Cobra 142 GTL Extra Channels

[Greg T]

A buddy and new guy to CB just bought a huge lot of radios and amps, and noticed two switches on the back of his 142 GTL. He says he knows it changes frequency, but the original owner can't find the channel sheet. Were there more than one of these kits, and if so, were they different frequencies? Does anyone have a frequency chart that may give us an idea of what these channels are? Yeah, we could map them out with a capable radio, but that would take eons. Anyone have a channel chart or a way to tell who built this channel kit?

 

[TM86]

Could be a kit or two, or just a couple of PLL pins being manipulated. Pics of the guts of the radio might help. Two switches on the back of the radio isn't a heck of a lot of info to go on.

 

[Greg T]

Could be a kit or two, or just a couple of PLL pins being manipulated. Pics of the guts of the radio might help. Two switches on the back of the radio isn't a heck of a lot of info to go on.

I know. I didn't get the pics from him yet. I told him to follow the wires and give me good pics

 

[nomadradio]

Too many possible ways to do this. Odds are that the original 11.125 MHz PLL crystal has been changed to 11.325. This allows the MB8719 PLL chip to go above channel 40.

The switches will be connected to pin 10, pin 11 and maybe pin 12.

But the exact hookup ends up being one of multiple flavors of secret sauce.

Too many ways to wire this up. You may need to put a counter in line with the radio and key each of 40 channels for all the separate combinations of switch positions.

A hand-written chart was a common thing to see on a "freebander"s radio back in the day.

73

 

[Greg T]

Too many possible ways to do this. Odds are that the original 11.125 MHz PLL crystal has been changed to 11.325. This allows the MB8719 PLL chip to go above channel 40.

The switches will be connected to pin 10, pin 11 and maybe pin 12.

But the exact hookup ends up being one of multiple flavors of secret sauce.

Too many ways to wire this up. You may need to put a counter in line with the radio and key each of 40 channels for all the separate combinations of switch positions.

A hand-written chart was a common thing to see on a "freebander"s radio back in the day.

73

Yeah, kina what I was thinking when I saw two switches. I have seen kits installed that used one switch, and even three switches, but have read about the custom use of the two switch method. I'm just going to suggest that he get a counter and map it all out. Thanx!!

 

[Cable Guy]

I agree with TM and Nomad, all 2 switch channel mods I have seen just manipulate pll pins, along with a xtal swap. Look up Poormans Channel Conversion for the MB8719. There are 2 methods provided. I prefer the pll switches as it's more stable on ssb, but the channels are somewhat mixed and overlap, whereas the expo mods keep channel linearity at the cost of some drift and stability from the added xtal bank and longer than optimal wires.

 

[BC Coyote]

The way I do this is with a scanner that has a frequency capture function. You go through the channels with the switches in the different configurations. On each channel you key up and read the frequency off the scanner.
You could also use a frequency counter if you have access to one, or even a HF ham rig with a VFO. Lots of ways to figure it out!

 

[Onelasttime]

Well it should have a MB8719 PLL. So it should match ever other MB8719 PLL two switch mod.

I spent less than 2 seconds on google so I am confident if you look you will easily find this info. Find out if it is a PLL hack or if it is the Expo type hi/lo/normal kit with crystal.

Good Luck to your friend!

 

[Onelasttime]

Well it should have a MB8719 PLL. So it should match ever other MB8719 PLL two switch mod.

I spent less than 2 seconds on google so I am confident if you look you will easily find this info. Find out if it is a PLL hack or if it is the Expo type hi/lo/normal kit with crystal.

Good Luck to your friend!

I think the crystal is a different freq. than the 148GTL or Uniden Grant XL. Outside of that it is similar performance.

 

[brandon7861]

If you want to use a 11.325 crystal in a 11.1125 radio, pretty sure you just lift pin 10. Vice versa going the other way. The washington and 142 with the 11.1125 crystal had pin 10 (P6) low while the 148 and grant with the 11.325 crystal had the pin floating (high, internal pull-up). Other than P6 (pin 10), I think the rest of the N code is the same regardless of crystal. It works in my MB8719 spreadsheet that way too.

What we need is a chart for whatever pin configurations you want for the crystal yours uses (regardless of whether it belongs there or not). What pin configurations do we want to try?

P0-P5 are controlled via channel dial, P6 is factory low (washington, 142) or high (148, grant) with pin not connected (internal pull up). There is a P7, but it is controlled internally by an inverted copy of P6 (more on this in a moment).

The simplest switch mod would be to have a high and low on P6 (basically ground it or not). A second switch would likely appear on P5. Since P5 is on the channel dial, having a switchable inverter here would be ideal, but some may just run that pin manual - isolating it from the channel dial (and screwing it up).

There is also the possibility for the MB8719 (not the RCI8719) to pull the voltage somewhere mid-rail on pin 10 (P6) using a resistor to ground. I have not tried it yet, but I have read that the threshold on the internal P7 inverter is higher than the input to P6 allowing both to be high at the same time given the proper voltage on P6. I think the RCI version removed that flaw. One of these days, I will lift pin 10 in my washington and see how many times my counter changes frequency with a pot to ground at pin 10, I just don't know the internal pull-up value so its a guess on the test pot range.

Anyone want to double-check this for me?


Edit: this chart shows one interesting thing, only the 11.325 crystal can get you to the triple nickel if the float trick doesn't work on your PLL. With the 11.1125 crystal, P6 and P7 must both be high to get the triple nickel, and that depends on the design flaw being present in the chip.

 

[Greg T]

I'll have to have him open it up and send some pics. I have no idea what he has and he's like 90 miles away.

 

[Onelasttime]

I'll have to have him open it up and send some pics. I have no idea what he has and he's like 90 miles away.

That is always tough helping a friend from a distance.

If the mod was done with crystals and a switch ala Expo or LC type board you get an entirely different channel map than if it is a hack of the stock pll. So sadly how the channels where come by does matter in what the channel map will look like.

 

[Tucker442]

A buddy and new guy to CB just bought a huge lot of radios and amps, and noticed two switches on the back of his 142 GTL. He says he knows it changes frequency, but the original owner can't find the channel sheet. Were there more than one of these kits, and if so, were they different frequencies? Does anyone have a frequency chart that may give us an idea of what these channels are? Yeah, we could map them out with a capable radio, but that would take eons. Anyone have a channel chart or a way to tell who built this channel kit?

There’s a few companies that make frequency counters that aren’t a lot of money that work off 12 V do you just use a power supply to operate run at your house and see what the frequencies are they’re all different. There’s a lot of different mods and some guys just put crystals in them or did a PLL replacement you really need a frequency counter

 

[nomadradio]

Fleabay frequency counters have gotten stupid cheap.

73