Author Topic: Understanding how CB radio works  (Read 11362 times)

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Understanding how CB radio works
« on: January 19, 2011, 12:21:13 PM »
If any are interested I like to start a thread on understanding how CB radios work. I have picked up some free time now and it may be something to help those that want to learn. There are a lot of folks here full of technical knowledge and I think this would be a good tread to help others that want to know learn and understand what is going on in these little setís.  I see so many folks trying to troubleshoot radios and do not even know where on the board to find the problem let alone on the schematic.

What I like to do is start with a chassis like the cobra 29.
Post a schematic with the various stages marked.
Post a description of components and what they are. For instance;
TR 1 (RF AMP),
IC 3 (TX MIXER)
Ecx.

Just to give the guys that want to learn a bit of a heads up.
Then in the end discuss how to make your own test equipment like using the 29 as a signal generator


I have uploaded the modified schematic. This is not my work. It was given to me and I updated it.
Buddy kc4umo

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Understanding how CB radio works
« Reply #1 on: January 19, 2011, 12:22:10 PM »
Cool, Well we will get started with the basic stuff this week.

Some things are easy to fix. Say the radio transmits power but there is no modulation from the rig. You would start with the simple stuff like checking the mic. Or the radio meter moves on receive but you hear nothing coming out, you would check the speaker and the external speaker jack.

But what if the radio has no trans mit or no receive? Or worse yet has neither  ???  ???
Instead of going in and shot gunning the problem it would most likely be easier to find out what is working. But to do this you will need a few pieces of test gear to do so. a good DVV, frequency counter, watt meter dummy load, and a test radio. And dont forget the test equipment you were born with.
Your senses.  
Do you see anything out of the ordinary like burnt traces, blown components?
Do you smell anything burnt?
Do you hear anything when you transmit to or from the test rig?
If you touch components do they feel hot?
Do you taste ..opps, we want go there :)

See, a lot of times with just using what you have the problem may stand out in front of you.
When a radio comes in that does nothing but light up, the first thing I do is check the 10.240 mhz crystal oscillator. Lots of time I fine the problem there. If you find no 10.240 check the solder connection to the crystal.  Failure of the 10.240 crystal does happen often. If all good there go to IC 4 pin 4. This is the Audio amp. pin 4 is input and pin 9 is output. With a metal probe touch pin 4. If all is good you will hear a loud roar you can expect  the audio chip is good. If the chip is bad it can short the voltage to ground and stop many circuits from running. The voltage to the TA7222 is on pin 10.
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Understanding how CB radio works
« Reply #2 on: January 19, 2011, 12:23:23 PM »
On the bench at this moment is a Uniden PC 66XL. This is the same radio as the Cobra 25. I still use the basic 29 schematic theme when checking these radios.

This radio has the no TX or RX problem. I am not just talking about audio, but there is no detectable 27 mhz signal on TX or RX.  This is when your test radio comes in handy. Key the radio under test with the test rig set to the same channel. Do you hear a slight signal? Now key the test radio and listen to the rig under test, do you hear anything? does the meter move?


First test was at pin 2 of the PLL chip. I used a frequency counter to check the 10.240.  And there is was. Good
Next test was at pin 4 of the audio chip with a metal probe. Yep, plenty of noise.
Next I used my frequency counter to check TP 3. Set channel selector to 19.  There should be a 16.270 mhz signal there. Nope, counter reads zero. Now lets go to pin 10 of the PLL chip. This should give a frequency of 5.140. That is half of the 10.240...The PLL chip has a diver circuit that produces this signal so it is internal to the chip. As I test this pin I find no 5.140 mhz signal present. With this condition there will be no 16.270 mhz at TP 3 to deliver 1st mixer energy to the RX mixer.  And since the signal is not present at TP 3 or pin 10 of the PLL, then we determined that the PLL chip is bad.

This took about 4 minutes to check after the radio covers were removed and power was applied.
This is why I say it is easier to check what is working in a radio instead of spending hours looking for  the faulty component first.
Buddy kc4umo

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Understanding how CB radio works
« Reply #3 on: January 19, 2011, 12:23:58 PM »
And Greetings to you  Handy Andy. I am honored that you chimed in. And please do so when ever you can.  

I agree on what you said there. Equipment is very expensive. And when on a budget, make the best of what you have.

 I am very old school myself.  Not very educated at that. So you want hear me using a lot of big words    Back in the day my work bench consist of a analog volt meter, (have to check those capacitors and transistors) soldering iron, and a SSB 23 channel radio as a test rig.  I took the SSB rig to a well known cb shop and had it tuned and aligned.  When tuning a am rig I switch the test radio to ssb and zero beat the signal to get the thing on frequency. That was the only thing I had back then to use as a frequency standard. I later converted and old RCA co-pilot (PLL 02A) to a signal generator.

Times have really changed since then. I will post a pic of my work bench again one day. But well after it is cleaned up.  Now my benches contain (3) B&K 2040 signal generators, Sencore CB 42, IFR 1200S service monitor, Motorola frequency/ auto tune deviation tone test rig, B&K 1801 and Heath 2410 frequency counters, Bell & howell scope, Kenwood SM 22- station monitor, and a Sencore SC61 scope. HP 3400RMS volt meter plus countless more pieces of test equipment.

But, all this is not needed for CB repair. I also repair business band radio and ham radio.  Anyone can become a decent repair tech if you stick with the basics, ei find what does work. Process by elimination.

Anyway, I am just an old country boy guys. Took a look in my first Cb radio when I was  12. I will do my best to work along with you guys to make this thread a learning experience.
Buddy kc4umo

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« Reply #4 on: January 19, 2011, 12:24:28 PM »
The Dead Receiver

Over the years I have had several techs call me trying to figure out a simple problem. They tell me the radio transmits fine. Has full power and audio out. But the receiver is dead as a door nail. No RX audio, no deflection of the S-meter.  One tech brought 2 radios over for me to look at. Sure enough, both had dead receivers. After 5 minutes a piece, both radio\'s were working fine and the receivers was singing away. Both radios were Cobra units. One rig was used in a pick up by a deer hunter the other was used in a big rig.  Both units had the same symptoms but different problems. one the speaker jack had cracked loose from the main board. The other had a cracked solder joint in the RX section.

Both units had non working S-meters. This is what I call the hibernating receiver.
Get you a known working S-meter from an old radio. Attach red and black wires with alligator clips. Then when testing a radio just clip the S-meter in. This may save you many hours of hair pulling.
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« Reply #5 on: January 19, 2011, 12:24:53 PM »
Thanks Klondike Mike.

In the next few post I want to start with a new subject. Getting to know you way around the radio.  I will take a pic of the inside of a Cobra 29 and break it down into 4 quadrants.
PLL section
Transmit mixer
Receive and output
Audio section.

There are actually 5 sections. The fifth section which is the middle of the radio contains  IF filters, the AM  demodulator and the squelch circuits. But we will leave this section for a later time.

Once you know your way around the radio this makes repairs a lot easier.
Buddy kc4umo

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Understanding how CB radio works
« Reply #6 on: January 19, 2011, 12:25:30 PM »
Ok, back to Getting to know you way around the radio

Please excuse the crude photo I am attaching.
As you can see I have divided the radio into 4 main groups. Remember, some sections do over lap with other sections. But that is fine for now. Mostly all radios today are built under this design. You will see a few that is a little different. But when you get the understanding of where things are in the Cobra 29 you will easily be able to identify other boards. In my first post is a schematic with certain area\'s labeled. Take the time to open that schematic and find the labeled sections on the schematic. That will give you a better understanding as to where the parts are in relation to the schematic.

PLL
In this area you will find the PLL chip and it\'s supporting components, 10.240 reference oscillator, The VCO and supporting components. You will also find the 5 volt regulator in this area.

Output and TX mixing
In this area you will find the begining of the transmit stage, the transmit mixer, predriver (buffer), driver , and the final

Audio and modulator
In this area you will find the audio stages, the AMC stages and modulation limiting circuits, voltage regulators for TX & RX functions..

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Understanding how CB radio works
« Reply #7 on: January 19, 2011, 12:26:02 PM »
What are all these components ??

I know when you are first starting out you look in a radio and say wow. These things are packed full of stuff. Let me see if I can help you out a bit.

TR 1- RF AMP                               TR 13- Squelch Clamp
TR 2- Buffer AMP                          TR 14- Final Amp
TR 3- Squaring AMP                      TR 15- Driver AMP
TR 4- Pulse AMP                           TR 16- PreDriver
TR 5- Pulse clamp                         TR 17- Mic audio AMP
TR 6- RF Gain Clamp                    TR 18- AMC Clamp
TR 7- RF AMP                               TR 19- ANT Ind switch
TR 8- 2nd IF AMP 1                       TR 20- REC/2nd IF osc
TR 9- 2nd IF AMP 2                       TR 21- TX supply switch
TR 10- 2nd IF AMP 3                     TR 22- Dimmer switch
TR 11- Squelch inverter                TR 23- Common 8 volt regulator
TR 12- Squelch AMP                     TR 24- ANT Ind inverter

FET 1- First RX mixer                    FET 2- Second RX mixer

IC 1- PLL chip
IC 2- VCO/Mixer
IC 3- TX Mixer
IC 4- Audio output / modulator

D1 & 2- Protection Diodes / Front end
D4- AM Detector/ AGC
FL1- First fliter
FL2- Second filter
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« Reply #8 on: January 19, 2011, 12:26:30 PM »
Tonight I am looking at showing you all how to convert a cobra 29 into a stable signal generator.
I will put what I can together and try to get this layed out. The generator will need to be 50 ohms, and produce a clean 100uv signal. This will be unmodulated. You will also have the option to drop the 100uv at 10 increment levels. this will be done with a simple divide by 10 network of resistors.

What we need.
A clean stable 100 microvolt signal. ( 100 uv is enough to set a S-meter to S9)
A way to drop the signal for testing the sensitivity and alignment of the receiver. (peak the stages)

The idea of the signal generator is that when you have a radio on nearby, and you turn the generator on with out being connected to the radio you want to test, you should not hear anything coming from the generator. This is going to take a bit of work and a lot of shielding to stop this from happening. The one I built years ago from the RCA co-pilot was mounted inside a steel chassis. The tuned circuits you build will also have to have there on fabricated boxes. These can be made of very thin tin sheets. As long as you can bed it and solder to it. No aluminum. You can also fabricate the boxes from double sided circuit board. This is easy to work with and has good sheilding.

So how do we start?
Looking at the cobra 29 schematic find the TX mixer. This is IC 3. Pin 9 of the mixer chip is your rf out. This goes to L20, the first tuned circuit in the output stage. From L20 we go to R63 then to C65. This is as far as we need in the circuit. Everything from the antenna side of C65 towards the antenna is useless. This is the section we will pick our signal off at and convert it to a usable signal.

At this point in the radio we are going to have around a 1000 ohms impeadance @ about 1 milivolt. As you can see we have to go a long way to get this to 50 ohms @ 100 microvolts (uv). The idea is to get this signal through a cable and to the radio under test and not through residual radiation from a poorly built leaky generator. So what we will do is take our 1 milivolt 1000 ohm signal and force it to 50 ohms @ 500 uv via a 1/2 divider network. To do this we need 3 47 ohm 1/4 watt resistors, 2 100 ohm 1/4 watt resitors.

If anyone see\'s error in what I am saying please jump in and let me know..
More later
Buddy kc4umo

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« Reply #9 on: January 19, 2011, 12:26:55 PM »
Ok,
Here is out network to complete the output stage of the generator. As you can see the divide by 10 stages can daisy chain from each other. Each section must be in it\'s own sealed box. All componet leads must be as short as possible. You can use the small teflon coated coaxial cable to connect is stage. Do this by drilling a small hole to pass the coax through then fold the braid back and solder to the enclosure.
Remember, ever component in this stage radiates RF.

I suggest finding and enclosure that will house the radio and other component. Also to house future upgrades that can be added. I would even reinstall the original covers on the radio. Everything must be grounded and sealed. So get you some aluminum tape.
More later

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« Reply #10 on: January 19, 2011, 12:27:25 PM »
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
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« Reply #11 on: January 19, 2011, 12:28:28 PM »
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.


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« Reply #12 on: January 19, 2011, 12:29:42 PM »
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?”
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« Reply #13 on: January 19, 2011, 12:30:08 PM »
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.
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« Reply #14 on: January 19, 2011, 12:30:33 PM »
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
Buddy kc4umo