Someone requested the truth chart for the Cobra 142GTL and I neglected to make a note of where that was posted. Can't find that thread right now.
I scanned it, and it occurred to me that a little explanation might be helpful.
The column marked Divider Input is the frequency you'll see at pin 17 of the MB8719. Divide this frequency by 10 kHz and you'll see the actual divider number inside the PLL. On channel 1 the chip divides the input frequency by 143 to obtain the internal 10 kHz reference frequency. The divisor for channel 40 is 187.
But the binary value of the six bits coming out of the channel selector shows decimal 15 for channel 1 and 79 for channel 40. Clearly the binary output from the channel selector is not the whole picture. The 8719 adds the number 128 to the binary input from the selector. 128 plus 15 from the channel selector gets you the 143 shown on the chart.
Pin 10 controls this internal number that's added to the selector's binary input value. In the 142, pin 10 is connected to ground. This selects 128 for that internal constant. Cut pin 10 loose from ground, and an internal resistor in the chip is connected to 8 Volts and "pulls up" pin 10 to a logic 1 state. This changes that internal constant to 64. This is how the factory wires this chip in a Cobra 148 or 2000, with no connection at all to pin 10. And this is why connecting that pin to ground moves the radio up 64 channels, making channel 1 27.605.
Hopefully this helps to make sense of the chart.
73
I scanned it, and it occurred to me that a little explanation might be helpful.
The column marked Divider Input is the frequency you'll see at pin 17 of the MB8719. Divide this frequency by 10 kHz and you'll see the actual divider number inside the PLL. On channel 1 the chip divides the input frequency by 143 to obtain the internal 10 kHz reference frequency. The divisor for channel 40 is 187.
But the binary value of the six bits coming out of the channel selector shows decimal 15 for channel 1 and 79 for channel 40. Clearly the binary output from the channel selector is not the whole picture. The 8719 adds the number 128 to the binary input from the selector. 128 plus 15 from the channel selector gets you the 143 shown on the chart.
Pin 10 controls this internal number that's added to the selector's binary input value. In the 142, pin 10 is connected to ground. This selects 128 for that internal constant. Cut pin 10 loose from ground, and an internal resistor in the chip is connected to 8 Volts and "pulls up" pin 10 to a logic 1 state. This changes that internal constant to 64. This is how the factory wires this chip in a Cobra 148 or 2000, with no connection at all to pin 10. And this is why connecting that pin to ground moves the radio up 64 channels, making channel 1 27.605.
Hopefully this helps to make sense of the chart.
73
