Looking for schematic for Cobra 19+ that uses a C5121 PLL IC

[enjoylife]

Does anyone have a schematic for a Cobra 19 plus that has the C5121 PLL IC, I have a couple radios I want to repair of this model but can't find a service manual or a schematic.

Thanks

[Sandbagger]

Jan 17, 2020 15:21:39 GMT -5 enjoylife said:

Does anyone have a schematic for a Cobra 19 plus that has the C5121 PLL IC, I have a couple radios I want to repair of this model but can't find a service manual or a schematic.

Thanks

CB tricks has schematics for a bunch of Cobra 19's, but none with a C5121 PLL. The 19+ there is an LC7131 PLL

[enjoylife]

From what I've read the late model Cobra 19+ used the C5121, I have 3 Cobra 19+ radios I bought at an auction for a little of nothing and they all have the 5121 PLL chip, its looking like I'll have to scrap them out as I have little luck without a schematic. I suspect the PLL IC might be bad in all 3 of them as they all act the same, no transmit or receive as the 10.24 osc is dead, I've replaced the crystal in one of them but it didn't help and trying to find a C5121 isn't an easy thing to do.

[Sandbagger]

Jan 17, 2020 19:13:17 GMT -5 enjoylife said:

From what I've read the late model Cobra 19+ used the C5121, I have 3 Cobra 19+ radios I bought at an auction for a little of nothing and they all have the 5121 PLL chip, its looking like I'll have to scrap them out as I have little luck without a schematic. I suspect the PLL IC might be bad in all 3 of them as they all act the same, no transmit or receive as the 10.24 osc is dead, I've replaced the crystal in one of them but it didn't help and trying to find a C5121 isn't an easy thing to do.

It's unusual for 3 radios to all have a bad PLL chip, as they typically don't fail on their own. I would suspect a bad cap or some other part that is killing the oscillator, or maybe it suffered from being worked on by a screwdriver tech. But finding a schematic for THAT exact radio design is the important first step. Otherwise, you're shooting blindly in the dark. Since it appears that Cobra changed the design of the 19+, it's going to be tough to find the correct schematic. The LC7131 PLL version evidently is the more popular version, but if you do a deep dive on the internet, you might be able to find your version service info.

[enjoylife]

I replaced the PLL IC, crystal and tuning capacitor that's on one leg of the crystal with parts that came from a 4th Radio I have that's identical that did work but had a broken front screen I'm parting out and that didn't help, I'm kind of new to CB radio repair, I've tinkered in electronics all my life but found these at a rummage sale so thought I would see if I could fix them..I have a schematic for a Midland 77-095 that uses a C5122 that is similar except for pin out. The PLL is unlocked as I assume its going to be without the 10.24 osc. I injected a 10.24 mhz signal into the RO pin on the PLL and I get noise through the speaker but by using a good CB on the same channel and transmitting I can't hear anything on the radio I'm working on so it must be picking up the wrong frequency.