The CB Forum > How CB radio works
Understanding how CB radio works
The Radio Shop:
Ok, Back to the radio...
No Receive.
Each question will be followed by a NO and YES answer.
Is there hash in the speaker?
NO Run you finger over the audio chip or do the metal probe on pin 4. If still no noise check pin 10 for 13.10 volts. You can check rest of the pins and make sure the voltages are as followed:
pin 1- 13.7 pin 2- 6.63 pin 5- 1.14 pin 9-6.86 pin 10-13.10 Rest of the pins should be at zero. For some reason you vave the same voltage on many pins look for a bad ground. If you have 13 volts (supply voltage) on 3 or more pins most likly the chip is bad. If there is no sound at all and all the above is good then make sure the mic is wired correctly. If ok then check ffrom the output of the chip towards the speaker. work your way towards the audio transformer and check C46. This is a coupling cap. Now work you way through the PA switch and the external speaker jack and then to the speaker. Make sure you check all caps along the way. These cap isolate DC from the audio line.
YES
Is the Receive indicator light on?
NO The receive LED is fed from R113. Check the 8 volt receive supply regulator TR23. Check C119. It is underated and fails often. Replace it with a higher voltage cap. Check the zener diode D22. It has a high failuer rate also. If D22 or C119 fails this will remove bias from TR23. That is all the voltage rail for the Receive supply.
YES
Does pin 11 of IC 1 have 5 volts?
NO Check D19. There should be 5 volts on it. If R124 is warm then most likely C112 has shorted. If the voltage is low check C97.
YES
Is the 10.240 oscilator working?
NO There are a lot of components in this circuit. We know the voltages are ok because of the checks that got us this far. Either the crystal is bad or there is a bad solder joint. D6 is a varactor diode. This is a diode capacitor combination. If the internal capacitance changes the circuit will not oscilate.
YES
More later
The Radio Shop:
Testing components is a very important part in radio repair. Test equipment can get a bit pricey. As LC mentioned you can check transistors and diodes with a simple analog volt meter or a digital volt meter. A lot of times you will have to pull the device out of circuit to test it. I use an old RCA transistor tester to see gain. But most of the times it is my old analog meter I check with. I will also show you a simple way to build a transistor tester. With only a handfull of parts.
9 volt battery and clip.
68 ohm resistor
56k ohm resistor
led
N.O. push button switch
Red test clip
Green test clip
Black test clip.
Small case to house the components
You should still be able to find most of these parts at your local radio shack.
This is a good way for quick checks on transistors.
The Radio Shop:
The Receiver
This looks so complicated. And over whelm the new person trying to read the schematic. So lets block this together to make it a bit easier.
Let’s get to know what is happening in the AM receiver of this Cobra 29.
Go here and look at the block diagram:
http://www.cbtricks.com/radios/cobra/29ltd_29ltd_st_29wx_st/graphics/29ltd-st_29wx-st_sm_pg1_pg11.pdf
RF signals are picked up by your antenna, converted into energy and sent to the so-239 on the back of you radio. We will call these 26.965 to 27.405 mhz signal. This signal is brought to TR 7. This is your RF amp. This amplifies the signal to make it more usable. The signal is then fed into FET 1. This is the first mixer. Lets say you radio is tune to channel 1. The frequency is 26.965 Mhz.. At the same time the PLL (IC 1) with the respect of the channel selected, IC 2 the VCO mixer sends the signal of 16.270 Mhz to FET 1 also. The signal produced at IC 2 will be from 16.270 to 16.710 (1 – 40).
The signal of 16.270 is then subtracted from the 26.965 Mhz signal yet producing our 10.695 Mhz signal.
26.965
-16.270
=10.695
From there the signal passes through the 10.965 Mhz filter (FL1) and delivered to FET 2. This is our second mixer. At this point we are getting a 10.240 Mhz signal from TR20 (10.240 OSC) This is mixed with the 10.695 Mhz signal
10.695
-10.240
= .455 Khz
The signal is then passed through the 455 Khz filter (FL2) and then sent to the second IF amplifiers, TR8, TR 9, TR 10. After the amplification the signal is passed through D4. This is our detector. Its purpose is to demodulate the signal for speech signal recovery. The recovered speech signal is sent through the volume control, to IC 4 (audio chip) and finally to the speaker. Where we plainly hear “Breaker 19, anyone got yer ears on comone?”
The Radio Shop:
The transmitter.
TR 20 (OSC) sends a 10.240 Mhz (Reference freq) signal to IC 3 (TX mixer). With the channel selector set to channel 1 a set of frequencies of 16.725 to 17.165 Mhz (fundamental freq) is also sent to IC 3. We now have our basic transmit frequency.
If we add:
16.725
+10.240
=26.965 Channel 1)
The signal is then passed to TR 16 (Buffer amp) TR 15 (Driver amp) and then to TR 14 (Final amp) From here it is sent to the antenna.
TX audio
The speech signal is pick up by the mic and carried to TR 17 (mic amp). This amplifies the speech signal and passes it off to IC 4 (audio chip). From there the signal is applied to the collectors of TR 15 and TR 16 for RF amplituse modulation.
The Radio Shop:
Now, what to do now eh?
You are probably thinking "thats not much to go on, you left out half the radio".
Well, yes and no.
This is the basic workings of a receiver and transmitter. If you will take the block diagram, the schematic, and the above info, you will see a whole bigger picture. But we are only starting with the basic radio.
It\'s the difference in knowing how to fix a radio or being a parts changer by the "shotgun approach". What is the shotgun approach? It is going in and changing parts without forst determining what is and what is not working. A lot of times you get lucky and replace the bad part. Most of the time you end up throwing the rig to the side.
The big three.
Radio may light up.
But has no transmit and no receive. That is what we call a dead radio.
Radio transmits but has no receive, Dead receiver.
Radio receives but has no transmit. Dead transmitter.
By using the above info you should be able to determine the problem in a matter of minutes. By process of ellimination.
Find the stages that are working. Then elliminate each one.
Again, this is where your test radio comes in handy.
Dead radio. Do you hear anything in the test radio when transmiting? Or hear anything in the receiver while transmitting on the test rig?
The most possible cause of a Dead radio is the PLL and voltage rail.
The dead receiver or dead transmiter can also be caused by a missing voltage but maily one of the mixers or OSC not working.
Weak receive can possibly be TR & bad or the protestion diodes just ahead of TR 7 bad. This happens when the front end is over loaded by strong signals such as some one keying a big amp up. Even static electricity on the antenna can take these devices out.
No output but can hear a 27 mhz signal in a test radio can tell you the final, driver , or prediver is bad. A completly dead transmitter can be as simple as the TX mixer has gone bad.
Other circuits we will discuss is :
RF Gain
Squelch
NB/ANL
AGC
Audio Circuit
Phase Lock Loop
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