CB RADIO Handy Hints
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.