Cobra 140 GTL channel mod question

[tubefan]

Picked up a Cobra 140 GTL with the mb8719 chip. It has a mod where when you switch the anl nb or the cb pa it goes below 1. Further down than the 26.815 I have seen on other 8719 radios. Thing is it has no uppers. If I remove the wires from the ic and reconnect pin 10 to ground so I am back to stock 40 what is the easiest way with 1 switch to get uppers only. No need for lowers. Thanks for any info.

[Sandbagger]

May 3, 2019 18:27:19 GMT -5 tubefan said:

Picked up a Cobra 140 GTL with the mb8719 chip. It has a mod where when you switch the anl nb or the cb pa it goes below 1. Further down than the 26.815 I have seen on other 8719 radios. Thing is it has no uppers. If I remove the wires from the ic and reconnect pin 10 to ground so I am back to stock 40 what is the easiest way with 1 switch to get uppers only. No need for lowers. Thanks for any info.

Some of the MB8719 chassis radios have mostly down channels and will top off at 27.445. The reason is that the oscillator mixer crystal is an 11.1125 Mhz. Replacing that crystal with a 11.325 Mhz will give you the usual 28.815 - 28.045 range.

[tubefan]

So if I install an 11.3258 crystal and open pin 10 I should get uppers?

[tubefan]

And if I install the 11.3258 will the regular 40 still be normal? I'm a little confused at how if I change the crystal to a higher one how the 1-40 will stay the same. Hope I'm making sense.

[Sandbagger]

May 3, 2019 21:34:48 GMT -5 tubefan said:

And if I install the 11.3258 will the regular 40 still be normal? I'm a little confused at how if I change the crystal to a higher one how the 1-40 will stay the same. Hope I'm making sense.

Everything will line up. The only difference will be the state of pin 10, which is normally internally pulled high (+8V) for the normal channels with the 11.325 Mhz crystal. It will be the opposite of what it was with the original crystal. And when you ground pin 10 with the 11.325 Mhz crystal it will go up all the way to 28.045. Some people mod their rigs to put both crystals on a switch so that you can get both uppers and lowers.

[tubefan]

Thank you. I appreciate it. Will let you know how I make out.

[2600]

In the stock 140, pin 10 of the 8719 is grounded. If you break the connection to the ground foil either side of pin 10 the radio moves down 64 channels.

Changing the crystal from the stock 11.1125 MHz to the 11.325 MHz moves the radio up 64 channels. With the 11.325 crystal, the radio will now move up 64 channels above the normal channels when you restore the ground connection to pin 10.

Grounding pin 11 moves the radio down 32 channels, no matter which crystal is in use. This will only be effective on the channels where that pin would otherwise be "high", or at 8 Volts. Those will be channels 15 through 40 on the channel display.

With the 11.325 crystal, pin 10 and pin 11 both grounded, this moves you up 32 channels, but still only on channels 15 to 40 as seen on the display digits.

Sounds simple, right?

73

[Sandbagger]

May 3, 2019 21:32:14 GMT -5 tubefan said:

So if I install an 11.3258 crystal and open pin 10 I should get uppers?

Yes. Exactly.

[tubefan]

I got it. Found a crystal in a junk radio. All is good now. Just need to align it now. Thanks for the help. Both of you !!

[Sandbagger]

May 3, 2019 23:05:22 GMT -5 tubefan said:

I got it. Found a crystal in a junk radio. All is good now. Just need to align it now. Thanks for the help. Both of you !!

That's what this forum is all about. Hope everything works out. Let us know how you make out when you're done.

[Hunter]

I am in the opposite boat.  I have a stock Cobra 140 GTL and my 11.1125 MHz crystal has become unstable.  I want to keep the stock 40 normal channels.  Does anyone have a working 11.1125 MHz crystal available?