Megajet 300 PTT diagram. Mechanical work during installation

Megajet 300 is a truly unique device in terms of reliability/failure statistics and price/quality ratio. Simplified to the extreme by the manufacturer, the radio station, however, retained the advantages of its older brothers (MJ 600, MJ 600+, MJ 800): an excellent receiver, automatic noise reduction, and a reliable push-to-talk switch. The device does not break because there is simply nothing to break in it. Over three years of operation in the hands of taxi drivers, hundreds of these devices have proven that they are an exception to the insurmountable rule that “a canary for a penny not only does not eat, but also does not sing, and after a while it begins to smell bad.” The Alan 100+ radio station, which is close in price, in comparison with them, resembles a copper basin complete with a stone axe. Among the disadvantages of the MJ 300, it is worth noting the following:
1. Lack of non-volatile memory, which makes it necessary to restart the device after each power outage in order to open. The device opens by holding down the AM/FM and Scan buttons at the moment you turn on the power with the volume knob and about a second after it.
2. The device has only three grids (C,D,E Alan"a/Megajet"a, grid C is indicated by the letter L, grid D is without indication, grid E is indicated by the letter H). Switching the grids of the opened device is done by pressing the CH9 button.
3. Lack of ability to switch channels from PTT.
4. Lack of S-meter.
5. Unlike its older brothers, MJ 300 has an output power of about 7 Watts (older brothers - 10 Watts, which is optimal for driving amplifiers such as KL 300, KL 500, etc.) However, this is not significant.. .
6. The device "from the factory" has an absurdly quiet and muffled modulation. However, this shortcoming can be easily corrected. In the photographs below (Photos 1, 2, 3) you can see the printed circuit board of the device before the alteration (the places where you need to fit are circled in red):

You need to do the following:

Following arrow 1 in Photos 4, 5, you need to remove the capacitor located between the direct and inverting inputs of the operational amplifier.
Along arrow 2 (Photo 4, 6) - remove the capacitor that shunts the output of the microphone amplifier to ground, and the resistor marked “272” (2.7 kOhm), standing in series with the output of the microphone amplifier in front of this capacitor (they form an RC low-pass filter , further spoiling the modulation). Place a jumper instead of the resistor.
In the direction of arrow 3 (Photo 4, 6), turn the deviation trimming resistor to the maximum. In the latest batches of MJ 300, these resistors rotate freely a full turn, since they do not have clamps in the extreme positions. This complicates the task somewhat: You do not know which position of the trimmer is the maximum volume. This is where you have to get confused. It is necessary to have another one next to the radio station you are tuning, included in the same channel. Turn on the radio station you want to tune in and turn the trimming resistor. The modulation volume will increase when turning in the right direction, and then suddenly disappear completely. At this moment, it is necessary to turn the trimmer in the other direction, but just a little, until the modulation returns. This position is the level of its maximum volume.
If you are going to rarely work in AM, or are ready to put up with the fact that when you release the PTT switch in AM, your radio station will start making a short squeak (this does not always appear), then in order to increase the service life and reliability of the PTT switch many times over, it is worth modifying it. Here is the view of the PTT before modification:

The microphone circuit in the push-to-talk switch is switched, closed, only for transmission. Considering that this circuit is very low-current and sensitive to any bounce of contacts, any minor dirt in the Receive/Transmit switch, on its contact group, will make working with you on the air disgustingly uncomfortable. At the same time, you will perceive everything as before and will not even understand why people keep asking you questions. The easiest way is to short-circuit this contact group. Photo 8 shows how to do this:

That, in fact, is all the modifications - remove two capacitors and one resistor, install two jumpers, twist one tuning resistor.

material from the site Ci-Bi.ru

This station is designed to solve completely different problems. Apparently, in this way, the RKK company decided to divide the market and earn even more money.

And I dare say they succeeded. The station is quite interesting, it is not designed to work in AM, although it can work in this modulation. Her path is frequency modulation, or FM.
So, if you are a CB taxi driver, or just use the band mainly in FM, and don't want to spend a lot of money on a walkie-talkie, this station is for you. In the harsh operating conditions of taxi fleets, which is where the MegaJet MJ-300 is most often used, the station showed its best side. It is extremely simple and reliable. Now, let's take a closer look at it.

The station is supplied in a glossy silver cardboard box. Like MegaJet, the cardboard is thin but durable.

The station is well packed. Inside the box there is a plastic insert that absorbs shock and is designed to divide the space for transportation.

The kit includes the radio station itself, instructions in English, a headset, a mounting bracket and fasteners.

Appearance
In appearance, the station is very reminiscent of MegaJet, the general pedigree can be traced well. The dimensions of the case remain the same.


The front panel is made of plastic. The plastic looks cheaper than . It is more slippery and glossy.


The case consists of a chassis to which all internal electronics are attached, two metal covers, top and bottom. Each cover is attached separately using 4 screws. As I already wrote in, this is not very good, because at high volumes such a design will rattle.


Ergonomics have changed. Now it is more convenient to use the station. The volume control and noise reduction control knobs are still nearby, but they are now located vertically, which, in my opinion, is more convenient. The handles rotate with little effort, but softly and without jamming. On the front panel there are buttons for switching channels, a button for changing modulation, a button for turning on channel scanning, this mode has been added here. The CH9 button performs many functions, which are discussed below.
The display, like the one, has a pleasant amber backlight. The indication completely repeats the indication of its younger brother. The channel number, grid letter, modulation and mode 0/5 are displayed here.


The display has the same disadvantages. It is clearly visible only from the front and above. You can't see anything from below at all.


The headset connector is modern 4-pin and looks reliable.


The headset itself is very simple, like the one, it only has a microphone and a push-to-talk button.


The headset wire is also very soft and should not stiffen in the cold.

At the back of the station there is a connector for connecting an antenna, a jack for connecting an external speaker and a power cable.


There is no quick disconnect connector on the wire.

The speaker is located on the bottom cover, like almost all civilian radio stations. There is also a marking plate with the serial number.

Functions
In general, the station repeats its younger brother in functions, but there are some features. The station has 40 channels in normal mode, and 120 channels in extended mode. AM/FM modulation, operation in zeros and fives (0/5), in addition, there is a channel scanning function, which can be useful in unfamiliar areas, the function is displayed on a separate SC button. The main distinguishing feature is the presence of automatic noise reduction with manual adjustment. If market sellers tell you that the station has a threshold noise suppressor, do not believe it! A real spectrum monitor, but with the ability to adjust, which is a plus.

To switch the station to multi-grid mode, you need to turn off the station, hold down the A/F button and turn on the station. After a couple of seconds, you need to release the button and the message EP will appear on the display, which means that we have switched to multi-grid mode. Grids are switched using the CH9 button. The grid correspondence is as follows: M – C, H – D, L – B. A total of 120 channels. The station has a built-in attenuator, which can be turned on by holding down the CH9 button. The letters Lo (local on) will appear on the screen, indicating that the attenuator is on. To turn it off, we do the same thing and LF (local off) should appear on the screen. This feature can be useful in noisy airwaves. To switch between fives and zeros, you need to turn off the station, hold down the A/F and CH9 keys and turn on the radio. The buttons need to be released after a couple of seconds. The number 5 will appear on the display indicating that the station has switched to zeros. The scanning function is activated by the SC button. Scanning will continue until the squelch opens.

Inside
The station is made on a printed circuit board made of green PCB. The installation was neat, no soldering flaws were noticed. Not washed flux either.


And the one in the photographs was left by yours truly during revision. I use no-clean gel flux. The components on the board are not labeled, which may make it difficult to find the right element.


Some of the parts are not soldered, which suggests that this printed circuit board is also used for the production of other radio stations. The output stage is made according to the classical scheme, using 2SC2314 and 2SC2078.


The overall impression of the filling is positive.

Stated characteristics

Frequency range: 26.965…27.855 MHz
Number of channels: 120 AM/FM
Transmitter power: 10 W
Power: 13.8 V
Dimensions: 138(w) x 40(h) x 125(d)
Weight: 600 gr

Modifications
About improvements to MJ-300.

Bottom line
MJ-300, like , is not suitable for all tasks. They share the market harmoniously and should be used in their intended niches. Due to the fact that the station has only one noise suppressor, an automatic one, using it in the truckers channel (15AM) is not advisable, since it will simply irritate you with its constant triggering for no reason. If they offer it to you for Channel 15, don’t take it! For these purposes, a threshold noise suppressor is most suitable, as, for example, in the MJ-100. The path of the MJ-300 is either work in a taxi, or use for other tasks where frequency modulation is used.

Introduction

1. Brief technical specifications and distinctive features

2. Main features and functions

3. Controls and displays

4. Basic modes

5. Small-size LCD display

6. Installing a radio station in a car

7. Installation and configuration of a car antenna

8. Rules for using the radio station

INTRODUCTION

Thank you for purchasing the MegaJet MJ-300 car radio. The MegaJet MJ-300 radio is easy to use and will provide you with safe, fast and reliable communication. Please read these instructions carefully before use. The information presented here will help you get the best performance from your walkie talkie.

The Megajet MJ-300 radio station belongs to the class of equipment of accessible civil radio communications range.

1. SAFETY INFORMATION

The following precautions must be observed when operating, servicing, or repairing this device.

Repair and maintenance of the MegaJet MJ-300 radio must be carried out by qualified personnel in a specialized service center.
Do not make any changes to the design of the radio!
Turn off the radio before entering areas where explosive or flammable substances are present.
Do not use walkie-talkies with a damaged antenna. If a damaged antenna touches parts of the body, there is a high risk of getting burned.
To avoid electromagnetic interference or compatibility problems, turn off your radio in areas where you are required to do so, especially where there are signs reminding you to do so.
Turn off your radio before entering a blasting area.
For vehicles with airbags: Do not place the radio in the airbag deployment area or directly on the airbag covers.

Functionality of the MegaJet MJ-300 radio station

Compact design
Microprocessor SAMSUNG 3P9228AZZ-QZR8
Separate frequency synthesizer on the LC7152N chip
Front panel with small LCD display
High power speaker (8 Ohm, 3 W, square 76x76 mm)
Reliable PTT with electret microphone
Scan mode (SCAN)
Emergency tuning to channel 9 (CH9)
Possibility of connecting an external speaker

1. BRIEF TECHNICAL CHARACTERISTICS OF THE MEGADJET 300 RADIO STATION

2. KEY FEATURES AND FUNCTIONS

FUNCTIONS

• Two kinds
• SC scan button
• Channel 9 Emergency Call (switch)
• Push-button control of UP/DN channels
• Non-volatile settings memory
• Adjustable automatic noise reduction system
• Built-in input attenuator

CONTROL

• Volume control with power switch
• Noise reduction threshold control
• Channel up/down buttons
• Emergency call button channel 9
• Scan by channel button

LCD DISPLAY

• Display CHANNELS/GRID/SCAN
• Indication of TX / SC / / L / H modes

3. CONTROLS AND DISPLAYS

Description of controls

1. 4-pin connector for connecting a PTT (headset)
2. Power on/Volume control VOL/OFF.
3. Threshold level regulator...
4. LCD display. All current radio operating modes are displayed.
5. Button for selecting the type of modulation AM / FM (AM/FM).
6. SC scan button. This button can be used to search for a channel where there is intense radio traffic. After pressing this button, the channels are scanned upward. When a signal is detected, scanning will be paused. After reception is stopped, scanning resumes after 10 seconds. Pressing transmit will stop scanning.
7. Button for quick switching to emergency channel CH9 / switching. This button is designed to quickly switch to channel 9 in the event of an emergency. If the radio is operating in multi-grid mode, this button switches grids. When pressed for a long time (more than 1 second), the built-in attenuator is turned on to weaken the received signal. In this case, the symbols “Lo” will be displayed on the screen. When you hold down the CH9 button again, the mode is turned off, and the “Lf” symbols will be displayed on the screen.
8. Channel up button UP
9. Channel down button DN

Description of the connectors on the rear panel of the radio

SC - Scan. Indication of the “SC” sign means the receiver is operating in automatic scanning mode. To implement this function, turn on the power of the radio and set the required volume. Adjust noise reduction. Press the “SC” button and scanning will begin. “SC” will appear on the LCD display. When a channel is detected whose signal exceeds the squelch level, scanning stops and resumes again after the signal disappears, until the PTT or “SC” button is pressed.

M,L,H – in multigrid mode, displays grids C, B, D, respectively.

5 – displays the inclusion of the frequency shift mode at -5 kHz (Russian standard)

6. INSTALLING THE RADIO STATION IN THE CAR

The MegaJet MJ-300 radio station is intended for use in cars with a 12-volt battery and a grounded negative. should be sufficient both to power the radio station and for the needs of automotive electrical systems.
Before installing the radio, check that your vehicle meets this requirement.
A mounting bracket and a microphone holder are used to mount the radio in a car.

INSTALLATION LOCATION OF CB RADIO STATION

The radio station is installed in the car in such a place that its use does not create inconvenience and does not distract the driver from driving the car. The best place for this purpose is under the dashboard of the car.

Attention: When choosing a location for installing the radio, make sure that it does not interfere with the driver or impair access to vehicle controls. When laying connecting cables, observe safety requirements. If the radio station or connecting cables are inconveniently located, you may lose control of the vehicle.

MECHANICAL WORK DURING INSTALLATION.

Step 1: Using caution, use the mounting bracket as a template to mark the mounting holes under the dash. To mark drilling locations, use an awl or other sharp tool designed for marking on metal.

Step 2: Drill two 3mm holes for each mounting plate mounting screw. Secure the bracket under the dash with the included self-tapping screws (see Figure 1). Particular care should be taken when drilling holes to avoid damaging the connecting harnesses and electronic devices located under the panel.

Fig.1. Attaching the radio using a mounting bracket.

RADIO STATION INSTALLATION.

Step 1: Insert the radio into the mounting bracket until it is aligned with the tabs (see Figure 2). Adjust the radio body to the optimal angle for ease of access.

Step 2: Attach the radio clips, making sure to provide a place to connect external cables to the rear of the radio.

INSTALLING THE MICROPHONE HOLDER.

Next to the radio station on the car dashboard, drill two holes to install the microphone holder bracket. Secure the holder with two 10 mm self-tapping screws from the kit to the radio.

CONNECTING AN EXTERNAL SPEAKER (not included in delivery).

On the back wall of the radio there is a socket for connecting an external speaker "EXT-SP". Insert the plug from the external speaker into it. This turns off the built-in speaker.

POWER CONNECTION.

Since the MegaJet MJ-300 radio station is equipped with a power supply filter to eliminate interference from the engine ignition system, it can be connected to the vehicle on-board network at any point. To reduce interference, it is better to use the wire in the form of a twisted pair.

Step 1: Disconnect the power cables from the battery terminals to prevent short circuits that may occur when connecting power to the radio.

Step 2: Securely connect the black “negative” wire from the radio directly to the vehicle body. For best operation of the radio, reliable contact with the metal of the body is required.

Step 3: Connect the red “positive” wire from the radio with the built-in fuse holder to the fuse block (“cigarette lighter” or directly to the positive terminal of the battery). Typically, the most convenient point to connect the radio is the fuse box. You can connect the power cable to the ignition switch contacts, in which case the radio will turn off automatically when the ignition is turned off, which will prevent accidental battery discharge.

Step 4: Reconnect the power cables to the battery terminals. Connect the power cord plug to the radio cable connector.

7. INSTALLING AND CONFIGURING A CAR ANTENNA

In general, there are two types of antennas for mobile CB radios - a full-length 1/4 wavelength whip (2.75 m) and a short matched whip antenna (from 0.5 to 1.9 m). Due to the large length of full-size antennas on cars, shortened antennas are mainly used in various designs with mounting through a hole in the roof, on a bracket behind the gutter flange or on a magnetic base (see Fig. 3).

Rice. 3. Typical installation locations for car antennas.

Antennas on a magnetic base have the advantage that they can be easily retracted inside the car when parked, and the strength of the magnet ensures its secure attachment during shaking and high-speed driving.

Its radiation pattern depends on where the antenna is installed. When installing the antenna in the middle of the roof, the radiation pattern approaches circular. If the antenna is installed on the right edge of the roof, then its maximum gain will be directed to the left of the vehicle axis. When placing the antenna on the rear trunk, its pattern will face forward.

In Fig. Figure 3 shows five typical installation locations for a car antenna: (1) rear bumper, (2) rear trunk lid, (3) water drain flange, (4) hood, (5) roof.
To receive qualified advice regarding the choice of antenna type and location of its installation, contact your retailer.

The installed car antenna must be tuned to resonance at a medium frequency. To configure and periodically monitor the antenna and antenna cable, a meter is used, which is connected to the circuit between the radio station and the antenna cable using a separate adapter cable of minimum length. Different types of antennas are configured in different ways, so you should read the instructions. As a rule, tuning an antenna that has reliable galvanic (or capacitive - across the entire area of ​​the magnet) contact with the conducting underlying surface (the area of ​​the cabin roof) at its location point is carried out by decreasing or increasing the length of the pin. Try to achieve a minimum SWR (one is ideal) in the middle of your chosen frequency range (for example, between the two most frequently used channels). And if, at the same time, at the edges of the “working” range you manage to get an SWR of no more than 1.5, then your choice of channels is correct and the antenna is tuned perfectly.

Carefully follow the manufacturer's antenna installation instructions.

Attention! Never turn on the radio station for transmission, even for a short time, with an untuned antenna or a “out-of-tune” antenna, when reliable electrical contact of the cable braid and the antenna “ground” with the conductive underlying surface at the antenna location is lost. This can lead to damage to the radio and electrical equipment of your car!

8. RULES FOR USE OF THE RADIO STATION

• Turn on the radio station by rotating the “VOL/OFF” knob clockwise. Set the required sound volume.
• Adjust the noise reduction threshold using the “SQ” control in accordance with the instructions in this manual.
• Select the desired channel using the UP/DN channel buttons.

Note: If you are not changing channels, please note that the emergency channel mode may be enabled. This mode is disabled by pressing the CH9 “Channel 9” emergency button.

TRANSMISSION MODE.

Press and hold the “PTT” switch on the microphone body. Hold the microphone body 5 cm from your mouth and speak in a normal, intelligible voice.

RECEPTION MODE.

Simply release the PTT switch and listen to your correspondent, using the volume and noise reduction controls to achieve the best sound quality.

Megajet 300 is a truly unique device in terms of reliability/failure statistics and price/quality ratio. Simplified to the extreme by the manufacturer, the radio station, however, retained the advantages of its older brothers (MJ 600, MJ 600+, MJ 800): an excellent receiver, automatic noise reduction, and a reliable push-to-talk switch. The device does not break because there is simply nothing to break in it. Among the disadvantages of the M300, it is worth noting the following:
1. Lack of non-volatile memory, which makes it necessary to restart the device after each power outage in order to open. The device opens by holding down the AM/FM and Scan buttons at the moment you turn on the power with the volume knob and about a second after it.
2. The device has only three grids (C,D,E Alan"a/Megajet"a, grid C is indicated by the letter L, grid D is without indication, grid E is indicated by the letter H). Switching the grids of the opened device is done by pressing the CH9 button.
3. Lack of ability to switch channels from PTT.
4. Lack of S-meter.
5. Unlike its older brothers, MJ 300 has an output power of about 7 Watts (older brothers - 10 Watts, which is optimal for driving amplifiers such as KL 300, KL 500, etc.) However, this is not significant.. .
6. The device "from the factory" has an absurdly quiet and muffled modulation. However, this shortcoming can be easily corrected. Below are two options to solve this problem.

Option 1

In the photo below you can see the printed circuit board of the device before modification (two red circles circle the places where you need to fit):

You need to do the following:

Following arrow 1 in photo 2, you need to remove the capacitor located between the direct and inverting inputs of the operational amplifier.
Along arrow 2, remove the capacitor that shunts the output of the microphone amplifier to ground, and the resistor marked “272” (2.7 kOhm), which is in series with the output of the microphone amplifier in front of this capacitor (they form an RC low-pass filter, which additionally spoils the modulation). Place a jumper instead of the resistor.
In the direction of arrow 3, turn the deviation trimming resistor to the maximum. In the latest batches of MJ 300, these resistors rotate freely a full turn, since they do not have clamps in the extreme positions. This complicates the task somewhat: You do not know which position of the trimmer is the maximum volume. This is where you have to get confused. It is necessary to have another one next to the radio station you are tuning, included in the same channel. Turn on the radio station you want to tune in and turn the trimming resistor. The modulation volume will increase when turning in the right direction, and then suddenly disappear completely. At this moment, it is necessary to turn the trimmer in the other direction, but just a little, until the modulation returns. This position is the level of its maximum volume.
If you are going to rarely work in AM, or are ready to put up with the fact that when you release the PTT switch in AM, your radio station will start making a short squeak (this does not always appear), then in order to increase the service life and reliability of the PTT switch many times over, it is worth modifying it. Here is the view of the PTT before modification:

Photo3

The microphone circuit in the push-to-talk switch is switched and closed only for transmission. Considering that this circuit is very low-current and sensitive to any bounce of contacts, any minor dirt in the receive/transmit switch, on its contact group, will make working with you on the air disgustingly uncomfortable. At the same time, you will perceive everything as before and will not even understand why people keep asking you questions. The easiest way is to short-circuit this contact group. Photo 4 shows how to do this:

Photo4

That, in fact, is all the modifications - remove two capacitors and one resistor, install two jumpers, twist one tuning resistor.

Option 2

So that you can speak while holding the PTT switch of the Megadjet-300 radio station at a distance of several centimeters, and not pressing it close to your mouth, add the gain of the microphone amplifier - change the 330 kOhm resistor between the 6th and 7th legs of the 4558 microcircuit to a resistor of about 5 MOhm and adjust the deviation trimmer resistor next to LC7152 (see photo 5).

Photo 5

The radio station has recently become quite widespread and enjoys well-deserved popularity due to its simple design and inexpensive price tag. It appeared on the market as a Chinese competitor to the Korean MJ-300, and in my opinion successfully competes with it. Statistics on failures during their sales are already available, which made it possible to systematize them. Many of these faults are also inherent in the Korean Megajet. But today we will talk specifically about the Chinese megajet.

1 - The most common burnout of this resistor is R114 face value 3.3 ohm. Sometimes it does not burn out, but peels off from the board due to heat. With this malfunction, the radio transmitter either works periodically or stops working altogether. Only nearby radio stations can hear you, since the output stage of the transmitter does not work, only the preliminary stage is turned on. As a rule, this malfunction is a consequence of a faulty or misconfigured antenna.

2 - Failure of the output transistor of the transmitter power amplifier. In the diagram it is indicated as Q38, the type of this bipolar transistor 2SC2078. Fails due to the antenna, high supply voltage or power connection with incorrect polarity. It often fails together with resistor R114.

3 - Protective diode D24 type 1N4002(protective diode) fails if the power polarity is incorrectly connected. Works in conjunction with a fuse on the radio power cable. The fuse rating should be 5A. When installing a “bug” fuse or a fuse with a higher rating instead of a fuse, as a rule, when the polarity is reversed, other parts of the circuit fail.

4 - Exit from standing ULF chip, IC1 type TDA2003, the phenomenon is quite common. The fact is that this microcircuit takes part in the operation of both the reception path and the transmission path in AM mode. Therefore, any malfunction in these paths can cause failure of this microcircuit. In addition, power is supplied to it almost directly from the on-board network, so faults in the power supply circuit also disable this microcircuit.

5 - Modulator transformer, like the ULF microcircuit, takes part in both reception and transmission, so they usually fail “in pairs”. The failure of the transformer can be easily determined by the increased current consumption in AM reception mode.

6 - Voltage stabilizer chip U11, denomination L7808CV As a rule, it fails due to problems in the power circuits or load circuits of this microcircuit.

7 - Failure volume control It is quite rare at new stations. As a rule, its cause is mechanical wear of the resistor track, or the mechanical power switch paired with this resistor. This malfunction appears after several years of operation.