can make a difference in receiver performance. There are two schools of thought about its location in the circuit path:

1. Placing the filter very early in the IF strip, such as immediately following the first mixer, will help establish the overall receiver selectivity. Many CB rigs only use a single conversion process for both SSB and AM. Or they convert down a second time for AM and FM later in the circuit. So placing the filter early in the IF strip means that it will work in all modes.

2. Noise is normally generated within the IF amplifiers themselves. If the filter is placed further down the IF strip toward the Detector end, such noise can be reduced. This results in improved sensitivity as well as selectivity. Noise is broadbanded in nature and the less noise that gets through to the Detector, the better the Signal-to-Noise Ratio of the receiver.

A few of the better base rigs already use two IF filters. All AM/SSB rigs use a multipole crystal filter, which is switched to operate on both RX and TX. This filter is the large metal can about 2" x 1" that you see on the main chassis. For RX, it provides basic IF selectivity. For the SSB TX mode, it filters out one of the two sidebands. The second filter (if present) is a simple monolithic device which looks like a regular crystal, with three wires instead of two. “Monolithic” means that it has two quartz elements sharing the same physical ground, in a single crystal holder. It’s a cheap way of using two pseudo-crystals in one can, instead of two individual crystals (which are sharper) like our filters use.

In most installations the best placement will be early on, immediately following the First Mixer stage. The extra selectivity will reduce extremely strong signals which might otherwise desensitize the receiver IF strip.

For best performance the filter must be correctly installed in the radio’s IF strip. This normally involves nothing more than removing an interstage coupling capacitor and installing the CHANNEL GUARD’s INPUT and OUTPUT bare wires into the correct empty capacitor holes. Then the middle bare wire is grounded, and the colored stranded wire is run to the +DC voltage source. Figure 2 shows the general idea. We’ve also included two hookup lists, one each for all the most popular 7.8 MHz and 10.695 MHz IF radios, showing the specific capacitor to remove, along with the correct INPUT and OUTPUT holes.  The 455 KHz filter installation is different and is described later.

NOTE: Many older AM CBs used an IF of 11.275 MHz. The CHANNEL GUARD won’t work with these models. However, you can order an 11.275 MHz crystal (specify “series” or 10 pF Load Capacitance) to really sharpen up the bleedover rejection. Install it in place of the correct IF coupling capacitor. Remember to solder a third ground wire to the crystal case, then ground it to the radio chassis. (We still have a small supply of 11.275 MHz monolithic crystals available at $15 postpaid; contact us for details.) A few old AM and AM/SSB rigs used a 9.785 MHz IF; we don’t have these crystals, but you could order one and use the same trick there.

HF FILTER INSTALLATION INSTRUCTIONS
Figure 3 shows schematically the general installation point between IF stages. With the parts side up and the colored (stranded) wire on your right, the three bare wires are (left to right): INPUT, GROUND, OUTPUT. Place the INPUT and OUTPUT wires into the two holes vacated by the coupling capacitor you removed. Solder the center GROUND wire to any convenient ground point. You can often tack it to the nearest metal IF can. Connect the stranded wire to a convenient source of DC operating voltage, usually +8-13.8 VDC. One easy connection is directly at the output switch lug of the ON/OFF/VOLUME control. This way no power is applied unless the radio is turned on.

NOTES:
1. We’ve included a piece of black plastic sleeving to slip over the bare INPUT and OUTPUT wires. The CG will often need to fit in a very tight space on the chassis, so always use this insulation to avoid shorts.
2. Reversing the IN and OUT bare wires is the most common mistake people make. This is usually obvious because the RX sensitivity as indicated by your S-Meter or speaker volume will drop way down. Also make sure you removed the right coupling capacitor!

7.8 MHz AM/SSB HOOKUP LIST BY PLL CHASSIS TYPE

µPD858 PLL (early Uniden SSB chassis)
Courier Gladiator PLL, Spartan PLL, Fanon Fanfare 350F, Midland 79-893:
   Remove C49. (IN = T5, OUT = R50/D20)

Cobra 138XLR, Palomar SSB500 early, Realistic TRC449, Robyn SB510D, Teaberry Stalker 101, 102:
   Remove C20. (IN = L5, OUT = R19/D4/C21.)

NOTE: Foreign mobile models like Stag 357 (LC7120 PLL), Palomar SSB500 late (MC145106 PLL), or WKS1001 (MC145106) are otherwise identical to Cobra 138XLR above. Remove C20, same as above.

Cobra 139XLR, President Adams, old Grant, old Madison, old Washington, Realistic TRC457/TRC458, Robyn SB520D, Teaberry Stalker 202:
   Replace existing monolithic filter “FT2” for best results. (IN = L5, OUT = L6.)

Courier Centurion PLL, Centurion 40D: Remove C22. (IN = L5, OUT = C23/R19/D5.)

MB87l9/MB8734 PLL (late Uniden single-conversion SSB chassis.  PC385, PC511, PC622, etc.)
Cobra 140GTL, 142GTL, Courier Galaxy, President McKinley, Washington (late), P400, Realistic TRC450/TRC490, Robyn SB505D, SBE LCBS-8,LCMS-8, early Superstar 360, Teaberry Stalker IX, XV, XX, Tram D80/D300, Uniden Washington: Remove C42. (IN = L7, OUT = L6/C44.)

Midland 79-900: Remove C43. (IN = L8, OUT = L7.)

MB8719/MB8734 PLL (late Uniden dual-conversion SSB chassis)
Cobra 148GTL, 148GTL-F, 2000GTL, 2010GTL-WX, Pearce-Simpson Super Bengal Mk II, President Grant (late), Madison (late), Stalker XX export, Uniden Grant, Grant LT, Grant XL, Madison: Remove C47. (IN = L7, OUT = L8.)

RCI8719 PLL (made by Ranger, exact clone of 148GTL above. PCB = EPT0148-11Z, -13Z, -14Z, etc.)
General Sherman, Texas Ranger TR296: Remove C47. (IN = L7, OUT = L8.)

µPD2824 PLL SSB chassis (early Uniden 7.8 MHz IF, PC582, PC585, PC621
Craig L132, L232, Wards GEN-719A: Remove C45. (IN = L7, OUT = L6/C49.)

Early NDC40013 PLL SSB Chassis
SBE Console V, Sidebander IV, Sidebander V: Remove C118. (IN = C115/Q103, OUT = T104.)

Late NDC40013 PLL SSB Chassis
Craig L131, L231: Remove C212. (IN = T203, OUT = Q204/R208.)
Johnson Viking 4740, Viking 352D: Remove C415. (IN = T404, OUT = Q404/R411.)
Johnson Messenger 4730: Remove C417. (IN = Q404/C415, OUT = L402/CR405.)
Pace 1000BC, 1000MC: Remove C314. (IN = T308, OUT = R343/CR303/R342/C315.)
NDI PC200, PC201, Tram D64: Remove C326. (IN = T305, OUT = Q305/R317.)
SBE Console VI: Remove C118. (IN = Q103, OUT = T105.)

Toshiba TC5080/81/82 PLL SSB Chassis
Browning Baron, Cobra 132/135XLR, Tram D62: Remove C28. (IN = Q5/R31/R32, OUT = Q6/R33.)
   NOTE: Watch excessive gain here; there are already several preceding IF amp stages.

Miscellaneous PLL SSB Chassis
Realistic TRC459: Remove C75. (IN = T10/C82, OUT = Q21/R83.)
Realistic TRC480: Remove C83. (IN = T10/TP6, OUT = Q22/R84.)

Miscellaneous 23-Channel Crystal Synthesizer SSB Chassis
NOTE: In most cases these are for AM improvement only, because there’s no common mode coupling capacitor. Some do have a separate SSB capacitor; send SASE for details if that’s what you prefer. These were all dynamite rigs in their day!

Browning LTD, Cobra 132A: Remove C55. (IN = Q9/T10/IC2, OUT = T11.)
Cobra 130: Remove C305. (IN = Q2/T3/C108, OUT = T8.)
Cobra 132/135: Remove C85. (IN = Q11/T10/U4, OUT = T11.)
Cobra 138/139, Midland 13-893, 13-895: Remove C11. (IN = T4, OUT = D2/R9.)
Courier Centurion, Gladiator: Remove C219. (IN = T9/C60, OUT = T12.)
Courier Spartan: Remove C21. (IN = D15/R32/C46, OUT = T5.)
Midland 13-893, 13-895: Remove C11. (IN = T4, OUT = D2/R9.)
Pace Sidetalk 1000B/1000M, Tram XL-5: Remove C313. (IN = R314/CR304/CR305, OUT = FL301.)
Pearce-Simpson Bengal: Remove C10. (IN = D15/R39/C46, OUT = T1.)
Pearce-Simpson Cheetah: Remove C98. (IN = D10/R33/C69, OUT = T11.)
Pearce-Simpson Panther: Remove C21. (IN = D15/R32/C46, OUT = T5.)
Pearce-Simpson Simba: Remove C219. (IN = D11/R29/C65, OUT = T12.)
SBE 12CB (Sidebander II), 16CB (Console II): For SSB only, remove C140 (IN = T4, OUT = T6).
  For AM only, remove C145. (IN = R148/C132/T5, OUT = D20/D21.)
  NOTE: No combo AM/SSB installation possible in above SBE models without defeating the
            Noise Blanker!
SBE 18CB (Sidebander III): Remove C120 (IN = D9/R113/D4, OUT = FL1.)
Tram D60: Remove C57. (IN = Q10/T10/IC2, OUT = T11.)
Tram XL-5: Remove C313. (IN = R314/CR304/CR305, OUT = FL301.)

10.695 MHz AM/SSB HOOKUP LIST BY PLL CHASSIS TYPE

NOTE: Either the 10.695 MHz or 455 KHz CG can be installed in most AM-only or FM-only radios. If interested, send us an SASE for details.

Cybernet PLL02A American & Canadian, PTBM048AOX or PTDM058COX
AWA/Thorn 1503, Boman CB950, Cardon Iroquois 40, Colt 480, 485DX, 890, 1000, 1200 (Excalibur), Gemtronics GTX77, G.E. 3-5825A, HyGain 2705 (V), 3108 (VIII), J.C. Penney 981-6247, JIL Citizen MPL-5, SSB-M6, Lafayette Telsat SSB80, SSB120, SSB140, Midland 78-976, Palomar 2900,
Pearce-Simpson Super Panther, RCA 14T302, Truetone CYJ4837A-87, Universe 5600:
A wire marked “C” runs from the secondary terminal of T10 not going to C111/R88, to a foil pad containing one end of C122 and D12, near T13. Cut this wire and install the CG in series.
    (IN = T10 end, OUT = C122/D12 end.)
Dak Mark X only: Cut gray shielded cable connecting T10/C121 to CFl/D12. Install CG in series.
    (IN = T10/C121, OUT = CF1/D12)

Cybernet PLL02A American, PTBM080/085COX & PTRF004/005DOX
G.E. 3-5875A Superbase, Midland 78-574, 78-999, SBE LCBS-4: Remove C279 on RF board.
    (IN = Q210/T207, 0UT = T208.)

Cybernet PLL02A Export, PTBM059COX & PTOS006 or PTSY016AOX
Ham Int’l Concorde, Jumbo, Multimode II, HyGain V, Lafayette 1200FM, Major M360, M588: A wire marked “C” runs from the secondary terminal of T10 not going to C111/R88, to a foil pad containing one end of C122 and D12 on the main PTBM059 board, near T13. Cut this wire and install the CG in series.
    (IN = T10 end, OUT = C122/D12 end.)

Cybernet PLL02A Export, PTBM121D4X, PTBM121COX
Cobra GTL150, Colt 320DX, 320FM, 1200DX (Excalibur), Ham Int’l Concorde II, HyGain 2795DX, Intek 1200FM, Lafayette HB70AFS,Tristar 747: A wire marked “H” runs from one secondary terminal of T12 (the terminal not going to C116/R90) to the pad marked “I” next to T13 (that contains one end of C120/D14). Remove wire from “I.” (IN = wire from T12, OUT = “I.” (C120/D14 junction next to T13.)

Cybernet PLL02A Export, PTBM125A4X or PTBM131A4X
Colt 1600DX, 2000DX, HyGain 8795 (V), Lafayette 1800, Major Echo 200, Midland 7001 (export),
  Pacific 160, Superstar 2000, Tristar 777: Remove C120. (IN = T11/Q19, OUT = T12.)

Cybernet PLL02A Export, PTBM133A4X:
Ham Int’l Concorde III, Jumbo III, Multimode III. Remove C117. (IN = T11/R96, OUT = T12.)

Cybernet PLL02A Export, PCMA001S
Argus 5000, Colt 2400, Cobra 148GTL-DX (fake), Falcon 2000, Lafayette 2400FM, Mongoose 2000, Nato 2000, Palomar 2400, 5000, Starfire DX,Superstar 2200, Thunder 2000, Tristar 797, Tristar 848:
    Remove C120. (IN = T11/Q19, OUT = T12.)

Early Uniden µPD2824 PLL SSB Chassis (10.695 MHz IF, PC833, PC965)
Cobra 146GTL, Midland 6001/7001 late, 79-260, Pearce-Simpson Super Cheetah, President P300, AR144, AX144, Realistic TRC451,Sears 663.3810, Uniden PC244:
    Remove C46. (IN = L6, OUT = L7/C47.)

Late Uniden µPD2824 SSB Chassis (PB062, PB105, PB122)
Realistic TRC453, TRC465, TRC850, Uniden PC122, PC122XL, PRO-640e, PRO-810e:
    Remove C40. (IN = L8, OUT = R58/D12.)

Uniden Multimode Export w/MC145106 PLL, PB010 chassis
  Cobra 148GTL-DX (late), President Richard, Superstar 360FM:
    Remove C61. (IN = L11, OUT = L12/C62.)

Uniden Multimode Export w/MC145106 PLL, PB042 chassis
    President Jackson: Remove R74 (470W). (IN = FT4/C16/C17, OUT = R75/TRI3.)

Uniden "clone" Multimode Export w/MC145106 PLL, EPT00JA-10Z chassis
    Superstar Jackson: Remove R74 (470W). (IN = FT4/C16/C17, OUT = R75/TRI3.)

Uniden Multimode Export w/MC145106 PLL, PC879 chassis
    Cobra 148GTL-DX (early): Remove C48. (IN = L6, OUT = L7/C49.)

Uniden Multimode Export w/MC145106 PLL, PC893 chassis
    Stalker 9-FDX, President McKinley export: Remove C57. (IN = L10, OUT = L11/C58.)

Uniden Multimode Export w/MC145106 PLL, PC999 chassis
    President Grant Export: Remove C61. (IN = L11, OUT = L12/C62.)

Uniden “clone” multimode copycat models using MC145106 PLL and with EPT3600-10A, -10Z, -11A, 11Z, -13B, -14B or similar Ranger-type chassis. NOTE: If in doubt about your specific chassis, check the other AM/FM “clone” listings below or send us the exact chassis details.
Cherokee CBS2100, NR100, NR150, Connex 3300, 3500, Dirland 3300, 3500, 3600, 3700, 3900, Eagle 2000, Excalibur (base), Excalibur Samurai, Galaxy 11B, Galaxy 2100, Galaxy II, Galaxy DX33, DX44, DX55, DX66, DX73V, DX77, DX88, DX99, Galaxy Jupiter, Galaxy Melaka, Galaxy Pluto, Galaxy Saturn, Galaxy Sirius, General Lee, General Stonewall Jackson, Mirage 44, Mirage 6600, Mirage 88, Mirage 9900, Mirage II, Mirage III, Northstar 3000, 4000, 6000, 9000, 9500, President Franklin, Ranger American Spirit, Super Galaxy, Superstar Grant, Superstar 3300, 3500, 3600, 3900:  Remove C58. (IN = L10, OUT = L11/C59.)

European-made version of Uniden "clone" chassis above (same circuit, different part numbers)
Euro 3900: Remove C64. (IN = L9, OUT = L11/C65.)

Early Uniden “Clone” AM/FM fakes w/MC145106 PLL & EPT3000-10A chassis
Dirland/Superstar 3000: Remove C53. (IN = L9, OUT = L10/C54.)

Early Uniden “Clone” AM/FM fakes w/MC145106 PLL & EPTMARS-10Z chassis
Dirland/Superstar 3500: Best-remove C38. (IN = L6, OUT = Q8/C39/R22.)  Also possible but not as effective: replace FL1 (IN = L7, OUT = R26/R27/Q9.)

Miscellaneous Newer CB or 10-Meter AM/FM/SSB Chassis
NOTE: Increasingly many newer 10M & “export” type multimode/multiband radios are now built using only
Surface-Mount Devices (SMD), including the main chassis.  Components and spacings are tiny!  You'll need a magnifying glass, special miniature solder iron, etc.   It’s not practical to use the CG in those models unless you’re an experienced pro with all the right tools and know exactly how to do it.  (SMD models noted here where known.)

Cherokee CBS-1000, Northpoint CBS-1000: Remove C32. (IN = L33, OUT = L34.)
Cobra 150GTL: Remove CF2-1.  IN = R91, OUT = CF2-2
Cobra 200GTL: Remove C75.  IN = T708, OUT = R93/Q23 for SSB only.
        NOTE!  Above for SSB only; there's no IF path to install that includes the AM/FM modes without defeating the Noise Blanker.
        To also filter on AM/FM, you would need another 10.695 MHz CG in place of their "CF2" monolithic crystal filter. For that
        one, IN = R91/R131, OUT = IC1, C622)
Cobra 148FGTL-DX+, Connex 4400, 4800, Galaxy DX45MP, DX48T, DX93T, DX95T, DX2517, General Grant,
  General Stonewall Jackson, Ranger 99, RCI2960, RCI2980WX, RCI6300, & others w/EPT6900 main PCB:
    Remove C61.(IN = L8, OUT = L9/C62.)
Galaxy DX949, DX959, DX2547, Texas Ranger 396, 399, 696, 699, “900” series & others
    w/EPT0696 main PCB:  Remove C61. (IN = L8, OUT = L9/C62.)
Courier Galaxy IV: Remove C30. (IN = T7, OUT= D10/R31.)
Courier Galaxy V, VI: Remove C58. (IN = L10, OUT = L11/C59.)
Eagle 5000, Galaxy Saturn Turbo, DX-2527, older RCI2950, 2970, 2980, 2990,
    Sommerkamp TS-2000DX w/EPT2950 main PCB: Remove C72. (IN = L12, OUT = L13/C73.)
Emperor TS-5010: Remove C115. (IN = L104, OUT = R304.)
G.E. 3-5826A: Remove C55. (IN = T7, OUT = T9.)
Magnum 257, 357, 457, Titan II, RPSY-485, ProStar 400, Superstar 4900*:
     Remove C87. (IN = T8/Q8/R14, OUT = T7)
Magnum Delta Force*: Remove C187.  (IN = L112/C117, OUT = D124/R204/R206.
    *NOTE: This is for SSB only; there's no path for the CG that includes the AM and FM modes
                without defeating the Noise Blanker.
Magnum S-980: (PCB both 0691R3 and R6.) Remove C58.  (IN = L10, OUT = L11/C59.
Midland 79-265: Remove C30. (IN = T7, OUT = R31/D10.)
Midland 79-290: Remove C57. (IN = T4, OUT = Q19/R69.)
Radio Shack HTX-100: Remove C43. (IN = L5, OUT = R56/D14.)
Radio Shack TRC485: Remove C32. (IN = T4/Q4, OUT = T5.)
Ranger AR3300, 3500: Remove C24. (IN = R21/C416, OUT = R20/D8.)
RCI2950DX, 2970DX, 2985DX, 2990DX, 2995DX w/EPT6950 main PCB:
    Remove C68. (IN = L8, OUT = L9/C69).
    WARNING: This has an SMD main chassis; leave this installation to a pro!
RCI6900FTB w/EPT9900 main PCB: Remove C68. (IN = L8, OUT = L9/C69).
    WARNING: This has an SMD main chassis; leave this installation to a pro!
Superstar 4900B: Remove C69. (IN = T204, OUT = T205).
Uniden HR2510, 2830, President Lincoln:
    Remove C15. (IN = L115, OUT = D105/R9.) C15 is between L115 and SSB filter FT102. Bend leads
    90° and install on foil side of PCB; criss-cross the IN/OUT leads and use the sleeving insulation
    provided. Put some insulating tape between filter and radio PCB.
Uniden HR2600: same as HR2510 above, except IN = L4, OUT = R9/D5.

The 455 KHz filter is designed to be soldered into the same three mounting holes as the radio’s original 455 KHz ceramic filter, and then connected to a 12 volt DC power source.

By placing three high-Q ceramic filter elements in series, the selectivity “skirt” of the IF response curve is greatly narrowed, as shown in Figure 4. This graph shows a typical single filter element (top line) vs. our narrow filter. A 10,000 µV sine wave test signal (modulated 30% @1000 Hz) is swept around the 455 KHz center IF frequency, and the signal attenuation is measured. In our 455 KHz filters we use even sharper ceramic elements of ±2 KHz, rather than the standard ±3 KHz element used in most CBs. This has no noticeable effect on the received audio quality of typical AM CB transmissions.

The area between the two curves represents the increased rejection of unwanted signals that fall outside the IF passband. (The ones you won’t hear!) The result: beyond about ±5 KHz from the center IF, the signal is barely detectable. And a sine wave test signal has far more average power than one modulated with a human voice.

INSTALLATION
1. Remove the existing 455 KHz filter. This is easily recognized as a small black or gray plastic block, about 1/4" square on the radio’s main PC board. It often says “Murata” on it. It usually has three leads: INPUT, GROUND, and OUTPUT. Unsolder the three leads using a solder sucker or solder wick.

2. See Figure 5. Install the CHANNEL GUARD filter’s three wires to match the same INPUT and OUTPUT holes from the removed filter. Ground the middle bare filter wire by soldering it to the nearest metal tuning can.

NOTE: A few original 455 KHz radio filters have four or five leads, not three. You’ll generally find that more than one of them is a ground. There will always be only one INPUT and one OUTPUT hole on the radio’s PC board.

3. If you don’t have the radio’s schematic, you can often figure out the INPUT and OUTPUT holes yourself. Figure 6 shows a typical 455 KHz IF amplifier stage. A metal IF transformer tuning “can” connects to the base of the IF amplifier transistor, either directly or through a resistor. The existing ceramic filter is in series with this signal path. Follow the circuit foil traces to reveal which is INPUT and which is OUTPUT. The GROUND is easy to spot because it’s the largest foil on the PC board, and will run all over the chassis.

The INPUT will always be the hole leading to the transformer can, and the OUTPUT will always be the hole leading to the base of the transistor, either directly or through a resistor.

4. Connect the colored stranded wire to +8-13.8VDC. The easiest connection is directly to the switch lugs at the ON/OFF/VOLUME control on mobile or base rigs. Use the lug on the switched side, so the filter is only powered when the radio itself is turned on.

CAUTION: Some very old tube-type base radios switch 115 VAC. Never connect to such a switch or the Warranty will be voided!

5. Avoid shorts by using the black plastic sleeving provided. Cut it into three equal lengths, and slip some over each bare wire for insulation.

6. Reversing the IN and OUT signal wires is the most common mistake people make. This is usually obvious because the RX sensitivity as indicated by your S-Meter or speaker volume will drop way down. Make sure you correctly followed the radio’s foil traces to their 455 KHz signal connections: the CHANNEL GUARD INPUT wire goes to the first IF amp’s output coil secondary, and the OUTPUT wire to the base of the transistor IF amp that follows.

You are done!

There are a few old CBs that have no 455 KHz IF filtering at all. These can benefit greatly by adding a CHANNEL GUARD. You do it by cutting the PC foil trace that connects one of the 455 KHz IF transformer secondaries to the IF Amp transistor (or tube) that follows it. See Figure 7. Make sure this trace isn’t used to power the transistor base (or tube grid) via the transformer secondary; otherwise you’ll kill the amp stage! In the case of tube rigs, check the schematic to make sure there's no high voltage present at the installation point; otherwise you'll have to add a DC blocking capacitor in series with the IN and OUT (or both) filter leads as necessary.  Install the CHANNEL GUARD in series, observing the correct IN/OUT path. There are two or three stages of 455 KHz amplification to choose from, and you can put the filter between any of these stages.

Finally, connect the colored stranded wire to +12VDC in the radio. For solid-state radios (including bases) use the switched ON/OFF VOLUME control lug again. On tube type rigs, there may be both low- and high-voltage DC sources, or high-voltage only. You can still connect our stranded wire to a high-voltage +DC source, if you use a series dropping resistor to get the required +12 volts DC. The CHANNEL GUARD draws about 3.5 mA total current @12VDC. You can therefore use Ohm’s Law to calculate the required series dropping resistor.

Example: Suppose the radio has (among others) a +250VDC power source.  You need about +12V for the CHANNEL GUARD, which means the voltage supply is [250 – 12] = 238V too much.  To drop 238V, the series dropping resistor would be:

R = E ÷ I;  R = [(250 — 12)] ÷ 0.0035 Amp = 68,000 ohms

A standard 68K carbon resistor could be used.  Its power consumption is:

P = E x I;  P = 238 X .0035 = 0.833 watts.

A standard 1-Watt resistor can be used. If your calculation yields some odd value instead of our nice round “68,000” you can always substitute the next highest standard value, in this case 75K.  Remember too if you don't have any 1-Watt resistors, you can always parallel two 1/2-Watt or four 1/4-Watt resistors of the right values to make the total parallel resistance needed.

* * * * * CAUTION!  HIGH VOLTAGES ARE LETHAL! * * * * *   If you have the slightest doubts about installing the CHANNEL GUARD in a high-voltage, tube type transceiver, let a qualified tech do it.

TECHNICAL SPECIFICATIONS, 455 KHz CG
(EACH INDIVIDUAL CERAMIC ELEMENT)
Selectivity: ±2 KHz @ –6dB; ±7.5 KHz @ –40dB
Maximum insertion loss: 6dB
Input/Output impedance: 2KW
Center Frequency: 455 KHz ±1 KHz

GENERAL TECH NOTES & INSTALLATION TIPS

1.     Always ground the center filter wire to the chassis ground or common point.  Any of the metal IF transformer cans will take solder, and these make a good hookup point.  This ground connection also provides the -DC return path to power the filter; the filter’s amp won’t be powered up without it.  And grounding helps remove stray signals that cause oscillations or other interference to proper filter operation.

2.     The following radios use non-standard IFs, for which we have no crystal filters: Browning Mark III, Mark IV/IVA, Tram D201/D201A.

3.     The following radios cannot use the high-frequency filter: Sears 934.3826/3831, 3827, and equivalent in J.C. Penney chassis such as 981- 6241, 6246, or 6248.  These use an 11.275 MHz IF which is not capacitively coupled.  Because of this, the filter can’t be installed in the signal path without breaking the DC supply to the mixer or IF transistor.

4.     Many radios that are AM-only can still use the 10.695 MHz filter.  The crystal filter is much sharper than the discontinued ceramic 455 types.  Almost all 40-channel PLL AMs use a 10.695 MHz first IF.   Exceptions are some older 23-channel AMs with the 23/14 MHz master synthesizers and an 11.275 MHz IF.&snbp; There’s no CHANNEL GUARD available, but our special 11.275 MHz crystal filter ($15 postpaid) can be added to improve selectivity in these models.  (We have a limited quantity of these left; send stamped SAE if interested.)

5.     The CHANNEL GUARD should not increase the overall gain.  It’s designed for a zero-loss, zero-gain effect.  Excess gain shows up as: higher than normal S-Meter reading, background noise, buzzing, squealing, or audio distortion.  In such cases you can try lowering the amp gain a bit by increasing the value of the emitter resistor slightly.  This is the resistor immediately below the transistor, normally a 560 ohm (green-blue-brown) value.  Try a 680 ohm or even 1 K-ohm to rebias the amplifier.  Also see #6 below.

6.     Many techs use the colored +12V power wire to drop the RF gain when extremely strong nearby transmitters block out or splatter over the desired signal.  By running our power wire through an existing radio switch or an additional SPST switch, you get a very convenient RF Gain control.  When the +12V supply is turned off this way, the gain drops so low that you can easily hear a nearby station but all others will not be heard, just like turning an RF Gain control all the way down.

7.     Be sure the filter is installed only in the Receive signal path, never in the Transmit path.  Correct points are already shown for the models on our list.   For other radios, the SSB TX signal passes through the radio IF filter, typically by use of switching diodes.  If you install the CHANNEL GUARD in this path, you may slice off part of the transmitted sideband, resulting in distorted audio or off-frequency operation.

8.     Recheck the alignment of the IF transformers just before and after the filter installation point.  Consult your schematic or SAMS Fotofacts®.  Make sure these coils are still peaked for maximum signal strength, as indicated on the rig’s S-Meter.  Filter installation sometimes has an effect on the alignment of these IF cans.

9.     Avoid shorts!  Black plastic sleeving is provided to slip over the #22 bare filter wires.  You may also need to put some electrical tape on the solder side of the filter PC board, if the filter has any possibility of touching other nearby metal parts.

10.   Sometimes it’s not possible to stand the filter up vertically from the radio’s PC board.  In such installations, bend the filter leads at right angles and don’t cut them off.  For extra physical support, solder another bare wire between one of the crystals and a nearby transformer coil.  This forms a bridge, with the filter supported at each of its long ends.

11.   Never install the filter within the Noise Blanker signal path.  Any sharp filtering there can stretch out the noise pulses, making the blanker useless.  This is a common problem in all the Uniden and Uniden-clone SSB IF circuits, unless you follow our specific hookup locations.  These radios all use a bandpass filter of two or three capacitively coupled coils following the first Mixer stage.  The NB signal input sample usually connects to one of these coils, and the NB output kill-switch to the coil immediately following it down the IF path.  Unfortunately in many of these circuits the only non-NB filter location works only for SSB; these are noted where present.  Use the radio’s  schematic if you have any doubts.

12.   Glorified CB type "Ham" rigs (Uniden HR2510, Galaxy 2517, RCI-2950, RCI-6900, etc.) use the standard Uniden dual-conversion IF circuit.  Our 10.695 MHz filter works wonders in these models!  We've included instructions for many of these on Pp. 4-5.   On the negative side, many of these newer RCI, Ranger, and Galaxy models now use Surface-Mount (chip) parts, often including the main PCB that has the IF capacitor you need to remove in order to install our filter. The incredibly small foil spacing means you must use EXTREME CAUTION and possibly even drill tiny holes into the main PCB when trying to install the filter into this area. Such installations are recommended for pros only!

HOW TO CONTACT US
For specific hookup of models not listed here, you can mail us a stamped SASE or send Internet email and the exact make/model to:

Published by:
CBC INTERNATIONAL  ·  P.O. BOX 30655  ·  TUCSON AZ 85751 U.S.A.
TEL/FAX: 888-I-FIX-CBs (1-888-434-9227), (520) 298-7980
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