Courtesy of CB Tricks
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I appreciate what these people do for each other - on a day by day basis.
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- any photos shown here - are respective © CB Tricks and the individuals that created them.
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A lot of these graphics are from datasheets, including the discussions within them. What is nice about this site is the wealth of information and the ability to access it. Much of these Graphics' edittings were done on this machine [an ACER ASPIRE One Netbook running Win 7 Crippleware / Starter] and PC paint. Sure it looks simple but if you were expecting something else - remember what you're dealing with and why you are even able to view this data for nearly free. I do give credit to those that take the time to photograph the situation and check for credits to those that are in these pages.
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re:Galaxy 33HML - Low Power Louder that High Power?
can be found here.
This was a thread that I found interesting, due to the nature of rebuilding a radio - many things done to it in the past, can come back and haunt you.The original series of posts dealt with trying to undo the mods done to this radio from the previous owner. As you view the thread, you'll see various people offer a hand or two to help this stranger in getting a radio back to stock and then re-work another set of mods to complete the radio.
One aspect that was interesting, I would like to work with further. This deals with upgrades to the AM regulator sections in nearly all of the Galaxy radio series. Whether it would be the Galaxy XXHML/VML, or the newer FCC Type-accepted chassis designs, Galaxy 9XX series. I once owned a 979 - and I liked it. [Got stolen a while ago - long story] They, Galaxy, have gone back to the starting block in their efforts and have done considerable refinements to their designs. They have come a long way from their older Cybernet days. In a good example, take a look at the schematic of a Galaxy 33HML [on right] and notice the use of what would work for SSB and AM toggle to turn on SSB and turn off AM modulation - is now being used to regulate AM power and the modulation envelope. They did this before, yes, but the refinements can be seen and hence an improvement from the original design.
In case you're curious, I'm talking about some Realistic and Cobra models use a 680 ohm resistor on the 8 Volt constant line BEFORE it went to the AM Power [VR10] and their use of swamping capacitance on the Driver [47pF]. It changes the overall response of the circuit. It is explained a little in my PC-122 pages, under References section.
Realistic and Cobra radios had used resistors where Uniden had used Jumpers. It created a different kind of problem that many to this day still can't seem to understand. It deals with input impedance from both source and ground references. If all the source inputs were not equal - power transfer would be affected and you would be unable to acheive the expected, or intended, results.
In regards to this thread and the PC-122 - I'm trying to save your radio - not destroy it!
Of particular note, is the loss of supply of components. Naturally as the years turn into decades, many of the transistors and semiconductors the original engineers designed the radios around [discretes], are now extinct - like a dinosaur. What was used in these chassis as their originals are now in parts bins that are rare to find and are often thrown away - tossed out because no one can use them except for X or Y and no one is beating down the door to get them as spares - yet. Nowadays, the supply of the components have been/are being depleted and people are turning to other components in an effort to provide similar or improved performance curves.
There is also the problem with trying to upgrade the design using these newer components and having to deal with their performance curves and operational characteristics.
Note the use of extra resistance from the constant 8 Volt supply feed. [See Graphic of 33's AM regulator above] There's a 100 ohm resistor [R237], and the 5.6K resistor [R239] after and in-line to the Mod amp bias control transistor, TR49.
They have modified this circuit to reduce the drive that the original design had.
People from the older radio pool/users liked the performance the previous designs had over these newer designs. This is not a big issue to me, for if it's necessary, I can change the components to reverse out and return to the original design. Thats' one of the nicer features of these radios, the ability to revert back if needed.
There are times when the original design and it's operational character actually improved the performance and reduced the need for extra filters - because they were aware of problems that this newer generation of engineers and radio components are not, one of them being what is discussed in this thread. There may be more to this story that hasn't been told yet. It's advisable to click on the linked thread to see if other changes and added posts may be there.
Considering the economy these days, this may be the new Uniden © Radio that people are going to be watching out for - with great interest. Galaxy has kept their styling which made them popular with many of the older radio enthuiasts, and at the same time, introduced newer platforms containing features that newer generations wanted. And on top of all that, they only needed to make two boards, one for the older traditional syles to appeal to that crowd, and use the same component and technology platform in another chassis to appeal to another. It appears that Galaxy learned a lot of lessons from earlier manufacturing practices, try to keep things simple -and if that can't be done, at least keep the platforms standardized and interchangeable for as long as possible.
Another aspect dealing with this thread, when the radio was working again, was the new owner needed to figure out why the radio wouldn't produce quality audio with carrier on high power. It would pinch up or lose audio drive. Not a good thing.
IMHO - I believe the previous owner made a lot of changes to the radio to de-convert back to the older chassis and mate it back to the old days and use the 2312, what the new owner found in it. It's sad in a way, because the previous owner wanted an older radio style and means of making it work. This puts the next person in-line in a lot of trouble with changes like this. Now, you have to rework this section just to see if you can re-create the original disappointment just to fix it/modify it to get the results the new owner wants.
As you can see in something like this - it's very easy to change one small part and get one type of transistor  to work, but extremely difficult for the next person to figure out how to return it to original value so you can install the original components and transistors to make it work as if it were stock.
This is one of the many frustrating aspects of purchasing a used radio. You don't know what to expect.
Once you decide to fix the problem, it may be better to leave it alone than to try and fix it. Is it worth the cost and effort? Or, are you better off to get rid of the radio? Now you know what I'm getting at.
Remember to take the time to do some research before you decide to tackle a problem like this. Don't buy a radio because it sounds good - especially when it's used equipment - because there may be an inherent problem with the original design in it's stock form, and the user had to put a lot of work into it, to make it sound like it does. Don't settle for the first one if you want to buy. Did I mention - RESEARCH?
This thread, was a thread in a thread [the same one about the Galaxy 33 - only just a different circuit], dealing with NPC-RC mod for the Galaxy 33 - it's location and orientation of parts schematically and on the trace side of the PCB. This mod is pretty simple and if you go back to the index page at my section - you'll see the PC-122 radio graphic - just click on it and go to Assymetrical section of those pages to show what it means and how to install these - it's not hard, once you know what to look for.
In this graphic, someone wanted to know how to install the NPC [Negative Peak Compression] Mod into the radio. As you can see in the graphic - once you've found the location points to install, the process is pretty straightforward.
Once again, you can call up the data on the website, edit it for the purpose to demonstrate or show what has to be done - and post for all the world to see and hopefully get, and/or give, answers. I pulled this one out of my own archives to show you the wide variety - and to give tribute to all those that participate in these forums.
I will EMPHASIZE...The subject of NPC-RC is still in heated debates and committes' as this is typed - will attempt to update you as this progresses. [O.P. - ED.]
This mod allows the user to adjust how the gain cell is charged and discharged - and deals with R241 [15 ohm] and C190 [100uF] and the diode used can be any common type - like 1N4148 or 1N914 - just as long as it can handle 50PIV and 1A surge rating. The resistor value is important and does affect how much audio is left in the signal that can be amplified. My personal recommendation is to use 330 ohms. Why? It's in how much of the audio waveform that swings below the DC bias voltage in the AM Regulator - higher values have less compression, but deliver more amplitude of negative envelope, than to use 100 ohm and lose more of the negative going audio envelope needed for the detector on the receiving end [those other users]. There are some benefits to this method, less cutoff disortion being the main one. But can reduce your audio punch and long distance listeners many only hear more of the air noise the carrier can bring in than notice the swing in the audio the radio produces.
Re: TRC - 419
can be found here
This little radio is interesting, if you can take the time, download the PDF files about this radio - particularly the schematic. It's important. There have been considerable consolidation of, along with a number of changes and refinements, to radios. As CB radio has evolved and tried to keep up with current technology, so has it's complexity to deliver more features. This radio is one of the last radios to contain older technology of discretes and became a template for modernization using SMD [surface-mount devices] and PLL/EPROM comsolidation of programming.
Then they added to it a frequency readout, and consolidated to a single-end dual-IF output and WX frequency generation - all reducing the need for extraneous components - which eventually led to a new type of construction technique using riser cards containing 1st and 2nd IF as well as VXO/VCO consolidation using SMD's.
This radio is a good example of how standardization has affected how radios are built. The simplicity and standard platform in radio designs and the components used, have now arrived to a point of; cost of assembly, to markup and what people are willing to pay.
The picture on the left is from a Midland 77-115 - showing how the TRC-419's two separate IC's got consolidated into one little chip with essentially the same functions and as a SMD or Surface Mount Device. One of Midland Radios' downfalls is due to taking the SMD Technology to the extreme. I know this may make it off topic, but I don't appreciate paying good money only to have it fail at some unexpected moment [Car Crash, Bad Roads and Road Construction and Traffic Jams come to mind] so I don't want to see someone take a good design like the TRC-419 and get micro-minturization on their minds and ruin some great theory and concept, because of the scarring it left behind in some peoples minds - it will never see the light of day again! [SIGH]
What they did is use Vertical Boards that contained the circuitry that the TRC-419 has, broken down into smaller boards that were soldered directly as if the board was a header. This made a lot of real-estate available for the maker to install WX system, Dynamite right? Not really... Because they were not able to withstand the rough-and-tumble abuse that occurs during real-life situations and end user wanted something durable. Lot of returns on this radio - due to failures caused by cracked and lifted traces and failed solder joints during normal use.
Notice the location of these cards are along one side of the radio - and slots were cut into the Poly-Carbonate-Board [PCB] and weakened the board to a point where the flexing caused by the lack of board support and stiffness to allow the boards to break off connections and the radio simply quit receiving.
Back to the TRC-419...
This radio is also one of the last chassis designs from the late 80's into the 90's that introduced discretes using PLL programming separated - the EEPROM latches what the PLL chip sees to encode the proper programming and the EEPROM also handled the display - done separately from the PLL chip. This radio is a good learning tool for someone that wants to understand more about PLL programming. This radio doesn't use a Binary Coded Decimal wafer switch - it's done electronically via EPROM and TTL outputs.
The gentleman that was asking about this radio just wanted to do some mods - so I gave him a few to work with and as I went over the schematic to give him the interpretation and -what was needed for what- for parts values I noticed several things that made me aware of this radio actually became a template, a predecessor, of a new generation of radios. I became more impressed with CB Tricks web-site simply due to having not only the schematic, but a service manual for this old dinosaur.
Use the following values:
I've found a different way to get around this. [See below schematic for details]
This is what makes CB Tricks website so much fun. You can not only post the mods or whatever you wanted about radios, but use a picture or two to show what you're talking about.
This radio shows many of the designs that are still in use today.
I cut my teeth on radios' like this. As radio evolved from crystal synthesis onto the PLL and eventually construction designs like this - it makes radio fun!
Here are some Transmit mods to help improve audio drive to the final.
These are simple changes to the bias and drive delivered to the Final from the Driver.
Note the SJ 52 tap point. This radio can take another type of modification using Asymetrical Modulation - which is dicussed in various forums and I can consolidate some of those mods into this section of the page. This jumper will be the point where Audio and TX will be put, or mixed together and presented to the pre-driver.
This mod is unique in the application of audio - the use of diodes and resistors to properly mix DC bias and Audio to form an envelope that the predriver will create into the next stage.
I'm assuming you've tuned-up the radio...
Read this well and understand it first, then apply this mod. For we are re-working a section of the limiter and how it responds to voice peaks. [As always, follow up - check for shorts and proper hookup and polarity e&c]
Then follow through - be willing to experiment here on this section to acheive the results by trial-and-error. I'm trying to help you keep the limiter intact because you can use it in your favor.
You'll need the following parts...
You can locate the schematic by Radio Information >> R >> Realistic >> TRC-419 and pull up the PDF schematic and serivce manual to help you as an aid...
Now it's math time...
Take the first reading, The total resistance needed to get to your wattage setting and subtract Total resistance reading with the diode in the circuit and you'll get your buffer resistor reading - of if you refer to the schematic of the two diodes together forming a T-ee type circuit - The Difference is your INPUT resistance in the circuit.
So your buffer resistor is now your INPUT resistance. Remember the above, when taking the two measurements? It will have to be 47 ohms or greater here for INPUT resistance, or the INPUT RESISTOR [Resistor Bias Trim] value.
Remember the second reading? [I told you to write them both down! So don't yell at me!] This Was the Total resistance for the diode and jumper together. Solder that resistor into the other hole with the free end going to the diodes' banded side. This will be PARALLELED with the resistance off the Audio Transformer - Reactance Modulator. You'll need two resistors of the same values, so solder one into the circuit here.
Ok, take a deep breath and relax, smoke-em' if you've got em', and then insulate the connections so they don't short to board ground or when you put back on the case covers. You can power up the radio, and verify that you're now back close to 4 watts or whatever the thing was originally putting out. Re-tune the entire TX strip starting with the pre-driver and using audio AHHH...-sound, tune for maximum audio and carrier. Do this step several times - up and down the strip and you'll see some extra swing and higher level of audio is now present in the signal.
This circuit comes with some history, this mod can be applied to voice-only radios as a means to reduce harmonics generated at the finals output caused by excessive modulation or modulation peaks that fall below bias voltages and can damage the Driver and Final. It was originally designed to be used in radios requiring low audio outputs.
What is being done, is to Reduce constant bias voltage using a Steering Diode and Dropping Resistor to reduce current draw, and carrier production as if we've reduce power being delivered in the Pre-Driver section. It's important to reduce the power to about 70% of the original power level.
Since the jumper supplies battery voltage when needed, See Schematic and PDF of Service Manual for this radio, the reduction of power gives us the overhead to add audio peak power to this circuit to help drive the RF with audio envelope power to reduce hamronic production that could have otherwise occured if the Driver and Final were simply driven directly and the AF limiter was removed. Not a good idea - especially nowadays with technology everywhere that could be interfered with, causing angry neighbors and complaints to the police - which involve you!
Audio power and bias voltages are sampled from the output of the Reactance Modulator - before it goes to another Diode. Only we don't want to use this diode, for we are using another set of diodes, along with resistors, to modify only a small portion of power that will be applied to the pre-driver as a power level, and hence - change the output using audio as a means of partially driving it with, and additionally to, the battery constant supply. Both used in proper ratio, keeps the pre-driver from affecting the entire signal - we do want just the audio peaks to compliment the carrier generation, and to remove harmoinics that could form then there's not enough RF to proprly shape a RF envelope at the Driver - before it goes to the final.
This diode prevents overmodulation of audio drive signal that can cause damage because the Driver and Final could become Reverse Biased when these NEGATIVE audio peaks go below Ground reference - or board ground, even though they are being powered by battery voltage being applied at the same time.
It is a good idea to review the Reactance Modulator, and note that there is no true ground reference at the transformer. If you know how a spark coil works, it uses an Auto-Transformer and impulses are generates across the coil terminals that are of different, higher, voltages when current and voltage is interrupted. In a radio the voltage and current may be steady, but audio impulses and waveforms are being applied at the same time.
Back in the days of FCC and FAA cooperation, they developed a means to communicate using this modulation technique to be used with Aircraft [Approval- imagine that?]. Since spectrum was limited, and FM had it's drawbacks [capture ratio and interference, extra bandwidth required] it was found that AM can be detected in it's weakest form better than FM due to noise levels.
AM and CW use the same concept, carrier, to encode and decode information.
So does FM, but due to the bandwidth required, you can't detect or decode information as easily and an inherent problem with FM was to squelch out noise and develop receiver quieting sensitive enough for detection without noise being brought in due to the deviation [spectrum bandwidth needed] required to encode and decode this information.
If a plane or any other craft would fail or crash and lives were at stake, if you had access to a radio using AM, you could transmit voice on the carrier, or at least a carrier. If you couldn't generate a voice, you could turn the carrier on and off, and you could make a string of signals like morse code, to help as a beacon. A beacon alone, dead carrier, would signal someone to help.
In regards to voice, Amplitude modulation can have audio enhanced into the carrier to produce more audio using the carrier information. This is what that circuit does, applies some audio waveform to the output side of the pre-driver to be mixed in at the starting level for carrier envelope. This may introduce some distortion into the carrier, but intelligibility of the signal has now been increased. You can hear the weaker signal because the carrier contains the information and can be decoded using CW or any AM radio - for longer distances from the station transmitting it.
But, using this tecnique has some drawbacks, one of them being induced distortion.
Another for the purpose of harming or intentionally interfering with others.
Use discretion here - this mod can do some damage - as in losing friends - if you take this mod as a means to purposely generate interference to other stations.
Cooperation gets you a lot farther than being mean when it comes to this [this being radio] medium.
Enjoy your radio!
Re: cbdoctor NPC-RC mod
Can be found here
This circuit is similar to the one discussed above, and can be used in, And Limited Only To, AM-only radios. So, with the above circuit already discussed, we can proceed to another application of Asymetrical Modulation circuit design.
NPC - RC can mean two things:
- Resistor and Capacitor [reduce carrier in AM-only via Driver]
- Resistor and Diode [Reduce negative envelop peaks using directional Diode and resistor AM-regulated]
This mod is still in considerble debate and explanation - and what makes CB Tricks web site such a neat place, is from the ability to participate and educate. It also provides a means to show others and invite those curious about the discussion to experiment and post the results for others to see. This helps the process of edification and enlightenment as the discussion progresses and refinements are made to the concept and ideas are exchanged.
This mod uses only two components and takes advantage of the Cobra 29 and Uniden PC76/78 platforms by providing a jumper - similar to the TRC-419 - which can be removed and the circuit installed in it's place.
The following will be needed:
The mod for Cobra 29 or PC78 is completed as follows -
The circuit is pretty simple, reduce the DC bias component coming from the Reactance Modulator and apply audio that is present on this signal - without being attenuated.
This may present a skewed signal that many may not truly appreciate. The new modified waveform contains more audio to drive the Final.
There is a cost to this, so you need to be aware that adjusting the bias or DC component here at the Driver will affect the level of carrier power and may produce some other artifacts - or distortion - to the signal. To adjust for this - adjust both the capacitance and resistance. Lower resistance increases carrier power and lessens the effect, but if you need to, you can lower the capacitance instead to improve the peak power and reduce some inherent skewed audio distortion due to the caps inherent limitations of rate of charge and discharge. The larger the cap, the more pronouced the effect. Too large of values of these two components can negate what you wanted to accomplish.
Just remember not to go too far in values. Start with low values and work upwards to acheive the effect. Each platform of radios Manufactured today, have an inherent response to RF that is different from models of other years. So, what works for one type or model and brand of radio - may not work on another platform, even though they are schematically identical or nearly so.
Re: Galaxy 959 poor RX
Can be found here
This thread demonstrates the ability of Field Effect Transistors to amplify low-level signals.
Unfortunately, they amplify too much. They're just too good at taking in small signals. They're good at control or switching but trying to use them for RF and raising the amplified output to a point that the analog-based signal can be heard - is a totally different ballgame. Their characteristics of how they amplify that makes using them for analog signal detection difficult to nearly useless. It can detect very weak signals and develop an output - to be processed further - they do not have a threshold or bias control to them that can be used to adjust input sense to reduce or remove background noise - so they need to have any signal amplification generated - processed further as in a Digital Speech Processor or be used in modes of reception where noise can be removed without detecting analog signals directly - like FM. That is due to the needs the engineers felt were necessary due to the features the radios' platform is based upon required sensitivity to the various modes, and conditions, the radio was to be operated in.
It's not anybodys fault, it's just using this technology generates some inherent limitations within circuit design that [well, maybe you can blame someone] the engineers should have understood before making a radio using this platform and RF receiver design. FET's, once understood in their behaviour, can be used - but not for this application - that being CB. Analog designs and Bi-polar transistors fit better than using something that acts more like a switch at this frequency range of RF spectrum.
A good example is when they're used in IF [Intermediate Frequency] amps and as a means to mix two signals together or during subtraction in filtering out the unwanted signal and amplify the desired signal.
The concept is not new, for the Galaxy series uses them with some success in FM applications, but due to their inherent on-to-off and off-to-on switching states, they produce a non-linear output that requires considerable amount of filtering for analog and audio-based envelope reproduction. They don't perform well for analog or amplitude signals that require output that is faithful to the input. They require some re-shaping of signal levels - and that can be accomplished to a certain degree by Reducing signal strength being presented to the Gate of the FET device. In the graphic, the RF amp [left side] is controlled not only by the RF gain, but it's input is also controlled using special purpose diodes - called PIN diodes. It is done by a process of feedback generated by a circuit that functions as a control - called AGC.
Let me show you what I'm talking about, refering to the next/above graphic on the left, you can see two FET transistors being used to accept Q17, the RF Amp, output [to the left on the graphic] through the use of a pre-selector/peaking section arranged like a PI-filter network and mix it with the IF generated at the PLL's VCO section.
It's output is then filtered using two pre-selector coils arranged in a PI-network filter to separate the IF from the RF image. The output side pre-selector is also used as a tap point for the Noise Blanker circuitry used to reduce impulse noise present in the signal - before it is processed further.
Many FET devices use a junction and substrate that is different from a bipolar transistor. The input control signals are applied to a layer of semiconductive material and may contain an insulating layer, or oxide, preventing current flow through the junction, or in the construction of the device, an insuating layer is created using an dissimilar material to form a blocking/insulating region - called a Gate. On the other side of this layer is the substrate which has a semiconductive material and has a directional characteristic. Referring to a Source and Drain. When a small signal is applied to the gate, the material, or substrate conducts in response. Allowing for small low-level signals to control a larger current device and have a higher gain and performance region or curve into the VHF and UHF region of the radio spectrum.
A typical bi-polar transistor uses 3-layer substrates and form Base, Collector and Emitter - having an inherent voltage drop and loading impedance requiring bias control support circuitry and have a response curve performance that degrades as frequency increases.
Benefits FET's have over Bi-polars include, but not limited to:
Drawbacks - referring to their use in this circuit.
The best way to put this is:
Due to the inherent operational curve and wide frequency response - the FET configuation used in the above radio is best suited for FM reception.
CB radio, as it stands at this moment, is AM and SSB modes only.
AM and SSB modes are Analog based signals and even though the FET's can easily handle the frequency of interest, their linear response curve is nearly non-existent - they do not linearly amplify AM and SSB signals. The FET configuration is based upon a 2 component, Push-Pull configuration and they have to use Diodes to prevent oscillation and keep them forward biased. They made a decision to make the receiver extremely sensitive. Perfectly fine, but no one bothered to tell them of the noisy ENVIRONMENTS these radios will be used in.
You would have to use Bi-polar and their required bias designs to make the analog signals present in the noise level, useable in subsequent stages of IF amplification and detection if you are to use such a Push-Pull configuration.
That's where AGC comes into play.
AGC is a means to control the amount of amplification the receiver applies to receive any type of signal.
Basically how it works is--:
AGC, or Automatic Gain Control, is a function that a receiver can use to detect signals at low-noise levels and if the strength of the signal was to change, either by arriving closer to the station or gaining distance from it - or through environmental effects like propagation, the ability of the receiver to capture and develop useful audio or signal level for a wide range of receiver input conditions is a result of using AGC to control the gain and reception of a receiver.
Every CB radio manufactured today, uses AGC as a means to receive signals, provide signal quieting [noise floor] protection from excessive signal input [attenuation - PIN Diodes - Overload], and remove noises [ANL/NB support circuitry] from the receiver and produce audio, detected from this received signal.
In todays radios' AGC is part of a feedback loop that uses received signal, obtain a reference value, and apply it to affect gain along the Receivers IF strip. It can also be applied at the same time to the RF amp and protection circuitry - known as PIN diodes - to reduce gain that had this control signal NOT been applied, would have been highly amplified and distorted, this is called signal overload. It means just that, that the received signal was too strong and the receivers own circuitry overamplified the signal and added distortion and clipping due to excessive drive into the next stages of the receiver.
Get your head around this concept first, by referring to the smaller schematics shown above as a reference - then you can proceed. AGC works with both the RF amp and these PIN Diodes to control the RF presented to the 1st Stage IF amp.
You may not be able to make FET's work as if they were a Bi-Polar transistor, you don't really need to. You just need to control how much signal is present, that's how it responds to input signals.
FET's - as you already know, are very sensitive to input signals applied to the Gate region. So, you may not have any true ability to make this logarithmic response component operate as if it were linear. So, what you can do, is make both the RF amp/PIN Diode section and the AGC work together to make the FET sense only signal, not the noise level or floor.
Refer to the graphic on the right, is the other half of the problem. This is a Quad Operational Amplifier. The discussion of how Op-Amps work is a bit beyond the scope of this essay. It is better to realize that AGC develops it's ability to control the input of the receiver through this area.
Note the highlited areas of the schematic and parts affected. We will focus on the AGC section, Red Arrow items and Green Arrow components at this time.
The Op-Amp takes in a sense signal from the detector - which is at the end of the Receiver Strip [R41 and D10, D11, D12 and R49 - there are other components that feed into the line going to this section of the Op-Amp that are for SSB] . This signal contains Audio and Carrier voltages at a very low current. They are filtered and applied to one of the inputs of the Op-Amp. R46 works like an impedance balancing component on the input line [Pin 12]. The signal level is so small, they use 2 Op-Amps, one to raise the signal to work the RF metering and then it's applied to the AGC - once the value is raised up - then some support components, like R50, C35 and C33, buffer and smooth off the waveform as a means to obtain a response curve that doesn't act too fast or too slow to get the receiver to develop audio even when the antenna is not receiving the signal at the same strength - moment by moment.
The gain of the sensed input is set by a ratio of R46 and R44 - and has an amplification, or multiplication factor, of about 3X.
Remember we are dealing with a Push-Pull FET design
The ideas used here are to generate a control signal based upon noise and signal present, and adjust accordingly - without being too aggressive with the detection to reaction time. Too much action can result in receiver noise and audio present within the signal - popping in and out and switching places - almost like as if it was Compressed.
When you use this design in a receiver, there is a tremendous amount of gain associated with it. This includes a noise level that many vehicles generate as a part of their timing and reference control signals to various sensors and modules located in various areas - serving specific functions - that communicate back to the ECU/ECM to maintain the vehicles systems - it's part of the emission controls - required by law.
Not only save the radio, but save your life too!
The best way to see what can be done, is locate the two green arrow components- and just swap them. This way, the Op-Amp multiplied, or amplified, output is adjusted to 1/3rd the rate of amplification and the FET's can do a better job of amplifying what is there at the signal input versus all the noise drowning out the signals you're trying to listen for.
What this swap does, is sensitize the receiver, making the AGC develop less aggressive response to the noise level, it raises the voltage output ratio to the sense input and allow the Push-Pull configuration work with the RF amp and PIN Diodes when the noise floor is the only thing that the receiver can hear at that moment. Working with what we have, we have changed the amplification of the 1st-Stage IF amp to improve it's dynamic headroom - pushing the inherent noise floor down - while allowing RF signals to be delivered above the threshold of the turn-on function of this amplifier configuration to be amplified in a way that is more linear and less likely to compress into a noisy receiver signal that the user can't understand and has to constantly adjust the receiver so they can at least discern, or attempt to understand, the person sending the signal.
You can do more for this circuit, and by reviewing the schematic snippets of 1st Stage IF amps and RF-Amp/PIN-Diode sections, there are ways to improve it, but doing so would violate warranty and if not properly done, can damage the radio.
Due to the complexity of the repair, it can't be done by normal means. But, using the snippets - you can obtain a working knowledge and attempt these yourself. I'm not endorsing you to do so, only saying that it is possible, but, it requires some skill many are not given unto.
To be continued....
Re: UNIDEN GRANT XL CLARIFIER MOD
Can be found here
Finally, a thread that I can appreciate! A gentleman was complaining about how the slide is skewed on a Grant XL after people have done some mods listed elsewhere that requires people to jumper a resistor.
First, locate the Voicelock/Clarifier control wires, and their color code - inside the radio. As an aid I've provided a photo of the location of those wires and two diodes D75 and D52 - note the wires just to the left side in the photo - those are the clarifier/Voicelock wires.
Grant radios use a header here and is a little different but use a VOM and you'll be able to locate proper color coding and where all the wires in the header go.
I have direct experience in both the Grant and 148GTL models. And as Uniden tried to improve their design over the original that both Uniden and Dynascan used, they [Uniden] started making wiring and harness improvements. The photos may not look like what is inside your radio - but are essentially the same - due to the foreknowledge that this chassis is very subceptible to RFI and poor ground loop issues - which Uniden finally tamed in their last Grant - that being the LT model and was their last version of this chassis. However, as these models progressed into their separate ways, RF and Audio issues and their locations on the main PCB stayed in the same areas for each generation. This made it easier for the techs most familiar with this radio to stay on top of issues like Clarifier and Final-Driver and most repairs.
Uniden Grant and Dynascan/Cobra 148GTL are essentially the same - or started out that way. So a simple clarifier unlock mod is easy to do and is essentially the same between them. Just locate the wires form the Clarifier/Voicelock and follow them - they should go to centerline of the board and towards the front.
Refer to the schematic too - you'll find the components listed above in areas that the Voicelock/Clarifier controls go to on the main PCB.
Using both the Schematic, and the photo shown above, orient the radio with the component side up and the front of the radio facing you. The rest of this section is based as if you are oriented so the the front of the radio is towards the bottom of the picture - as if you were actually looking at it from the front - to inwards. You'll find a lot of photos I do, are from that perspective and are oriented with the bottom of the photograph is towards the front of the radio.
It's a good idea here to use the VOM and check voltages at the Clarifier/Voicelock terminals - this way you can locate source - out - and to board ground terminals and all you need to do is follow the source RX/switched side to the main PCB - in this center harness/wiring location - and there would also be a wire leaving it, back into the harness and down towards the front right section, just behind the meter display. A potentionmeter labeled VR3 will have this wire going to a feed point of RX source voltage here - and powers the Clarifier/Voicelock circuit from this location.
Once you've located the power feed wire, it's color and location in the wiring harness, you need to remove the power - and let the radio discharge itself - off to on then to off again - to ensure the radio doesn't have any stray voltages or charge left on a capacitor that could short and discharge into what you are doing and damage the radio - as in an expensive repair. It's a good idea to use proper lead dress and use the wiring harness as much as possible to reduce stray RF issues from goofing up your work. Just thread the wire from VR3 through the harness strap and bring it down to the board at the PLL side. You shouldn't need to trim off any wire - but if you want to make a professional looking result, follow through and keep it clean.
Just unsolder or cut the wire off close to the board. Since I didn't have a header to worry about - note the hole left behind - I just remove the wire by unsoldering it instead of cutting it - to make a neater appearance. What I do to ensure connections remain good, is remove these jumper headers - you can keep VR3' header in place - you're just pulling a wire off of it - but to ensure positive contact for the life of the radio and for ease of assembly - you can re-do the jumper harness at the clarifier/voicelock section if you so desire. Then re-tin the lead of the free-end wire - and resolder it to a location by the PLL as shown above. This board is from the older model of the Grant/Cobra 148 [pre-1994]. These boards were designed with extra holes in key points - not just for testing but - for things than can be installed in them. Some of these holes had more than one purpose and this hole could also be used to power a frequency counter display if the user wished. So, if something is already there - check it out, but you can always locate another 8 Volt constant source by the PLL. If you look hard enough, you can count at least two more in that area. So, obviously, they had plans for expansion.
Remember D75 and D52 photo? Just remove one of the diodes here, that breaks the TX connection and the TX frequency trimpot. If you'd like better temperature compensation, you can use the diode you removed and put it in-line with R175 [by audio amp right-side panel near centerline by right chassis panel]. The banded side of the diode is towards the resistor - unbanded side to the wire itself. This will help keep the clarifier more stable and less likely to drift too far on a cold winters night. I'm mentioning this because you may need to have this radio work for you in an environment that normally would de-tune this radio and prevent others from hearing you because the Clarifier/Voicelock circuit drifted too far off of center of the channels' frequency so they simply could not hear you.
You're essentially done with this for now. Check and verify wiring, look for and remove any stray wires and solder blobs - anything you touched - make sure it's clean and in good condition - and you should be able to power up the radio and the clarifier/voicelock control should work, and tracks both and Transmit and Receive in all modes.
Site page still under construction - please check again later.
You DO NOT have to do all the mods listed on these pages. If possible, and I know I'm asking a lot, but I want you to read about these mods first. A lot of these modifications came directly from email exchanges [sometimes heated debates] about how people wanted more out of their radio. A lot can be done to it, but radio is not everything to everybody. It simply cannot be done!
Some, not all of these mods are a cullimation of when I trying to help a group on CB'ers become Amateur Radio Operators. Being an Amateur myself, I have a responsibility of helping others - it's a part of the License the FCC gives you. And in some aspects, it's backed-up by what this nation was founded upon - freedom to assemble, pursuit of happieness, freedom of the press, and freedom of religion. If those rights were never excersized, you and I would not be here today.
I'm not trying to be preachy about this - nature abhors a vacuum. Remember what this set of pages is for. Place it in the perspective and context, or the premise, of what these pages are about.
They are for Educational Purposes Only
It is my desire to level-out a playing field that has been skewed in favor of those that are hell-bent on being number one [At what? I don't know] far too long and we suffer long-term damage from ignorance. It will take a while to undo and correct what is wrong - to heal the land and it's people. But it is my effort to reveal-that which many have taken for granted and the simplest way to explain some of this - is to review what is in these pages.
If you see my car, give a toot on the horn - seems that everybody else in the neighborhood does it - so why not you?
:+> Andy <+:
/ Created August 2008 / version 0.1.a / Current Version 0.1.b /
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Although the greatest care has been taken while compiling these documents, we cannot guarantee that the instructions will work on every radio presented.
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