CB radio Cobra Handy Hints
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. | 