Hands-on Electronics – Signal tracing a simple transmitter
Hands-on Electronics – Signal tracing a simple transmitter
Or, How not to be an appliance operator
Is ham radio still about electronics? If you think so, then this posting is for you.
You may want to read our nearly related article on the Heathkit radios (Collecting Heathkit Models SB-101,102 & HW 100,101 ). That posting contains a high level conceptual explanation of how the transmit section of those radios work. We think that the Heathkit folks missed an opportunity in the education market.
Heathkit had a great platform in the SB and HW series of transceivers to serve as a course in electronics – learn as you build. But that was not to be. Perhaps an objection is that there were high voltages in the Heathkits – 800 volts on the plates of the tubes. So, sticking fingers in the wrong place in a Heathkit is going to get you zapped. So maybe that would undermine ones joy of learning.
A learning strategy
If the goal is to learn about electronics there are many ways to go about it. Too much theory and not enough practice is sub optimal. Too much hands-on, and not having a clue as to what you are doing from a theory and design perspective is just as bad. A good balance of theory and practice might be the best way to get you there.
So, here’s the pitch. Get yourself a simple working radio, match it with theory and design models, and have at it. That is, get your fingers in there and see what’s going on. Read the theory, see how this is applied to a design in practice in a real radio, and then inspect a working example.
From there you should be able to use what you have learned in troubleshooting a non-working (simple non-working) radio and see if you can apply what you know to get it working.
And from there, use what you know to make changes to existing circuits to fit your needs. And from there you are on your way to designing from scratch or modifying an existing design.
This is one view of the progression of learning. You may have your own strategy.
Choosing a representative transmitter (or, finding a victim)
So, what do you start with? You need a simple radio where there are a limited number of things going on. You want a simple working model which does not make things overly complex which frustrates learning. Oh, and you need the radio to be cheap; if you break it it should not really matter, and you don’t want to play with a tube rig ( like the Heathkit ) which can bite you with 800 volts if you put your fingers in the wrong place.
The poor beleaguered CB radio
Old CB radios are easy to come by. You can sometimes find a guy at a hamfest with a box of CB’s that he is willing to part with for $5 to $10 each. “Does it work? Yes, the last time I used it” – 40 years ago. If you don’t want to wait for a hamfest then go on eBay. If you have a CB sitting in the basement then you might go for that.
To be the least frustrating learning experience you should start with a known working radio. Even if you had to buy one that works from eBay they can be had for $30 or so – your mileage may vary.
Meet the Guest Radio – PACE CB144 (uh, the victim)
We dug up a bunch of old working CB radios. The one we picked for this posting is an old 23 channel CB radio from the 1970’s. Why did we pick this radio?
- Its crystal-controlled. That is, no IC’s so you can see the electronics in the raw. No mysterious black boxes (chips) for this posting. This radio has all discrete components.
- It was available.
- It works – mostly – enough for this posting
- We could get the full schematics for the radio
- We could get the service manual for the radio
The latter is important. Without the schematics and service manual you are not going to have a pleasant time. So we don’t recommend you do our “hands on” with a radio for which you do not have the schematics and service manual.
For CB tinkering, there is a great archive site of 1,000’s of service manuals and schematics at
Meet the test equipment
- Frequency Counter
And these items come in handy
- Magnifying glass
- Good lighting
- Alligator clips
- Dummy Load
- A large workspace
- Limited interruptions
- Nylon tuning tool (if you want to mess with the alignment)
I suggest that you enlarge the schematic so you can easily see it and mark it up. We assume that you have the service manual as a JPEG or PDF for the radio you are working on. If you have it as a JPEG then you can use Microsoft Paint (standard as part of windows) or the image processing program of your choice to enlarge the schematic. I made mine 6 pages (3×2) and taped it together so I could easily see it all at once and mark it up.
Print the manual if you don’t already have it as hardcopy. You will want to mark it up with notes. Keep a notebook. If you are going to do a learning exercise then write things down – for yourself or to share with others so they can learn from what you found out. If you have open questions – then jot those down as well so you can follow-up later.
Getting to know the radio
Most service manuals have an introduction that gives you a brief overview of all the sections of the radio. Generally they start with a block diagram to show you how everything is related. Then, depending on the manufacturer, the manual may give you a detailed description of each of the functional blocks. Once you understand the functional blocks you can map this to the schematic. And, from the schematic to the parts in the radio.
Other parts of the manual you will need are:
- Board layout. This will show you how to map schematic to the parts layout on the board and identify any Test Points that the manufacturer has supplied.
- Board X-ray. This is useful as it will show you the board traces and the parts in one view. Its like you are able to “see through the board” – like an X-ray. You will need this to get to test points that are not specifically called out on the board by the manufacturer. If you don’t have this, and the board is thin, you can hold the board up to a very strong light and see the “X-ray” – components and traces from the solder side so you can identify test points of your choice.
Hands-on Tracing the signal in a simple transmitter
- Read the conceptual section in the service manual
- Mark out the relevant section in the schematic
- Using the component layout in the service manual, find the components of interest on the board.
- Identify Test Points identified by the vendor in the service manual – or your own.
- Check the service manual alignment procedure for the section in question and be prepared to validate/inspect or make a change.
- Follow the signal front to back checking/inspecting the signal as it gets processed in each functional block. If a functional block test fails, be prepared to subdivide the block into smaller chunks and do sub testing.
Conceptual overview of the transmit section
For the PACE CB144 23-channel radio these are the functional blocks in the transmit section
- Master Oscillator
- Transmitter Oscillator
- Transmitter Mixer
- RF PreDriver
- RF Intermediate Power Amp
- PI Output Filter into Antenna Jack
In an AM radio like CB the modulator is a separate and distinct section. For this “hands-on” we’ll just trace how the carrier is produced. At the end we will show how the modulation of the carrier takes place – but this is not the subject of this posting.
Physical aspects of the PACE CB144
Old School, no chips. This radio is “old school” (but not as old school as “valves”) and this is why we picked it. This radio has no CPU, no memory, no programmed EEPROMs, and no fancy integrated chips of any kind. This is full-on discrete electronic component radio. No magical black boxes. All the electronics that make this radio work are in the raw for you to see and mes with.
Crystal Controlled. The Pace CB144 is 23-channel – crystal controlled (no PLL). Notice that there are two banks of crystals. Each bank of crystals plays a role as will be discussed below
Channel Selector Rotary Switch (23 channels). Notice that the rotary switch on the front panel. This rotary switch has two wafers, or sets of contacts. When you turn the rotary switch a finger runs around the wafers and makes or breaks contacts. The rotary switch has two independed wafers. One wafer controls the master oscillator and the other wafer controls the transmit oscillator. As you rotate the rotary switch the fingers on the two wafers switch in/out different crystals from the two crystal banks
Description of Functional Blocks
Here is a handy copy of the schematic
Master Oscillator. The master oscillator is build around transistor Q16 and a bank of eight (8) switchable crystals. The master oscillator runs all the time and is used for receive as well as transmit. The eight (8) crystals are switched in and out of the circuit through one of the two wafers on the channel selector rotary switch on the front panel.
Transmit Oscillator. The Transmitter oscillator is built around transistor Q20 and a bank of six (6) switchable crystals. The six (6) crystals are switched in and out the circuit through one of two wafers on the rotary switch on the front panel.
Mixer. This is where interesting things happen. One frequency from the Master Oscillator and one frequency from the Transmit Oscillator will be “mixed” (Q21) in a non-linear way to develop one of 23 channelized frequencies in the CB band. (See our two previous articles on the non-linear mixing process and what goes in and what comes out)
Band Pass filter. The output of the mixer will go through a bandpass filter to select only one of the mixer products. This selected frequency will be in the CB band 26.965-27.255 (23 channel band plan)
RF Driver, RF Intermediate Power Amp, and Final RF Power Amp. This section amplifies the signal from the output of the Mixer. This is made up of transistors Q22, Q23, and Q24. Q23 and Q24 are heatsinked and easy to spot on the board.
Output Filter. This is a PI Filter to filter the final output into the antenna. It is made up of coils L9, L10, and L11 (and caps). Coils L9 and L10 are adjustable via a ferrite tuning slug.
Special note on the mixing process
Its essential that you understand how the the final frequency is developed by mixing two oscillator frequencies together. Study these two tables imaged below. The first table shows what is in the crystal banks. The second table shows how the master oscillator frequency and the tranmitter oscillator frequency is combined (mixed) to produce the final transmit frequency.
The front panel rotary switch has two wafers. One wafer chooses the crystal for the master oscillator. The other wafer chooses the crystal for the transmit oscillator. Turning the rotary knob selects 1 of 23 channels and thus a crystal from each back to be used by each of the oscillators.
Crystals X1-X6 are on one bank on one wafer; (37 MHz frequencies)
Crystals X7-X14 are on the second bank on the other wafer; (10 MHz frequencies)
This is what is in the radio – Note the 2 banks of crystals and the rotary switch.
The manufacturer has provided a handy sticker to show you the frequency of each crystal in the bank.
All crystals are socketed and have the frequency written on the case. We pulled one at the end of the bank.
Click to enlarge the images.
This table shows how the frequencies mix to produce one of 23 channelized frequencies.
As an example, for channel 14 (= 27.125 MHz)
The rotary swich selects crystal X4 on one wafer and crystal X8 on the other wafer
Mixing these freqs produces ( 37.750 – 10.625) ) = 27.125
Connecting all the parts – Hands-on Inspection
Here is the Pace CB144 with three test points. These test points are provided and identified by the manufacturer in the service manual as part of the service/troubleshooting process. We put colored “grabbers” at each test point.
The three test points are as follows:
- Red: TP1 – We expect to see the master oscillator frequency at this TP for each of 23 channels
- Yellow: TP3 – We expect to see transmit oscillator frequency at this TP for each of 23 channels
- Green: TP4 – We expect to see the mixed and final filtered frequency at this TP for each 23 channels
The signal path is straight forward. When the microphone key is pressed these circuits come alive.
1. Check/Inspect the Master Oscillator. The two wafers on the front panel rotary switch select one crystal from each bank in the crystal bank. With the particular crystal selected from each bank, the Master Oscillator and transmit oscillator each generate a particular frequency.
Here is the schematic fragment for the master oscillator (Q16) and the transmit oscillator (Q20)
Master Oscillator at 37 Mhz
Note the rotary switch selects the crystal.
The output is on the secondary of T9 and is accessed via TP1 (see full schematic at point B in RX section)
So, lets see if Q16 master oscillator is running. We hooked up the scope and the frequency counter to the red grabber on Test Point (TP) 1. The channel selector is set to channel 14.
Checking the mixing table above – if the oscillator is running correctly – we expect to see a master oscillator frequency of 37.750 ( rotary switch will switch in crystal X4 (37.750 MHz)
So, lets see, “close enough for government work”
At this point you can check the oscillator through all the 23 channels. Each of the positions on the channel rotary switch will select one of the six crystals in the 37 MHz crystal bank.
If this test fails you can suspect a faulty rotary switch wafer, broken connection, bad solder, bad crystal and so on. You can do a lot of troubleshooting here.
This is a very basic and fundamental test. If the master oscillator is not running or not running at the correct frequency then functional blocks down the line will not produce the right outcome. We are tracing “front to back”.
On the PACE we found that one of the positions on the rotary switch was intermittent. After 30+ years these contacts may have oxidation on the contacts. A spray of Contact Cleaner (De-Ox-It) did the trick.
2. Check/Inspect the Transmit Oscillator (Q20)
Transmit Oscillator at 10 MHz
The output is on T10
This is Test Point 3 – the Yellow grabber
On channel 14 we expect to see (check the mixing chart) 10.625 MHz
“Close enough for government work”
If you have a scope you can look at the signal
As above with the master oscillator, the transmit oscillator will run at different frequencies depending on the position of the channel selector rotary switch.
Run through all 23 channels, check the mixing chart, and make sure the oscillator is running at the correct frequency. There are 8 different frequencies depending on which channel you select.
As far as troubleshooting at this point, if you can’t get both the master oscillator and transmit oscillator running at the correct frequencies consistent with the mixing chart and the channel select rotary switch then… “stop, do not pass go, do not collect $200” before you fix the oscillators.
The two frequencies, one from the Master Oscillator and the other from the Transmit Oscillator are mixed (non-linear) in transistor Q21. Frequencies generated will be F1+F2 and F1-F2.
3. Check/Inspect the mixer and band pass filtering. This next test will test many things at once. It will test the mixer (Q21) and the bandpass transformers ( T11,T12, and T13). The transformers act as a band pass filter and will select the mixing products in the 23 channel CB band and reject the others. (There will be a mixing product at about 47 Mhz (Master Osc + Transmit Osc) and this must be rejected).
This is TP4 (Green grabber). On channel 14 we expect to see the final synthesized frequency of 27.125
“Close enough for government work”
In the schematic fragment below the output from Q20 (transmit oscillator) is fed into the emitter of the mixer Q21.
Note the convenient test point that the manufacturer had supplied !!
The Master Oscillator frequency (Q16) is fed into the base.
T11,T12,T13 select only the difference frequency of the mixing products.
If you don’t see the final frequency at TP4 – then there is no sense in going any farther. Suspect the mixer transistor or the transfomer chain. To identify the fault you will have to break up the mixing function and the bandpass function and test those separately.
4. Check/Inspect three stages of RF amplification.
Push the final frequency through three stages of amplification. The pre Driver, the Intermediate Power Driver, and the final power amplifier. – Q22, Q23, and Q24
For this test the manufacturer has not provided test points. You have to make your own if you want to check each stage. You can test at the Collector to Base coupling at each stage – you should see an increased clean signal at each stage. To find the right point on the board look at the parts layout and find the location. If you can get a grabber on your test point from the top of the board then good for you. If you can’t you can get to a test point at the bottom of the board – solder side. To find the right place on the bottom of the board you need the board X-ray – which the service manual does not supply. You have to discover this for yourself.
If the next test works then you can skip testing each stage of amplification.
5. Push the amplified signal through a PI filter (harmonic rejection)
This is L9,L10, and L11 (and the caps). From here its out the antenna jack via the TX/RX relay. Your test point is the antenna and your wattmeter into a dummy load.
Let see what we get at the final test point (antenna jack)
Wow. That’s nearly 4 watts – just what it should be.
If you did not see anything at the antanna jack then you are going to have to work backwards. If you are following along then you can see we tested (“forward traced”) up to the output of the mixer and got good results. We skipped testing each of the RF amp stages since the manufacturer did not supply a convenient test point.
If you got no RF output at the antenna jack you should continue with you tracing. To trace back see if you have RF before the PI section. If you do, then you know the problem is the PI section. If there is no RF into the PI section then you are going to have to dig into the RF driver, the intermediate power amp, and the final power amp. This is not a lot of components. Break them down into each stage and check each individually.
I hope it doesn’ work in your case. This means you got some nice exploratory work to do and if you isolate the problem then you learned a lot. You can often learn more from what does not work than from what does work and this builds your diagnostic skills.
6. The whole transmit strip.
This is the whole transmitter strip (lower section) . Click to enlarge
So what’s next?
On AM what you have now is 4 watts of carrier. How do you modulate with intelligence (your voice)?.
(Amplitude) Modulation in this radio is accomplished via an audio amplifier which modulates the collectors of the RF IPA and power transistor via a transformer. Your voice into the mic drives the amplifier. The output of the amplifier goes into transformer that can drive a speaker (PA setting on the radio) or modulate the RF collector voltages of the TX section. The lower winding on transformer T17 modulates the RF Amp and thus modulates the 4 watt carrier
We won’t discuss this, we’ll just post the relevant part of the schematic. This is a classic push/pull amplifier design. Q12 is the preamp, Q13 is the driver, T16 splits the signal (180 deg out of phase), Q14/Q15 are the push/pull amplifier pair, T17 combines the signal; the lower winding of T17 modulates the RF amps Q23 and Q24. (see full schematic)
Where is the power in AM
An unmodulated 4-watt carrier when modulated at 100% by a sine wave adds 2 watts of power to the carrier. There is one watt in each of the two sidebands which makes a total of 6 watts. Power is proportional to the square of the volatge (or current) which means that 1/4 of the power of the carrier is in each sideband.
So, expect the audio amp to generate at least 2 watts of audio for AM modulation of a carrier at 100%.
Here is an excellent discussion on AM Modulation by WA5BXO, WA3WDR, W5TOB, and K4KYV
The goal of this positng is to get you to stick you fingers in a real working radio and find out what’s going on in there. For this posting we picked an “old school” crystal-controlled transistorized radio for these reasons: a) these old CB radios are cheap, or free, and thow away and the financial investment in minimal; b) these radio are running on 12 volts so you will not get zapped by high voltage required by a tube rig; c) this particular radio is of a vintage before PLL so all the electronics of what makes the radio work are out in the open – no black boxes of magical integrated chips; d) Users guides, schematics, and service manuals for these radios are readily available for free on the Internet.
So, get in there and understand how things work – none of this appliance operating stuff – no excuses. Use this as a stepping stone to understand more complex circuits. For example, a PLL-based (Phase Locked Loop) radio will have much of what you see in the Pace CB144 design integrated into a single chip. So, learning acn be incremental and what you can learn on a crystal controlled radio can be applied when you conceptually tackle the PLL.
How much can you learn? The limit is only yourself.
We have a book recommendation: Understanding and Repairing CB Radios: For the Professional Technician
The reason we like this book are: a) this book is laser focused on real circuits of real CB radios. That is, it is not a general purpose book on theory – it does contain theory but only in the context of practical use. b) It is written at a level that asumes a minimal amount of electronics knowlege. It’s takes a “teach as you go” approach. c) the book is comprehensive (400 pages). d) the author, Lou Franklin, knows how to communicate (teach) to a audience of “hands on” technicians and has the right balance of theory and practice.
This posting assume you know some basic electronics. If you don’t know basic electronics you can do a self-paced, self-study course compliments of the US Navy. Here is Module 7—Introduction to Solid-State Devices and Power Supplies
You can get links to the full Navy Electronics Training here – http://cbtricks.com/miscellaneous/tech_publications/neets/index.htm
You can find 1,000’s of schematcis and service manuals here –