Cobra 29 LTD Classic & Chrome FM Radios are Here

[CbRadiosPRONTO]

Cobra 29 LTD Classic & Chrome FM Radios are Here

Well, that’s a little anticlimactic. It’s the same radio, feature wise, except FM. Push the channel selector for FM.

Hope it’s ok, I’m quoting NZ8N from another post about the 29LX

——————
NZ8N [begin quote]
The PLL in this latest generation of Cobra 29LX and the new 29 AM/FM models are serial programmed, this particular PLL outputs the VCO frequencies direct.

Here are FCC submittal pictures for the new AM/FM Cobra 29 radios out now. This also uses the same PLL/VCO daughterboard as what the 2nd Gen 29LX now uses.

https://fccid.io/BBO29LT01/Internal-Photos/Internal-Photos-6305096

On Page 11, shows what is under the soldered down metal lid of the PLL/VCO daughterboard, a Beken BK4802P, which is a repurposed FM/ASK chip.

https://www.pi4raz.nl/download/BK4802N_Datasheet_V1.1_EN.pdf
[end quote]

 

[NZ8N]

Another thing I noticed, Case Ground is no longer capacitively coupled to Board Ground, it is now direct. So, Negative Ground Only.


This is from the C29 Classic AM/FM I got last weekend.

 

[NZ8N]

When comparing the new AM/FM radio to earlier model schematics, other than the PLL daughterboard section, and FM daughterboard section, the rest match up (mostly) to the same on the schematics as, say, the 29 LTD Classic. C65 (.01uF or 103) feeding the TX buffer TR16 and most of the rest of the TX and RX, TX Switch TR21, etc... match up.

I did verify that the yellow wire going to the Ch9 switch to the Ch9 pad on the CPU board, when moved to the Ch19 pad next to the Ch9 pad, does make the radio go directly to Ch19, so that would be a good idea for the truckers radios if they wanted instant Ch19. The faceplate won't say Ch19 above this switch, but it works.

The PLL outputs 37.660 MHz on Channel 1 in RX, and 38.100 MHz on Ch40. TX frequencies are direct to 26.965 (Ch1) and 27.405 (Ch40). Both RX and TX "samples" are present at C65 (input to TX Buffer) and JP39 (14mm) which feeds R17 then FET1 (RX 1st Mixer).

 

[Techno1]

Does it look like they are using a crystal oscillator programmable for the mixer frequency. I dont see that area on this new board

 

[NZ8N]

There is no mixer frequency. No offset like older 29's.
These use a direct PLL signal of 37/38 MHz to the RX mixer, and direct 27 MHz to the TX buffer stage.

 

[Techno1]

So it looks like no extra channel mods for the newer 29's

 

[NZ8N]

I have a couple of ideas.
Stay tuned....

 

[Crossbow]

Looking at your pictures I see something new VR7 what is that controlling?

 

[NZ8N]

VR7 is FM Deviation.

 

[Shadetree Mechanic]

VR7 is FM Deviation.

Does the deviation adjustment effect the rx or is it just tx?

 

[groundwire]

Does the deviation adjustment effect the rx or is it just tx?

fm deviation trimmer adjusts the tx deviation. adjust for 5khz, 2.5khz narrow band is lame

 

[BlackPearlRadio557]

Looks like a 5351b version might do the trick.

 

[Enterprise312ok]

fm deviation trimmer adjusts the tx deviation. adjust for 5khz, 2.5khz narrow band is lame

I am confident fm cb will catch on, there may not be any forward swing but you can turn up the audio.

 

[Shadetree Mechanic]

 

[BlackPearlRadio557]

Any traction on frequency modifications?