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
7
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
7
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
7
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
23,070
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
488
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.
 
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