Tony G0MQG Tips

Time for another of radio tech guru Tony G0MQG's Tips, this time solving a display fault on a CB...                                
CB radio channel display

   A club member brought a cb radio to me to have a look at (This was a radio from the 1980's)

Most cb radios from the 1980s had the channel display wired directly to the channel selector switch via current limiting resistors and this one had one segment of the display out.

The displays can be either common cathode or common anode depending on the manufacturer but the principle is the same, eg if the display is common cathode then 13.8 volts is fed to the channel selector switch common terminals,  the 14 switched terminals (7 for the “tens” digit, 7 for the “units” digit) are fed via the 14 current limiting resistors to the dual 7 segment led display module, the common of which is then taken to the common rail.

The displays on these are usually fairly reliable so I had a careful look at the wiring from the display module to the channel selector pcb.

The channel selector pcb on this radio is linked to the display led module with a short ribbon cable and careful inspection revealed that one of the cables from the display to the switch pcb had broken at one end.

I decided to re-wire the whole cable as it was easier than trying to re-terminate one wire and this cured the problem.

It should be noted that sometimes the problem is the selector switch and so can be difficult to cure as i've yet to find a supplier of new replacement switches.
Tony g0mqg

 

Christmas 2017

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