NEW AT THIS, Learning, Where to inject 455Khz into a Cobra 29 LTD ,and or injection itself for diagnosing???

Thread Starter

baconater4749

Joined Dec 17, 2020
23
I am new and learning to align and repair CB Radio. One question I have is where do I inject a 455Khz signal into a transceiver, Cobra 29 LTD to see if I get receive back. I do have a Oscilloscope, Signal Generator a Function Generator, Frequency Counter. I am a bit embarrassed to ask this but I do not want to blow up anything or get zapped. Looking at the Schematics, what do I look for to inject a signal. I inject a signal in the antenna port for aligning the radio, but not sure where to hook up for signal injection, do I use a C=Ceramic Capacitor that is in the beginning of a circuit? Is there any information that I could read to understand how to do this, or is it in front of my eyes. Thank you in advance for any help repairing this radio,ready to work, Thank you, Mark
 

Thread Starter

baconater4749

Joined Dec 17, 2020
23
The schematic diagrram http://www.cbtricks.com/radios/cobra/29ltd_29ltd_st_29wx_st/graphics/cobra_29ltd-st_29ltd_sch.pdf

Because it is a double conversion the Intermediate frequencies are: 1st: 10.695 MHz, 2nd: 455 KHz

video shows the 455 KHz slug tuned shielded inductors.
Thank you., what my issue is, where or what electrical part do I hook the tone to? I understand how to search for a signal, but not sure where to input signal in that stage. Now, do I turn the radio on or search without having power, or do I keep the juice on to be able to hear the tone change?
 

Thread Starter

baconater4749

Joined Dec 17, 2020
23
I would not touch the tuning slugs. It would be a bugger to align properly.
Why do you want to inject 455kHz?
Hello, diagnosing a no TX or RX problem, as someone already played with the slugs, I did the alignment, as I am learning. I have my 10.240 and 10.695, just want to inject the 455Khz and see if I would get TX OR RX, make sure the 455Khz is mixing? I have the tools, learning on how to use the Oscilloscope, Function Generator,Signal Generator also, Ans I have a tinySA. Signal Analyzer, which may take me a while to learn, but I want to learn CB repairs for a hobby. I worked on cars for 30 years, the back and neck are all rotted and fused so I needed to find something less back braking and mellow during the National Grounding from the government, so I am a Ham, and I figured this would be cool to learn, and a mellow job for me to do, but this is a lot of learning,reading,and interesting, just need some hands on training, so I could get the picture. So, yes, I did move the slugs, and now I have a light hissing out of theCobra 29 LTD. This is where I am at, I have a Signal Tracer, but I am waiting for Amazon to drop off a speaker for it tomorrow, I want to see if that will help me out any..Talk soon ,Mark
 

MrChips

Joined Oct 2, 2009
24,200
Hello, diagnosing a no TX or RX problem, as someone already played with the slugs, I did the alignment, as I am learning. I have my 10.240 and 10.695, just want to inject the 455Khz and see if I would get TX OR RX, make sure the 455Khz is mixing? I have the tools, learning on how to use the Oscilloscope, Function Generator,Signal Generator also, Ans I have a tinySA. Signal Analyzer, which may take me a while to learn, but I want to learn CB repairs for a hobby. I worked on cars for 30 years, the back and neck are all rotted and fused so I needed to find something less back braking and mellow during the National Grounding from the government, so I am a Ham, and I figured this would be cool to learn, and a mellow job for me to do, but this is a lot of learning,reading,and interesting, just need some hands on training, so I could get the picture. So, yes, I did move the slugs, and now I have a light hissing out of theCobra 29 LTD. This is where I am at, I have a Signal Tracer, but I am waiting for Amazon to drop off a speaker for it tomorrow, I want to see if that will help me out any..Talk soon ,Mark
Fair enough. I think that you should tackle one problem at a time, TX, RX. I wouldn't think that TX and RX are related.
Hissing, isn't that what Cobras do? :)
 

sparky 1

Joined Nov 3, 2018
579
I agree that a non- invasive approach is better not not adjusting anything. I also agree breaking it down as a receiver
you can start with carrier and voice coming in at the antenna on the schematic rather than tackle the frequency mixers come back to that later for transmit.

The video was posted because it clearly indentifies the IF cans. Becoming familiar with the circuit and all it's parts.
The schematic can be explored getting familiar with all the parts on the circuit board may need more experience.
The IF can is where 455 kHz should be resonating when powered on. It does not take much to blow out a live board.
Usually the microphone cable is twisted and has an open and needs surgery. Probing with a multimeter might find
that a power mic needs a battery.
 

sparky 1

Joined Nov 3, 2018
579
Having high SWR has been known to damage CB. A repair can start at audio.
video on diagnosing a CB for audio
 
Last edited:

Thread Starter

baconater4749

Joined Dec 17, 2020
23
Hello,I am looking at a crystal issue,I am having problems getting 10.240mhz in my first stage of diagnosing a receive on a Cobra 25 LTD,I am pretty sure this 10.240 MHz crystal is out of wack,I cannot get the crystal to change frequencies. I am sure I need a 10. 240 MHz crystal. Does any body have a store,electronic store that carries these MHz crystals? I checked on the solder side of the board,and still stuck on 10.560 MHz, also the PLL is reading the same out of wack frequency,and will not adjust. In advance,I appreciate any ideas or a store for a crystal I can purchase. Non at this time on eBay,if I can trust the seller that the crystals are good,and were not thrown or dropped,I hear this will screw the crystal up. Thanks in advance to all who may help with a place to purchase a crystal,a,reliable electronic store. Thanks Mark,Cedar Lake,IN.
 

Thread Starter

baconater4749

Joined Dec 17, 2020
23
I would suspect the PLL synthesizer circuit where the frequencies are generated if no Tx or Rx.
This is a late response as I am back on this CB project,it is a 10 240 MHz crystal problem,the crystal will not adjust,I believe I need a crystal before proceeding with the alignment,the thing will not adjust to my begining frequency,stuck on 10.556 MHZ,I have to start with a crystal,the PLL is reading the same as a the crystal that is in the Cobra 25 LTD,this is where I think I should start at, Mark
 

Thread Starter

baconater4749

Joined Dec 17, 2020
23
This is a late response as I am back on this CB project,it is a 10 240 MHz crystal problem,the crystal will not adjust,I believe I need a crystal before proceeding with the alignment,the thing will not adjust to my begining frequency,stuck on 10.556 MHZ,I have to start with a crystal,the PLL is reading the same as a the crystal that is in the Cobra 25 LTD,this is where I think I should start at, Mark
I put a PLL in the radio,reading the same as the old one,every voltage reading is the same as the one I took out.The PLL was $9.00,did not break the bank for a starting guess,
 
Top