Cybernet PTBM 135 AOX
No TX/RX until the rig has been on for a few minutes - It most often appears to be the capacitors next to the VCO coil becoming very temperature sensitive.
Try replacing the 82pf capacitor. Re-tuning the VCO seems to cure the fault but it might loose channels when hot.
Cobra 148 010 chassis (2)
To silence the roger bleep disconnect C162 near the microphone socket (a 2u2/25v electrolytic which isn't numbered on earlier boards but C270 on later
ones) or make this capacitor switchable. The tone of the bleep is also controlled by this cap.
The regulator transistor often blows (appears like output transistor gone but can be spotted by no mod on AM) and can be replaced by a TIP42 which is
PNP. If you fit a NPN by mistake it will oscillate and howl through the speaker.
Similarly Am modulation transistor is also PNP and may be replaced by a TIP42. If an NPN is used there will be no AM modulation on TX.
Suffers from lazy needle - Short out either D7 or D8 (not both) which are in series near the center of the board. Adjust VR1.
Bad modulation - on TX can be caused by loose or missing PCB securing screws. Carrier on SSB, no mod on AM/FM, SSB red light stays on during TX -
Can be converted to UK channels quite easily since the channel selector contains a section with no "jumps"(Alphas) . Use a 20.39MHz X-Tal. 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 Type Rigs
PCMA001s - Nato 2000, Tristar 797, 848 (PCCW001S bleep board)
PTBM048AOX - Colt 483DX Black Shadow
PTBM059COX - Ham Multimode 2, Ham Jumbo(PTOS006AOX crystal and VCO board) Major M360 (PTSY016AOX crystal and VCO board)
PTBM092COX - Colt 870 (PTOS008AOX crystal board,PTZZ033AOX FM board)
PTBM094COX - Ham Viking, Formac 88 (PTOS008AOX crystal board and PTZZ033AOX FM board)
PTBM106AOX - Lafayette HB940, New Formac 88, Formac 120, Colt 720 (PTOS008AOX crystal board and PTZZ033AOX FM board)
PTBM121D4X - Concord 2, Hy-Gain 5, Tristar 747, Clt Excalibur (PTOS11AOX crystal board)
PTBM125A4X - Tristar 777, Superstar 2000 (PTZZ080AOX bleep board)
PTBM133A4X - Concord 3
Ham Multimode 2 - 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" end of the switch. The off position wire to post 5 (do not remove wire already there)
Fitting x-tal filters
PTBM059COX - 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 the filter
PTBM125A4X and PCMA001S - Replace C120 with filter
PTBM092COX and PTBM106AOX - Replace C63 with filter
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 is joined to R29). The audio AGC action is fierce so, to reduce it, cut the track between D38 and the
other components (C125 C173 R150) or lift one leg out of the PCB. Connect a 1K preset in series with D38 and adjust for the most pleasent modulation.
The Altai DM315E echo mic works well. The AGC action can also be reduced by soldering an 18k resistor 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 can be caused by burning of the RX coils which can be quite
difficult to spot. When you replace them keep the power output down to 5watts.
Extend the Tuning range - by replacing L1 (2u2H) on PTOS012AOX crystal board with a 3u9H inductor and remove capacitors C5 and C7. Note that the
RV14 is a limiter for AM and RV13 is the Amplitude Modulation depth adjuster. Using these to "apparently" turn output power up will replace modulation
depth to almost Zero.
Lights dimming or fuse blowing on TX can be caused by D1 on the PTZZ080AOX bleep board
PTBM106AOX - common fault is no TX or intermittent TX caused by faulty D13 near modulation transformer.
PTBM048AOX - 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 (this adaption makes SSB a little harder to tune in)
PCMA001S - (Nato 2000) RX ok but erratic RF power output or no output can be caused by the demise of D1 on the PCZS0001 board, replace it with a
1N4001 (In case you didnt realise the two "IC's" on this board are not IC's 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 bleep on and off - Insert a switch in one leg of C601. To shorten the duration of the bleep change C601 (10uf) to 3u3F. To lower the tone add
22nf across C605.
Improve Modulation - on Superstar 2000, Tristar 777, Nato 2000 by adding 100nf across C151 (22nf) and C149 and C155 (47nf)
Improve on Colt 747, Hy-gain 5 by adding 100nf across C160 and C162 (22nf)
PTMB121D4X Colt Excalibur - Add a switch in line with R185 (330k) or C207 to make roger bleep switchable.
Ham Jumbo/Excalibur/Marko - LED can be replaced with one from a Fidelity 3000 homebase (Finally, a use for it ;o)
Concord Mk 2 - No TX on Am/Fm but SSB ok. Change voltage regulator 2SD837 on the back plate of rig. This is a darlington pair transistor and must be
replaced by the same type.
Ham Int. Rigs - No TX or RX. Check for 5.8v on pin 1 of PLL02A, if low then replace Zener diode.
President KP77 - (homebase PC902AA) TX power loss with "motor boating" on some channels can be caused by faulty TR202 on PSU board (PC410AC)
replace with TIP41. Thsi radio was originally made for the 23ch German market and has been factory adapted for the UK market. Thats why theres a
separate PLL IC board.
Maxcom/Midland Rigs - Often suffer from "Spurious emissions"(good word that) during TX which can easily be seen on an Oscilloscope. Symptoms
include sending SWR meters wild and kicking out a seemingly good signal but only being heard locally. In bad cases it can also send frequency counters
haywire as more than one frequency is being transmitted. Somewhere along the radio spectrum your voice will be heard like your actually on that channel.
(in fact you are) The effect seldom causes any serious problems with your equipment and the cause is normally mal-adjustment of the RF can nearest the
Left hand side of the chassis (looking at the component side with front towards you)
Replace - The existing ceramic filter with a PR Goolage 1K crystal filter. Remove C110 (27pf) and R189 (390R) and re-tune T8
Sensitivity - Can be improved by de-soldering one leg of C207 (but leave it in place)
Power Output - Can be increased by changing R147 to 1ohm.
Lazy Needle - Can be cured by changing R128 (4k7) to 1k. Cut brown wire to signal 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 4w with
Midland AM - No TX or low TX caused by R330 (10R) going open-circuit removing supply voltage from the RF driver. Also check R322 (In front of O/P
transistor) as this can burn out and be hard to spot.
Freq Adjust - Can be achieved by adjusting L204
Midland 2001 - No squelch, try replacing MC3357. No TX no RX no incoming signal on meter, try replacing 2SC7808 on side of rig.
Midland 77- 104 - Very common fault. RX and TX go on and off. Dry joint on center of board or voltage regulator 7808.
Midland 77- 099 - No channel display or only counts up to 9. Try D302 (8v1 Zener diode) or R310 which burns out. Both are located at the back of the mic
Increase RF power to 8w. Link the two legs of RV103 together (ignoring the center leg) and re-tune the RF output stages.
Faint modulation - Replace Q306
Midland port & pac - LED driver is TL489CP (IC2) No TX power - replace R147 (resistor supplying power to output stage)
Cheiza - Acadamy - Fidelity 1000 - Sapphire X2000 - Lake Manxman - Commtel - Tokai - Elftone - Barracuda
Quite possibly the worst CB radios ever made. Terrible broadband, also to avoid, Binatone route 66 and communicators NI440DX/Planet 2000/Serpent
In these rigs the 10.7 ceramic filter sits between T1 and T2 and
may be replaced directly with a crystal filter. You wont even need
to make any adjustments
Low Power Output - or no RF output is often caused by R65
(8r2, 0.5W) which goes high resistance. RF 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 TX is that a long screw has been forced
into L6 (Use proper thumbscrews for mounting) can be replaced
by a Toko 199ccal127ek but do not connect its centre pin.
No Channel display - no TX/RX - Replace Q9 with a 2SD325
or BD241 also check Zener diode D11
VCO Range - Can be increased by adding an 82pf ceramic
capacitor in parallel with C126 (27pf) to 22pf
Signal Meter - Indication of signals but no incoming sound,
Transmitted Sound - Low or zero, replace Q11
No Squelch - Try replacing Q7 in front of audio IC.
No Signal Movement - but otherwise ok, replace D5 R65 burns out
(8.2ohm) causing no TX. This resistor is located close to Q19 output
transistor. Replace with a 1 watt resistor or 2 x 0.5watt 18ohm in parallel.
Stuck on TX - but frequency is incorrect, this fault is caused by Zener diode D11 going open circuit. D11 is located in front of Q9 voltage regulator.
General Fault - No sound from rig. If mic is ok check speaker. Occasionally the connections corrode away on the cardboard cone otherwise check for
broken track on the audio IC. Be aware that sometimes the ext. speaker socket can be faulty (dry solder joint or dirty)
Microphone trick - If you file a groove in the opposing side of the mic plug the same mic can be used for a cybernet wired rig.
Cobra 29-LX-EU - Comes from the factory with quiet FM modulation. The pot for adjusting this is VR4 which it towards the back right of the component side
and to the left of the AM modulation transformer.