Handy Hints for UK rigs
Cybernet PTBM134A0X chassis
Remove the front leg of R60 (10k) from the board (RHS front of PCB). Solder a wire into this hole (connects to Q14 collector) and solder the free end to the post near the front of the board which has a wire (usually blue) going to the top (loud) end of the volume control.
Power output lower than usual
- replace L4 (22uH), L5 (68uH) and L7 (24 turns of 28SWG enamelled copper wire on a 27R resistor).
Almost no power output on TX
. Monitor rig reveals weak, noisy TX. Try turning CT1 slightly because maladjustment can cause this effect.
Volume control turned down but continues to give sound
dependent on squelch setting! Try replacing C109 which might be open circuit.
No RX no TX
- try replacing Q18 (2SC1318).
10nF cap across vol. control acts as treble cut if rig has no tone switch.
To alter the 10.24 crystal frequency
by several kHz connect a Toko TKACS34342 discriminator coil in series with it. Use the two outer pins of the three.
CH9 to CH19 for mobiles, disconnect track from LC7137 pin 9 and reconnect to pin 8. Add 2 off 1N4148 diodes with cathodes (bar) connected to common ends of D251 - 256 and anode ends to c' and b' segments on CH display. On rigs without a CH9 switch, use a changeover switch to disconnect power from CH switch and CH display and to connect power (up to 8v) to pin 8 and to the diodes added to produce "19" on CH display.
I read and tried the channel 9 to 19 conversion you had included on your CB radio information page. It worked, but I found that the display was giving strange numbers etc. I have made a modification to your original and attached it as a PDF document, THIS WORKED PERFECTLY!
THE RADIO I PERFORMED THIS ON WAS A HARRIER CBX.
Download Dan's document HERE
Windows users should "right click" a DOWNLOAD link above and choose "save target as" to save the PDF file without launching Acrobat Reader. If you "left click" then Acrobat Reader may launch and act as a "helper" for your browser, preventing the file from being saved in its original form.
If you do not have "Acrobat Reader" you will also need to download that from here:-
Remove C67 (2pF) between T8 and T9. Fit the crystal filter in its place with the centre lead soldered to T8 or T9 can. Tune T8 and T9 for optimum signal strength, then readjust slightly for best sound on a loud but distant signal. If T9 is not present, fit a 85FC4402EJ 7mm Toko coil.
Cybernet PTBM135A0X chassis - no TX or RX until rig has been on for a few minutes - I haven't traced this fault to the exact component but it appears to be one of the capacitors next to the VCO coil becoming very temperature-sensitive (try replacing the 82pF cap). Retuning VCO appears to cure fault but then the rig can lose channels when it is hot.
Cobra 148 (later version) : to silence the "Roger Bleep", simply disconnect C162 near the microphone socket (a 2u2/25v electrolytic which is not numbered on early circuit diagrams and labelled C270 on later ones!). The duration of the bleep is controlled by the value of this capacitor.
Cobra 148 (early version) Regulator transistor TR39 often blows and can be replaced with a TIP42. Note that this is PNP. If you fit an NPN by mistake it will oscillate and produce a howl from the speaker. Similarly A.M. modulation transistor is also PNP and may be replaced with a TIP42. If an NPN is used there will be no amplitude modulation on TX.
Suffers from "lazy needle" problems
. Short circuit either D7 or D8 -- NOT BOTH -- which are in series near the centre of the board. Adjust VR1.
Bad modulation on TX
can be caused by PCB securing screws loose or missing. Carrier on SSB, no mod on AM FM SSB, RX LED stays on during TX - replace TR37.
Cobra 150 GTL can be converted to UK channels quite easily since the channel switch contains a section with no "jumps". Use a 20.39MHz crystal. Remove the green and blue/white wires from the channel switch board and join them to the purple/white position on the board.
Ham International Rigs
(PCMA001S) Nato 2000, Tristar 797, 848 (PCCW001S bleep board)
(PTBM048A0X) Colt 485DX (Black Shadow)
(PTBM059C0X) Ham Multimode II, Ham Jumbo (PT0S006A0X crystal & VCO board), Major M-360 (PTSY016A0X crystal & VCO board)
(PTBM092C0X) Colt 870 PT0S008A0X crystal board, PTZZ033A0X FM board)
(PTBM094C0X) Ham Viking, Formac 88 (PT0S008A0X crystal board, PTZZ033A0X FM board)
(PTBM106A0X) Lafayette HB-940, New Formac 88, Formac 120, Colt 720 (PT0S008A0X crystal board, PTZZ033A0X FM board)
(PTBM121D4X) Concorde II, Hy-Gain V, Tristar 747, Colt Excalibur (PT0S011A0X crystal board)
(PTBM125A4X) Tristar 777 and Superstar 2000 (PTZZ080A0X bleep board)
(PTBM133A4X) Concorde III
Ham Multimode II : The Roger Bleep can be switched off by using a SPST switch as follows: Take the wire off post 3 and solder it to the slider of the switch. Connect post 3 to the "on" tag of the switch. The "off" position should be connected to post 5. (Do not remove wire already there)
Fitting crystal filters
PTBM059C0X Replace C122 (27pF) with filter.
PTBM121D4X Replace capacitor labelled C128 (27pF) with filter. ALSO, for all these rigs, cut the track to T13 and connect this side of the filter to the spare pin of T13. Retune T10 and T13.
PTBM133A4X Simply replace C117 with filter.
PTBM125A4X and PCMA001S replace C120 with filter.
PTBM092C0X and PTBM106A0X Replace C63 with filter.
PTBM121D4X - modulation is seldom brilliant on FM. Firstly ensure that the TX frequency is absolutely right. Use a frequency counter with at least 7 digits. Lift out R29 (100k) near the VCO block and cut the track between TP1 and the VCO block. Solder a 4k7 resistor across the break in the track. Connect a 1M resistor from the VCO block side of the 4k7 to C32 (where it joined R29). The Audio AGC action is fierce so, to reduce it, cut the track between D38 and the other components (C175, C173, R150), or lift one leg out of the PCB. Connect a 1k preset in series with D38. Adjust for the most pleasant modulation. The Altai DM315E Echo mike works well. The AGC action can also be reduced by soldering 18k in series with C175 (3u3) next to RV14.
No TX AM or FM
caused by DC switch buffer Q36 going open circuit (2SD837). Poor TX caused by burning of RF coils - quite hard to spot. Make sure you rewind them with the same gauge wire! (And keep the output power down to 5 Watts).
Extend the Tuning range
by replacing L1 (2u2H) on PT0S012A0X crystal board with a 3u9H inductor and remove capacitors C5, C7 and C7.
Note that RV14 is the limiter for A.M. and RV13 is the Amplitude Modulation Depth adjuster. Using this to (apparently) turn output power up will reduce modulation depth almost to zero!
PTBM125A4X - lights dimming or fuse blowing on TX can be caused by D1 on PTZZ080A0X bleep board.
PTBM106A0X: Common fault is no TX or intermittent TX caused by faulty D13 near modulation transformer.
PTBM048A0X KC shift - remove D4 and D5. Connect lead from middle tag on
clarifier to D4 position on board, negative end. Connect spare tag on Clarifier pot to 9v regulator on board.
PCMA001S (Nato 2000) : RX OK but erratic RF power output or no output can be caused by the demise of diode D1 on the PCZS0001 board. Replace it with a 1N4001. (In case you didn't realise it, the two "ICs" on this board are not ICs at all: one is empty and can be thrown away. The other has shorting links across opposing pins.) The same fault on a Tristar 777 is caused by D1 on the relay board.
- To switch the bleep on and off, insert a switch in one leg of C601. To shorten the duration, change C601 (10uF) to 3u3F. To lower the tone, add 22nF across C605.
If you use a 1k impedance crystal filter from P.R. Gollege, simply replace the ceramic filter and retune L4 and L3. If you use the early Ambit 3k Ohm crystal, replace L4 with a TOKO KACSK3892A Transformer (red) and retune L4 and L3. The new L4 will usually need to be screwed downwards. If distortion is evident disconnect the track between the filter and L3 and reconnect the filter to the unused pin on L3, which is a higher impedance point. If you use the later 1k8 Ambit filter you may get away with the standard L4 but, if distortion is evident, replace it.
RF Gain Control
The smaller rigs have no R.F. Gain control: unless you happen to snip through the wire of the resistor in series with the squelch pot! Use the tone switch to disconnect this resistor and you have a squelch or R.F. Gain control at the touch of a button. Very useful for fox hunting.
The R.F. Gain control on the big rigs is very abrupt in operation. Replace R2 (1k5) with about 6k8 for smoother operation. Also, D1 diode goes short circuit; results in loss of sensitivity. May need to replace with 2 ordinary diodes in series. D9 open circuit can cause the same symptoms.
The squelch control on the Uniden/Audioline rigs is very wishy-washy. The operating point can be made much sharper by replacing C5 (10uF) with 1uF. C5 is near the centre of the PCB, just behind loudspeaker clearance area.
The modulation tends to be muffled and a modification was offered in What CB Magazine, as follows:-
change from to
R69 10K 8K2
R70 12K 47K
R71 10K 4K7 Also, add a 4n7 capacitor
R80 560R 330R in parallel with R81 and add
C74 47n 100n an 82pF capacitor in parallel
C92 68n 33n with R79.
C93 680p 330p
C98 1uF 100n
C99 22n 100n
Tricky Uniden Faults
VCO locks on TX but not RX
- replace TR18 2SA105.
No RX and both RX and TX lights on
- replace TR17 2SC1815.
RX jumping in and out when in TX
. Both lights on but set will not lock. Removing RX wire from mike socket allows normal TX operation. Fault is caused by Audio IC - replace.
President KP77 homebase (PC-902AA)
TX power loss with "motor-boating" on some channels can be caused by faulty TR202 in PSU board (PC-410AC). Replace with TIP41.
Amstrad 900, 901, Mustang CB3000, Fidelity CB 2001FM, Harvard 402 MPA, Spinneytronic and Harvard H-407 homebase .
No RX no TX
may be caused by the small copper foil burnt out on the mike socket PCB from the common (0v) pin.
The square FET
(K107E1K) can be replaced with a 2N301 or a 3SK45. Fit so that the metal tag is to the left as you look from above with the fascia towards you.
(2pF) with the crystal filter, as in the Cybernet 134 board (see above).
Make the squelch action snappier
by changing C99 (1uF) to 100nF. C99 is hidden beneath the cableform towards the front of the PCMA002F board.
These rigs use an inductor to adjust the 10.24 MHz oscillator. If the inductor becomes damaged, replace with CIRKIT part 119ANA5874HM. The value of C18 (33pF) may need to be changed.
on high volume: replace C107 (1000uF) next to audio IC.
Remove RF power transistor and solder up the gaps in the chassis to improve heat sinking and reliability. The big 3R3 can be shorted to increase power output but an extra heatsink will then be needed.
Maxcom/Midland rigs - often suffer from spurious emissions during TX, which can be seen on an oscilloscope connected across the dummy load. Symptoms include sending frequency meter or SWR meter wild and possible interference to radio receivers. The effect seldom causes any obvious problems with transmitted modulation, but power output may read less than 3 Watts. Cause is usually maladjustment of the RF can nearest the left side of the chassis (look at component side of board, knobs towards you).
Replace the existing ceramic filter with a P.R. Golledge 1k crystal filter. Remove C110 (27pF) and R189 (390R) and retune T8.
can be improved by desoldering one leg of C207 (but leave it in place).
Power output can be increased
by changing R147 to 1 Ohm.
"Lazy needle" can be cured by changing R128 (4k7) to 1k. cut brown wire to "S" meter and insert a 270R resistor. Connect a germanium diode from the other tag of the "S" meter to the 270R resistor. (The diode cathode band should be at the non-meter end of the resistor.) Adjust Power meter to 4 Watts with RV103.
Midland AM - No TX or low TX caused by R330 (10R) going open circuit, removing supply voltage from the RF driver.
Cheiza, Academy, Fidelity 1000, 3000FM, Sapphire X2000, Lake Manxman, Commtel,Tokai, Elftone, Barracuda.
In these rigs the 10.7 ceramic filter sits between T1 and T2 and may be replaced directly with a crystal filter. You won't even need to make any adjustments.
output or no R.F. output is often caused by the demise of R65 (8R2) which goes high-resistance. R.F. output is invariably low on these rigs in any case and may be increased slightly by replacing Q17 with a 2SC1815. Note, however, that the most common reason for NO R.F. output is that a long screw has been forced into L6 (Replace with a Toko 199CCA127EK but DO NOT connect its centre pin!)
No channel display, no TX, no RX
- replace Q9 with a 2SD325 or BD241. Also check zener D11.
VCO range can be increased by adding an 82pF ceramic capacitor in parallel with C216 (80pF). A further increase may be achieved by changing C224 (27pF) to 22pF.
Telecom TC9000/Grandstand Gemini/Grandstand homebase crystal filter. Replace the ceramic filter with a 1k filter and remove R85 (330R).
Harvard 400M - poor RX - replace 3N201 near trimmer next to o/p transistor. A 3SK45 can be used.
Interceptor crystal filter- Fit crystal in place of FT-1; Replace transformer RX4 with KACSK3892A; fit 1k resistor between filter and RX3 transformer.
Shogun - LED bar driver LB1415 can be replaced by LB1405.
Icom ICB1050 varies its RF gain as you use the squelch. This is due, helpfully, to a diode not shown on the circuit diagram! Luckily it is the only diode underneath the circuit board, so cut it out if you don't want this "facility".
- has a tendency to go "deaf" because T101 goes open-circuit. Replace this transformer with TOKO 113CN2K159DZ from "Cirkit".
Bremi BRL210 linears are potentially LETHAL because the mains earth goes to a 1" wide by 5" long strip of PCB. The PCB cracks at the supporting bolt and leaves the case unearthed. Drill the case, scrape away the paint and secure a good earth with a solder tag and bolt.
B30 Linears - Low TX can be caused by poor earth between case and PCB.
Bremi BRS31 PSU 2200uF capacitors fail causing low voltage output on-load.
Quasar CB - Switching transistor (2SC1815) behind mic socket goes s/c causing low audio hiss on RX with TX light remaining on.
Radiomobile 202 - PLL not locking - replace D2 (6v2 zener).
- Low audio - replace Q15 (2SA733).
Communicators, Manor Kestrel, Planet, Serpent - Stuck on TX - Q22 s/c, replace with TIS91.
- No RX and TX stays on when keyed - check D13 near relay.
LCL/DNT 2740 - relay dropping out - replace Q11.
LCL ECONOMY 40FM :- Small resistor in series with voltage supply track to LC7137 pin 18 burns out. Replace with 270R. Mixer coil adjuster can apparently screw itself in causing loss of receive. (Coil is towards rear and has small adjuster).
Binatone Speedway - Low audio, poor sensitivity on RX, low power on TX - replace Q18.
Harvard/Alba 40 Ch handset - no Freq. mod. although the carrier is perfect and the Roger Bleep may just be audible. Locate the diode (D20) closest to the HI-LO power switch. This diode is under-rated and should be replaced with a 1 Amp type such as 1N4001.
HARRIER CBX HQ has only two-core mains cable. Always fit 3 core cable, for safety!
Pegasus 7 : Change C184 to 2n2, R155 to 2K2 to improve echo.
Colt 444s and Ham International FM rigs et alia: no TX at all. Locate and replace the black diode, in front of the modulation transformer, with a 3 Amp type.
- crystal filter- remove ceramic filter and the 220 Ohm resistor connected to it. Fit a 1k5 resistor and crystal filter instead. No retuning necessary.
Murphy CB1500H home base rig - low power o/p on TX can be caused by burnt out L7 (27uH). You can rewind this yourself, if necessary, with 68 turns of 30SWG enamelled copper wire. (The plastic cover can be cut away and discarded.)
Grandstand Hawk, Lowe TX40
Replace C65 (3pF) with the crystal filter. Retune L16 and L17.
Falcon, Compact 40 - T1 can be replaced with a Toko 113KN2K241DC.
DM315E microphone using DM432 board - two unsoldered tags on slide cause hum. Solder them to 0v.
HAM RELAX hand power mike. This mike is normally very bassy and sounds muffled on most rigs. The following mod can be used on other battery-powered mikes, too. Remove old mike insert and fit Tandy electret mike (270-092B), cushioning it in foam rubber. Connect screen to 0v ground. If a resistor is used between original insert and PCB, remove it. Connect inner core of Tandy insert to the PCB via a 10uF tantalum capacitor. Connect the +ve lead from the Tandy insert to battery +ve (VCC). On the Ham Relax mike, remove the top-cut capacitor from between the F.E.T. and the volume control slider on the component side of the board. Volume control housing can cause hum. Solder its tags to -ve rail to eliminate hum.
Altai power/echo mikes - Echo disappears when battery voltage drops.
Sirtel 2002 and 313P microphone - the mike clip screw on the back can be overtightened to cause a short which results in intermittent speech. Fit a "sticky fixer" under screw base.
Realistic Noise Cancelling Mike - the switch can return too far and mute the audio. Build up the inner case or the trigger with hot glue.