Magnum 357DX: Review, Conversions, Adjustment Locations, Control, Main PC Board Layout


The best way to start this review is to give a little background on the radio and the part my customers played in developing the Magnum 357DX of today. The first Magnum 357DX radios were developed in 1998 and didn’t have memory channels.

Review Of The Magnum 357DX

In addition, instead of the step function used to change the frequency in 1KHz steps, the radio had a 5KHz button that jumped (+) and (–) from the center frequency, changing the full frequency display to a +5 or –5 display.

It also required the call button to be pressed as the radio power was turned on to access the extra frequencies.

Around a dozen were produced and sold through Bob’s CB & Wireless. Although it is more difficult to use than the newer version, it’s a fine-performing radio.

These radios were developed with the same idea in mind: a small radio deck with a separate RF deck that mounts in a remote location up to 9 feet away.

This setup delivers over 150-Watts PEP without the space required by other high-powered radios in the 100 to 150-Watt category. Being fully integrated, there is no RF sniffing circuit that keys the amplifier once power is present at the input.

Instead, the unit is hard-wired to the push-to-talk circuit. This eliminates a choppy RF signal on the sideband, as well as the delay before the receiver signal is present once the transmission is ended.

Additionally, the ALC (automatic level control) is sensed from the amplifier output. This is the best way to control the output and prevent over-driving the amplifier stage.

The original 357DX has an 8-pin microphone connector at the RF deck end of the cable. The new 357DX has a computer-grade D-connector with three shielded coaxial connections at each end of the cable.

It was interesting to be involved in beta-testing these radios for Magnum.

Bob’s CB & Wireless was able to offer a number of pre-production radios at a substantially reduced price to customers interested in being part of the evaluation program. The customer feedback is what molded the final product.

This is one of RF Limited/Magnum International’s ways to ensure the public gets what they want. Both decks are supplied with mounting brackets. The microphone is an electret condenser type with a wide frequency range and with more output than needed to drive this radio.

Inside the microphone is a modulation potentiometer, which can be adjusted for your particular needs. If your vehicle has a high cabin noise level, the gain can be lowered.

If the noise level is low and you prefer talking away from the mic, a higher level can be set. The mic also has up/down buttons for frequency selection.

Normally, they will change channels in 10KHz steps, but they also work in the step command changing the flashing digit on the display. The CPU changes functions under different conversions.

The radios can be converted in many ways for worldwide markets, and the CPU can be used in many radios. One conversion is the CB-only conversion.

This is a 40-channel-only conversion used on their CB radios. The next conversion is the full coverage conversion with limited 10-meter functions.

The step button only covers 1KHz steps, and the split function doesn’t work with this conversion.

The upside is that the radio turns on to the last frequency used. The third conversion is the most common conversion. This conversion maintains the 10-meter functions as they are out of the box.

However, each time the radio is turned on, the radio powers on for 10 meters regardless of the last frequency used. The function/clarifier control must be momentarily pressed, and then the call button must be pressed for 2 seconds in order to access 11 meters.

Once memory channels are programmed in the 11-Meter band, the process is easier. Turn on the radio, load the 11-meter channel, and go from there.

Until one or the other procedure is done, the radio won’t go out of the 10-meter band. This may be desirable if you’re concerned about revealing a converted radio. The following table lists control locations, functions, and instructions for the Magnum 357DX. They are also applicable to the Magnum 257.

Magnum 357DX Control Descriptions

Magnum 357DX Control Locations
Magnum 357DX Control Locations
  1. Microphone Jack
  2. Microphone Gain Control
  3. RF Gain Control
  4. STEP/NB/MEM 1
  5. LCR/RPT/MEM 4
  6. CALL/MEM 2
  11. Frequency Selector
  12. On-Off/Volume Control
  13. LCD Display
  14. Squelch Control
  15. RF Output Control

Mic Jack

A 6-pin connector with a locking ring. Power to the EMC element, TX, RX, TX Audio, and Up/Down functions are controlled through this jack. The mic wiring is as follows.Pin 1 = Microphone Audio InputPin 2 = Receive N/C Switch ContactPin 3 = Transmit N/O Switch ContactPin 4 = Channel Up/Down InputPin 5 = GroundPin 6 = 13.8 Volts


Microphone Gain Control

This controls the gain of the microphone, allowing optimum setting of modulation for any situation.

RF Gain Control

This controls the gain of the receiver, enabling the operator to reduce background noise to a minimum while still hearing stations above the background level.


This allows the frequency to be controlled by 1KHz, 10KHz, or 100KHz steps. Each time it is pressed it flashes the next digit controlled. The frequency selector or the microphone Up/Down buttons change that digit. To exit from the step mode, press the step button until there are no flashing digits or momentarily press the microphone PTT button. In 11-Meters it only allows 1KHz steps.

The noise blanker reduces or eliminates pulse-type electrical interference. Momentarily press the CLAR/FUNC switch, then press the STEP/NB/MEM 1 button.MEM 1: Memory Channel 1. A frequently used frequency and mode can be programmed into this memory position 1. See (9) M.SAVE and M.LOAD for a description of how to save and load memory channels.


Last Channel Recall. Press the LCR button to return to the last channel transmitted on for 3 seconds or more.

RPT: Repeater Access Tone. Most repeaters require an 88.5 Hz tone burst for access. To activate this tone, momentarily press the CLAR/FUNC switch, then press the LCR/RPT/MEM 4 button. To deactivate, repeat the process. This feature doesn’t work in 11 meters.MEM 4: Same as MEM 1 except functions in position 4 of memory.


This button jumps the frequency to 29.300MHz FM in the 10-meter mode. It is also used with the function switch in one conversion to toggle between 10 and 11 meters (See Conversion Article). In both conversions, it is used as a band selector. Each time it is pressed, the radio increments one band of 40 channels.

In addition, in the function command in converted mode, momentarily pressing the CALL button will toggle the display from frequency to channel display.

MEM 2: Same as MEM 1 except functions in position 2 of memory.


Each time this button is pressed, it steps to the next operating mode and is indicated on the front display. Modes of operation are FM, AM, USB, and LSB.T. LOW: Tone Low.

Momentarily press the CLAR/FUNC switch and then press the MODE/T.LOW/MEM 5 to toggle the tone on and off. This low-tone filter cuts the high-frequency response, substantially reducing static noise. MEM 5: Same as MEM 1 except functions in position 5 of memory.


Frequency Scan. Scans in increments of 10KHz. There are two ways to scan using the front panel controls.

1) Receive Audio On Scanning. With the squelch control fully counterclockwise, press the SCAN button. The scan rate is one step every 5 seconds. To stop scanning, press the scan button again, or momentarily press the PTT button on the microphone (scanning will stop without transmitting).

(2) Receive Audio Mute Scanning. Carefully rotate the squelch control until the receive audio is off. Press the scan button. The radio will scan at a rate of five frequencies per second.

When a signal breaks the squelch, the scanning will pause and resume once the signal drops below the squelch level. Scanning can be stopped in the manner described above.SHIFT: Shift Offset. This button is used to program and enable the shift offset feature to operate repeater networks.

The shift offset allows the 357DX to transmit and receive on different frequencies. This feature doesn’t work in 11 meters. To program the offset, momentarily press the CLAR/FUNC switch then press the SCAN/SHIFT/MEM 3 for 3 seconds. The three-digit offset frequency will appear on the LCD.

Rotate the frequency control until the desired offset frequency is displayed. This is the amount that will be added or subtracted from your receive frequency during transmission.

To exit this function, repeat the steps above that enabled this function or momentarily press the PTT button on the microphone (the transmitter will not be engaged).To activate the programmed offset frequency, momentarily press the CLAR/FUNC switch, then press the SCAN/SHIFT/ once. +SHIFT is displayed on the LCD.

This indicates the offset programmed will be added to the displayed frequency during transmission. Repeat the step to display -SHIFT to subtract the programmed amount from the displayed frequency during transmission. Deactivate this feature, momentarily press the CLAR/FUNC switch, then press the SCAN/SHIFT/MEM 3. Repeat until the SHIFT indicator is no longer on the LCD.MEM 3: Same as MEM 1 except functions in position 3 of memory.


Memory Load (recall). To load or recall any saved memory channels, press the M.LOAD/M.SAVE button. A L will appear on the LCD next to the frequency for a couple of seconds. While the letter is displayed, press one of the desired memory buttons 1-5.

The programmed mode and frequency will be entered and displayed on the LCD.M.SAVE: Memory Save. To save a frequency and mode to memory, select the desired mode and rotate the frequency knob to the desired frequency. Momentarily press the CLAR/FUNC switch, then press the M.LOAD/M.SAVE button.

An S will appear on the LCD next to the frequency for a couple of seconds. While the S is displayed, press the desired memory channel button (1-5). The mode and frequency are now saved in that memory location. If the S disappears before the memory button is pressed, the information will not be saved, and the process must be repeated.


Clarifier. The clarifier shifts and tracks both the TX and RX frequency approximately +/- 1KHz from the center frequency. This small range makes it easier to fine-tune sideband signals. If a larger shift in frequency is desired, see (4) STEP feature.FUNC: Function.

This shifts the function of the six front panel buttons to the blue function name imprinted on the button. A momentary press of the clarifier control enables the function feature, and FUNC is displayed on the LCD.

The function command will deactivate once one of the six buttons is pressed or the clarifier control is pressed again. Remember, the CALL button has a function feature once the radio is converted.


Rotate to select the desired frequency. It is also used to select the desired repeater offset in the (5) RPT feature.


Turns the power to the radio on and off.VOL: Adjusts the receiver audio gain.


Liquid Crystal Display. This displays frequency, S/RF meter, FUNC, SCAN, RPT, +/- SHIFT, NB, T. LOW, TX, MEM L & S, mode, and once converted, the channel number and last frequency digit or 1KHz position. It’s the information center of the 357DX.


Squelch. Used to eliminate background or “white noise” when monitoring strong signals. Also used to activate (8) SCAN feature. To properly adjust the squelch control, rotate clockwise until the received white noise is muted. For best results, rotate the squelch control to the minimum amount necessary to mute the white noise.


Variable RF Output Power. Rotate clockwise to increase RF output power. Rotate counterclockwise to reduce the output power. Used for close proximity communications or matching drive input to linear amplifiers.

If the button press beep is not desired, press the microphone PTT button while turning the radio on. To turn the beep on, repeat the process. The S/RF meter is an LCD bar graph that has many segments, but they are grouped into five active segments. It’s equivalent to 5 segments on the meter.

An external analog meter can be added with a little modification, covered in the Workman MD-4 article of this issue.

The Magnum 357DX memory backup will hold the memory information for about a week once disconnected from its power source.

Having used both the pre-production and production versions of this radio, the newer version with memory channels is much more user-friendly.

The older pre-production radio did have a couple of benefits over the newer version, though. The 5KHz shift was very useful and quicker to use than the step feature. The receiver seemed a tiny bit quieter as well.

I’ve used a 357DX with a roof-mount Wilson 5000 in my mobile for two years.

It has performed remarkably well with no problems. Although the stock mic works very well, I decided to modify an Astatic 575M6 by adding UP/DOWN buttons and eliminating the battery by adding regulation and filtering to the battery input connection.

The microphone receives its power from pin 6 of the mic jack and is regulated to 10 volts, supplying the mic preamp. I’ve recently replaced the 357DX with a 257, which is the same basic radio without the RF deck.

I have modified the stock mic by adding a longer, more flexible cord that will reach across from the passenger side to the driver’s side and adding a tone control inside the mic. The stock mic is much smaller and lighter than the 575 mic. I’ve asked the regulars I talk to how the mic sounds.

Everyone says the same thing, “I can’t tell the difference.” The radio is mounted inside the passenger side dashboard with the MD-4 meter next to it. The meter is about two-thirds the size of the radio and makes for easy reading from the driver’s side.

My good friend Willie did the custom installation job. This is something that can’t be done with larger radios or in the newer cars with airbags in the dash.My decision to use a Magnum 357DX was because of the performance, not the size.

It has one of the most natural-sounding sideband receivers I’ve heard in this market. Early on-air testing brought questions from everyone about what I was running. Many operators thought it was a big-name HF rig. When they came into the store to see it, they looked with amazement.

Some jokingly questioned my honesty. If you’ve heard one, tuned properly, you know what I mean. Out of the box the SSB output just hits 150-Watts PEP, FM is 100-Watts, and AM is 50-Watts dead-key with a swing to 120-Watts. The RF power control works in all modes, and the range is very good.

If you intend on using an additional amplifier, it must be designed to work with a 100-watt radio. All the commercial-made amplifiers for amateur radio will do fine.

The majority of black market CB amplifiers will get cooked, as this radio, like other 10-Meter 100-Watt and above radios, produce instantaneous spikes of high power even at low power settings.

If you want to drive a linear amp, use a Magnum 257; it will sound better and last longer. One thing I’d like fixed is the RF gain circuit. It’s not only the Magnum radios that work in this manner; it seems all 10-meter radios share this characteristic.

Most of the reduction in gain occurs in approximately 1/8 of the control rotation. In mobile operation, especially, it would be far better and easier to set if the gain were more evenly controlled.

In these heavy skip conditions we’re currently in the middle of, the RF gain is a great tool to reduce the background level without fear of missing a call. This is another great radio from the makers of the Clear Channel Corporation Ranger AR3300 and AR3500, the first 100-watt 10-meter radios.

Magnum 357DX Conversions

As mentioned in this month’s review of the Magnum 357DX, there are different conversions that can be performed on this radio. Two will be covered in this article.

Remember to always disconnect electronic equipment from the power source before doing any modifications.

The most common and simplest conversion is cutting the diode lead on the front panel CPU PC board. You can see the diode location in the photo below. After the diode is cut and before the power is applied to the radio, short out the two Reset Pads illustrated in the photo below. The conversion is now complete.

Every time the radio is turned on, the radio comes up in the 10-meter-only band. All the functions operate as described in the Operating Manual. To toggle to 10 & 11-Meter operation, momentarily press the CLAR/FUNC control, then hold the CALL button for 3 seconds and release.

This will be required each time the radio is powered on until an 11-Meter frequency is programmed into memory. If an 11-Meter frequency is in memory, simply load it in as described in the Magnum 357DX review or Operating Manual.

The diode conversion retains the three-position STEP command in the 10-meter-only band. Each time the step button is pressed, one of the digits on the LCD frequency display right of the decimal point flashes. Either the UP/DOWN buttons on the microphone or the frequency selector on the front panel will change the digit from 0-9. Only that digit will change under this command.

The next digit to the left will remain unchanged while in this mode. The operator can select and change the 1KHz, 10KHz, or 100KHz digit while it is flashing. To exit the STEP command simply press the step button until there are no flashing digits or momentarily press the microphone PTT switch.

The second conversion is the full coverage conversion. The downside to this conversion is that the radio can’t be toggled back to the 10-meter-only operation, and none of the commands associated with this operation can be accessed.

The upside is the radio powers on with the last frequency used without the additional steps required in the first conversion. You must decide which is best for your needs.

To perform the full coverage conversion, a resistor must be added to the circuit traces shown in the photo below. The diode need not be cut, but if it is already, it won’t have any effect on this conversion.

The spacing of the pads is for an SMT 1/10 watt resistor. The value required for this conversion is 47KOhms. Good soldering skills are needed to perform this modification. The pads are delicate and will lift off the board with excessive heat. A low-wattage tipped iron is essential.

If you choose to use a standard resistor, a 47K 1/8 Watt is preferred for its small size, but a 1/4 Watt is OK. It’s recommended that the circuit traces be followed to other components already attached to them and the standard resistor be soldered to them at those points for added strength.

Mounting a standard resistor to the pads meant for the SMD part means the resistor will have to be mounted vertically. The additional stress in mobile operation could and most likely would damage these circuit traces over time with the vibration of the moving vehicle.

After the resistor is added and before the power is applied to the radio, short out the two Reset Pads illustrated in the photo below. The conversion is now complete.

Magnum 357 DX Front Panel Printed Circuit Board


Magnum 357DX Adjustment Locations

“S” MeterRV-1
Power MeterRV-8
AM Power Maximum AdjustRV-16
FM Power Maximum AdjustRV-14
AMC – AM ModulationRV-4
FM DeviationRV-3
Driver BiasRV-13
Final Bias AdjustmentsRV-11 & RV-12
Carrier BalanceRV-6 Unmarked Near Center Of PCB

Magnum 357 DX Main PC Board Layout