Is there a 10kc drop-mod for the Cobra 2000?

[ShakyMcQuiver]

Isn't there a 10kc drop mod for he Cobra 2000? Mine has the MB8719 PLL.
If memory serves, the older 148 boards are identical (or nearly) and they had the mod done by pulling pin 10 high or low.
My Grant-XL has a drop switch. It's a lot easier to do it this way rather than a super diode and a re-align. You really only need 5kc, but what the hell - might as well get it to do 10 for the Alpha channels.

I'm digging through my books but I don't have much on the Cobra 2k. Most mods out there refer to full channel mods, but not a drop switch.

Thanks,

 

[nomadradio]

If memory serves, pin 16 of the MB8719 PLL chip is the least significant bit of the binary input from the channel selector. When this input is high, the divisor for that channel is odd. When pin 16 is low, it's an even number. The divisor for channel 1 is 79. Channel 3 is 81. The RC channel that will get skipped has a divisor of 82, but the channel selector skips that.

You can't add 1 to the binary "81" code and go up one channel. Pin 16 is already high. But we can subtract one from the channel 4 divisor of 83. Pulling the input to pin 16 down to ground gets us a binary zero even if the channel selector is sending it a high input. The foil trace between the channel selector and pin 16 gets cut. A 1k resistor is strung across the gap. This isolates the channel selector from the switch we'll use to force pin 16 up or down.

The next RC is above channel 7, at 27.045. The divisor for channel 7 is 86. Because this is an even number, we'll need to bump the PLL up one channel for this RC. That's done by forcing pin 16 high.

This up/down sequence continues. channel 11A comes down from channel 12. Channel 15A goes up from 14, and channel 19A gets the channel selector on 20, and pin 16 pulled down.

So 3A, 11A and 19A all get pulled down one channel. 7A and 15A get bumped up one channel.

A SPDT center-off switch will take care of this. The center pin of the switch goes to pin 16, after the cut/resistor are in place. One side goes to the chip's power-supply pin, the other side to ground.

Not exactly the convenience of a "plus ten" switch, but the two 4008 chips that make THAT feature possible are even more trouble to wire up than this.

It's as close as you can come with just this PLL and switches.

73

 

[ShakyMcQuiver]

Thank you for your very informative explanation. Reminds me of the EPROM book I bought from CBCity years ago. Made your head hurt at first, but eventually I learned how the programming worked.

Eprom programming is muscle memory too (at least for me anyway). If you don't use it.. you lose it! I can recall my early days of messing with various configurations and crystals. I thought I was into some next-level James Bond stuff when I was able to make some old Walkie-Talkies transmit on one weird frequency and receive on the other. LOL... loads of fun. Probably why my old CB hobby will never die. They can sit on your shelf and still be fun years later. Always learning.
_____


I did find a few interesting sites on the MB8719 pin 16 mod. It wasn't Cobra 2000 specific, but interesting nonetheless.

This first schematic... what is its purpose? Most pin reprogramming I've seen don't use a transistor or multiple resistors. Most I've seen use your suggestion of 1 resistor and a simple toggle switch.



And here's another one that you run across on many mod sites. So what's the advantage (if any) of the transistorized version above?