2 cobra 29 ltd classics, modulation and carriers tank

Hello to those who read this! I'm fairly new to the cb scene and this is my first post here so forgive me if im ignorant. I tried searching for my issue but couldn't seem to find a solution.

Simple description: no mod after d8, no carrier on pin 9 of mixer.

I have two cobra 29 ltd classics that are exhibiting the same problems. One is Philippines and the other is China. The first was receiving and transmitting great until i received the new meter for it today and installed it. Now it seems to receive on the wrong frequency with no transmit and no power out. The second i got to receive but I may have messed that up in the first place though i thought it was working when i got it. It has no transmit no power out.

I have an oscope and can trace voice modulation all the way to the other side of D8 where it disappears. I get good frequency waveforms all the way to pins 1 and 4 of the tx mixer. when i key up, i get 27 mhz out of pin 7 up to R102 heading to the 2816 chip but loose it on the other side. i do not get it out of pin 9 were i believe it's supposed to feed out to the tr16 and on.

What I've done on the Chinese: I changed the final driver and the final because they were both bad. Split down the middle, I've recapped the left side looking at the component side to get the waveforms to stay at pins 1 and 4. I was playing with l19 because i noticed it was receiving but my frequency was off according the schematic so i dialed it in where it at least picks up my other radio just fine and receives on channel. This is why i think the other is receiving just on the wrong freq. I also changed C63, 64, 65, 111, and 110. I've spent two days poking and prodding and tracing to find the above. Although one final driver brought a Uniden 78 back to life for me, I was skeptical about them being good, so I changed this one again to find nothing different. After a bit, I unsoldered the legs on the final and final driver and probed just to make sure the two wasn't bad and sinking my signals. I even jumped pins 7 and 9 to see if i could just hop it on over but no. I've checked D8 and even lifted the one leg to make sure my signal was flowing through it. It was and when I resolder, gone.

The Philippines: Nothing really except fix a cold solder joint in the added echo board to get it working again. I recapped all the electrolyte caps for fun and changed all the pots on the front cause someone glued the knobs on and I broke several of the pots getting the knobs off. The unit was working fine afterwards, and I talked on it for hours one night and even used it to check my power/swr meter a different night because I found that my personal cobra loses power on the upper side of 20 on the high side of the expo kit. The meter in the cobra was bad, and I changed that out today happy to be done with it and wow, suddenly no receive or transmit. Checked out a couple pin spots and got irritated because it suddenly has the same problem so gave up for tonight.

Since both seem to have the same problem, I was hoping this was a common issue someone could point me to the solution. I suspect I may have killed the mixers somehow and was going to swap the one out of this working Uniden just to see, but that wouldn't explain (to me) why the modulation tanks.

Thanks for any help!

brandon7861 said:

Did the final/driver going bad also take out their bias resistors or L17?

Whats the voltage on pin 15 of the PLL?

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I'll double check em when I take lunch but I don't remember any burnt resisters. And that was only in the one. How can I check l17 without an LC meter.?

I usually unhook the speaker and hook an external up when testing, would accidentally grounding the speaker wire cause anything like this?

brandon7861 said:

Just make sure the secondary to the driver has continuity. I was just worried about a bad transistor putting DC through it and burning out the coil.

Click to expand...

ok so it would appear that on the china version (ill call radio-c)that i have gotten to receive, the #16 pin, or vcc according to the radiomods website is just short of 5volts, L17 seems to be intact and resistors around the driver and final test ok.

on radio-p, i do not have any voltage at that pin, which i assume is why it quit working. L17 and all resistors appear to test ok.

brandon7861 said:

Pin 15, not 16.

Click to expand...

my apologies, it's also just short of 5v. 4.89 volts is what my meter is reading.

So on radio-c, i traced my 5 volt all the way through r92 where i loose it on the pin 16 trace. took the time to change r92 but it wasn't bad and i checked voltage with the cap there out because it was one i had already changed but still nothing. Does this mean the pll is bad or is there something else that could be killing it? my crystal is dead on 10.240mhz according to my oscope.

Also, I can't help but notice that my 13volts jumps up 20+ on radio-p when i key up and will then settle back down after unkey. somehow the driver was burnt so i replaced it with a known good one out of another radio but still no transmit.

To answer my own question, no it does not mean it's bad. swaped it for a good one and put the other in the doner radio to find it is fine. Also my mixer is too.

so to answer my questions even further for anyone who uses copilot (as asking it questions it immediately quoted this thread back to me), they were both just out of tune. Aligning L22 brought radio-p to lock and spring back so that I could tune the rest even further. I couldn't get the full range of the expo kit to tune. It would either cut half the top or bottom bands but i managed to get it to just role off at the top and bottom at just above half a watt.

with my scope hooked up i was able to tune L20 and L21 on radio-c to bring it back to life, but now im hunting down an oscillation issue that recapping didn't help.