Cobra 29gtl/ltd upd 2816 channel mod ?

[dss56]

Been messing around with the 29ltd for low channel mods with the 2816 pll.

I have all the expo kits for these radios and all the xtals for 40up/40dn.

My question is on the pll 2816 i isolated pin 9 and got all the low channels 26.950 to 26.510 all half channels. I can easily fix to get back on the full channel.

Problem is when this is done the receive does not follow the low channels it still receiving on regular 1-40. But transmitting on the low channels.
Is there something else that needs to be done to get the receive to work or it cant be done.

thanks

 

[dss56]

I just saw an old post on here where it says
The only "non-crystal" mod I've ever heard of won't move you down 45 channels, it moves you down 45-and-a-half channels. This trick involves taking the transmit/receive pin (#9) on the 2816 loose, and "inverting" the signal that feeds into it. Takes a DPDT switch, a NPN transistor like a PN2222 and three resistors.

Would anyone have this mod info documented that you can share/post here.

thanks

 

[dss56]

Anyone have any info if you can get the receive to work on the low channels by lifting pin 9 to go low to get the low channels?

Main question is in post #2

thanks

 

[TM86]

Let's look at why it's doing that 455 Kc "shift down" on TX.

When the radio is keyed normally, the VCO gets shifted up by 455 kc to mix with a 10.240 TX carrier signal. This a buffered output of the same 10.240 MHz that drives the PLL. So, say for channel 1, the VCO will be generating a 16.725 Mhz signal. Add that to 10.240 MHz and you get 26.965 MHz, the frequency for channel 1.

In receive, the VCO would be generating 16.270 MHz into the first RF mixer, and the output of that would be 26.950 MHz - 16.270 MHz, or 10.695 MHz, which is the first IF frequency. The 10.240 MHz signal comes into play here again, as it is used to mix with the first IF frequency to give the second IF frequency of 455 kc.

By disabling the T/R pin on the PLL (pin 9), the radio is not getting the command to move the VCO up by 455 kc when transmitting, so it keeps the VCO at the receive frequency of 16.270 MHz, which gets added to the TX 10.240 MHz, giving you 26.510 MHz. Which you've already mentioned seeing.

So, yes, there's something else needed to get the receive to move to where the transmit is.

The 5.12 MHz out of pin 10 on the PLL feeds into a tripler coil that puts out a 15.360 MHz signal. Replace that with a 14.910 MHz signal to go under channel 1 or a 15.810 MHz signal to go over channel 40. And reconnect pin 9.

There's also the options of installing a DDS-VFO or one Gerhard Maurer's PLL replacement modules.

Sorry it's not as easy as some of the earlier chips. Don't blame me, blame the FCC.

 

[dss56]

Thanks TM86 yes i have the expo kits for the 15.810 and 14.910.
In post #2 someone mentions about inverting" the signal that feeds into it. Takes a DPDT switch, a NPN transistor like a PN2222 and three resistors. Have you seen this done or would you know of this mod?

thanks

 

[TM86]

Thanks TM86 yes i have the expo kits for the 15.810 and 14.910.
In post #2 someone mentions about inverting" the signal that feeds into it. Takes a DPDT switch, a NPN transistor like a PN2222 and three resistors. Have you seen this done or would you know of this mod?

thanks

Sorry, no. Never seen that one.

 

[dss56]

This trick involves taking the transmit/receive pin (#9) on the 2816 loose, and "inverting" the signal that feeds into it. Takes a DPDT switch, a NPN transistor like a PN2222 and three resistors

Well Im just messing around as I do have many expo kits for the 29s and 25s and many other radios. Also the Galaxy N kit for the 29s to go from ch 42 to 104.
was interest in finding out about this mod with the resistors and transistor.

Hope someone chimes in with some info,

thanks

 

[Cable Guy]

I found this mod in a 25 (different pll) and altered the schematic to reflect the mod. They are different pll's but the same idea I think. You invert the t/r pin and switch in some capacitance on the 10.240 xtal to get back on the nickels. I haven't tried this on a 29 yet, it may not work but I don't see why not.

Thread 'Cobra 25gtl channel mod, help.'

May 18, 2025

Someone gave me a radio to fix. It had a dead reference xtal, no biggie. Board pc-417ac. Tc9106p pll. He said it had lower channels, switchable from the front. Well, it sure does, and without any xtal kit. I've never seen this mod and it's kinda confusing to me. Looking for any info.

Underneath 10.240 xtal. A track has been cut, a cap bridging it and wires from that to the switch.

Okay. When switched on, the gap and cap are bypassed. When off, an extra 33pf is added between xtal and circuit. I guess to pull the xtal, I got that

Then there is this...


A...

• Cable Guy
• Replies: 8
• Forum: CB Radio Modifications

 

[Cable Guy]

I see the 9106 pll has its tr pin low for rx, hi for tx. The 2816 is reversed, hi for rx, low for tx. Still, I can't imagine it would matter as the mod just reverses what the radio tells the pin to do.

 

[nomadradio]

the mod just reverses what the radio tells the pin to do.

Yep.

73

 

[nomadradio]

When we had teenagers at home decades ago I put a CB in each of the cars. Used 23-channel mobiles just to reduce the pain if one of them got stolen. Changed the channel 1-4 crystal so they all had the same 4 channels below channel 1, but "in between" on the zero, not the five. Figured it reduced the chance of getting stepped on by skip. Maybe it improved my chance I got heard at the other end. Didn't hurt.

73

 

[dss56]

I see the 9106 pll has its tr pin low for rx, hi for tx. The 2816 is reversed, hi for rx, low for tx. Still, I can't imagine it would matter as the mod just reverses what the radio tells the pin to do.

Thanks Cable Guy for the post wish there was a write up post or directions of the resistor values and the transistor and the switch wiring. Interesting just to try on a 25 and 29. thanks for the post.

 

[Cable Guy]

Thanks Cable Guy for the post wish there was a write up post or directions of the resistor values and the transistor and the switch wiring. Interesting just to try on a 25 and 29. thanks for the post.

Dss, I may try this on a 29 this week and if successful, I will write up a guide. With the 25gtl mod schematic to reference, and a schematic of the 29, it should be easy to apply this mod. Resistor values around the added transistor would be the same, just locate the t/r resistor going to the pll and a ~7 volt, unswitched power source, which a dmm would show easily. Hacking a dpdt switch or adding one. Then there are things to do around the reference xtal. I will check it out soon.

 

[dss56]

Cable Guy

Thanks for the update and hope you will have luck getting it to work and document the mod

 

[Cable Guy]

Cable Guy

Thanks for the update and hope you will have luck getting it to work and document the mod

Unfortunately, the 2816 won't take the t/r pin inversion and get what you want, at least I haven't seen it yet. Yes, inverting will get you the tx frequency, but the rx frequency is 910khz higher, 455khz ABOVE the standard channel. So, channel 1 regular 26.965, with pin inversion, TX would be 26.510 (what we want) and RX would be 27.420 (no good). I'm trying to figure out why. This chip can't divide the vco directly like the chip in the 25gtl can, so it triples 5.12Mhz (half of reference, from pll) and subtracts that from the vco to make the mixdown signal for the divider to compare. Interesting, the mixed down signal for channel 1 is .910 MHz or 910khz, the same difference between the tx/rx I see when the t/r pin is inverted. Digging in deeper now.