Menu:



Version:

April 16, 2025:
Revised: v1.1

Converting the MRS-1 Portable HF SSB Transceiver

to 80m and 40m for Amateur Radio Operation

This page describes how to convert the crystal controlled Mountain Radio Service MRS-1 portable HF SSB transceiver to VFO operation on the 80m and 40m HF amateur radio bands using the new “SC+M” Si5351a-based SugarCube-Plus digital PLL control board. 

Figure 1 : The completed modified and upgraded MRS-1 radio

Introduction

The Mountain Radio Service MRS-1 portable HF SSB transceiver is a compact portable battery-powered HF SSB transceiver with a nominal output power of 4W PEP. The transceiver was originally designed and built in New Zealand around 1983 for the Mountain Radio Service, an organization consisting of volunteer groups across New Zealand. MRS provided extensive radio communications services to support people engaged in the ever-popular hobbies of hiking, tramping and mountain climbing. Portable HF transceivers were highly desirable for these hobbies which often take place in very remote areas of New Zealand.

The MRS-1 was designed by Vern Lill, a very talented New Zealand engineer, using discrete components. A total of 120 of these were made for MRS using Tait Electronics production facilities. Later in the transceiver's life, a more compact version, the MRS-3, was made. Using a near-identical circuit, around 50 were made for MRS in a diecast case with smaller components. Finally, a commercialized version, the SR-3, was made for a number of public safety and government departments, and several hundred were also sold to international customers. Some remain in service around New Zealand.

The MRS-1 transceivers were extensively used by MRS members and hired out to the public until the Mountain Radio service finally closed down in 2024. It must therefore rank as one of the most successful radio designs after a continuous service life of over 40 years.

Conversion to Amateur Radio

After my successful conversion of two other yellow public safety portable HF transceivers to amateur radio operation, the Codan/Condor 8833 and the AWA TR-105, discussions took place between MRS and Christchurch Branch (“Branch 05”) over the possible conversion of these MRS-1 transceivers. The Radio Frequency Service, the NZ government’s spectrum licensing body, was also consulted during this process to ensure compliance with regulatory obligations.

This webpage describes the original MRS-1 transceiver, provides an outline of the conversion of the MRS-1 to amateur radio use on 80m and 40m, and provides links to instructions about the detailed conversion procedure. The goal was to modify the MRS-1 such that it would provide full coverage of the 80m and 40m bands in New Zealand while continuing to meet or exceed all existing applicable MRS-1 specifications. An important part of this modification process was to also make it as simple and inexpensive as possible.

In practice, I believe these objectives have been met. It’s the sort of project that a small group of hams might like to undertake together in a series of sessions at a local ham club. Based on our past experience, the actual time required for the work can extend out a bit. That's due to the chats, cups of coffee and other socialising going on during the process. But then, that’s a key part of the process, too!

Inside the MRS-1 Transceiver

The MRS-1 is a conventional single conversion filter-type SSB transceiver with a 10.7MHz IF. The block diagram of the original transceiver is shown in Figure 2.


 Figure 2 : The MRS-1 features a VERY high performance 8MHz 7-pole elliptical low pass filter
to avoid the need to fit a series of switched channel-specific bandpass filters.
(Right-click on this figure to view the full scale image)

The four channel capable MRS-1 5W PEP SSB transceiver was originally designed for operation from 1.6 – 6MHz. They were originally fitted with two 3MHz crystal-controlled channels. Another crystal was used in the carrier/BFO oscillator to give Upper Sideband (USB) operation.

An external hand speaker/microphone was directly wired into the radio. A selective calling keypad speaker/microphone was fitted to some transceivers to provide selective calling features similar to those defined in CCIR Rec. 493-4.

The radio featured a black engraved Formica front panel with two mini-banana socket connectors for a 3MHz dipole antenna. The radio had a volume control with power switch, a channel select switch, a ‘battery’ LED to flag flat batteries, and an ‘RF’ LED to show transmitter operation. No receiver clarifier (i.e. fine tuning) was provided.

The radio is built using a single-sided PCB which slides into the very robust but relatively light aluminium case. It measures 280 x 115 x 55 mm. It is showerproof but not waterproof. The transceiver weighs about 1.1kg without batteries, and is compliant with NZPO/RFS RTA-19 (Now AS/NZS4770).

Figure 3 : The MRS-1 is built on a single-sided PCB. The selective calling equipped keypad microphone is seen here along with some transceiver modifications fitted to radios used in fixed mountain hut service.

MRS-1 Receiver

Looking at the block diagram, and starting with the receiver, the antenna signal passes through a low pass filter that is shared with the transmitter and into the AGC-controlled MOSFET RF amplifier

Note: The schematic of the MRS-1 can be found in the Download section at the end of this webpage.

The transceiver’s RF band pass filtering on transmit and receive is provided by a combination of the antenna 3-pole low pass filter and the RF amplifier’s 5-pole input high-pass filter. The signal is then passed to an elaborate 7-pole low pass filter before passing to the first mixer, a double balanced diode mixer.

A crystal oscillator and buffer also fed the first mixer using high-side injection to convert the RF signal to the IF of 10.7MHz. A standard 10.7MHz SSB crystal filter provided most of the selectivity at this point in the IF stage. The IF signal is then amplified by another AGC-controlled MOSFET and a conventional cascode transistor stage.

The signal is then converted to audio using a MOSFET mixer. The crystal oscillator used a crystal either 1.5kHz above or below the IF crystal filter centre frequency of 10.7MHz for USB and LSB respectively.  This is followed by a conventional audio preamplifier and AGC stage, and an LM386 speaker amplifier.

MRS-1 Transmitter

The microphone audio is amplified in another LM386. A diode and FET are used to provide a moderate level of speech compression. This signal is fed to a conventional discrete diode balanced modulator along with a buffered 10.7MHz (+/- 1.5kHz sideband offset) carrier. This generates the double sideband (DSB) transmitter signal. Much of the carrier is removed in this modulator, but further carrier attenuation occurs as this DSB signal passes through the subsequent 10.7MHz crystal filter which also removes the unwanted sideband.

The low level 10.7MHz SSB signal then passes through the double balanced diode first mixer and the 7-pole low pass filter before passing to the first MOSFET driver transistor in the transmitter power amplifier chain. This dual gate MOSFET is also used to provide the ALC function on transmit.

The output of this stage then passes through a two-transistor buffer and on to the final power amplifier (PA) stage consisting of a push-pull pair of HF SSB type power transistors. The temperature of the PA stage is monitored by a diode sensor which reduces the gain of the previous stage if conditions require it.

The transmit PA chain is a relatively broadband design with a nominal 50 ohm output impedance. Measurements show that transmitter power amplifier gain gradually reduces slightly over the 2 – 6MHz design bandwidth.

The output of the PA is connected to a 3-pole 6MHz elliptical LPF. If the channel is below 4MHz, a further 3-pole LPF is added in series with the first LPF using the channel switch. An alternative 2.8MHz LPF was also notionally available to be used should such channels be fitted. These components have not been encountered in any radio to date.

Finally, the transmitted signal passes through an antenna current detector before passing to the antenna connector. This detector connects with a front panel LED to verify transmitter operation.

Original MRS-1 Antenna

A lightweight 3MHz dipole was usually used with the MRS-1 transceiver connected via the front panel antenna panel connector. Since this was cut to frequency, an antenna tuner was not required.


Figure 5 : An original MRS-1 without the keypad microphone pictured with its typical lightweight 3MHz dipole (MRS original photo)


Modification of the MRS-1 Transceiver

Figure 6 shows the block diagram of the modified transceiver.

 Figure 6 : Changes include the addition of the “SC+M“ SugarCube-Plus PLL oscillator and control module. It also handles the OLED display adding a near-linear S-meter and 3-digit battery voltage meter. The antenna lowpass filters are also modified.
(Right click on the diagram to see it at full scale)

As noted earlier, the transceiver was modified for use on the 80m and 40m ham bands. In New Zealand, these cover 3.50 – 3.90 MHz and 7.00 – 7.30 MHz respectively. These band edges are programmed into the VFO but can be reprogrammed by the builder to suit amateur band assignments in other countries. The SC+M VFO was redesigned for the MRS-1 based on earlier SugarCube Si5351a/ATtiny85 designs.

In addition, the original antenna low pass filters were replaced by band-specific LPFs, the 7-pole LPF was returned to permit 40m operation, and the selcall microphone was modified to remove the unwanted functions.


Figure 7 : The new SC+M module is fitted in the location of the original channel crystal oscillator (Top centre) and wiring is run to the MRS-1 and to the new front panel (Right).

You can read a detailed description of the SC+M and obtain construction details here.

Feature Summary:

Currently, the modified MRS-1 supports the following features:

General:
Front panel controls:
Performance:
Other:

SC+M Software

The SC+M VFO firmware was developed from earlier SugarCube-Plus software. Minor changes were required for operation with the MRS-1. Figure 8 shows the SC+M operating in one of the modified transceivers.

 
Figure 8 : At power-up, the software briefly displays the MRS-1 logo before proceeding to display the
frequency and other functions as shown here. This panel was made using a 3D-printed faceplate.

Again, the SC+M VFO and its construction is described in full on its own webpage.

MRS-1 Modification Procedure

The following procedure assumes the availability of the MRS-1 ‘s purpose-designed SugarCube-Plus (“SC+M”) module. See HERE for details.

The modification process includes:

This conversion procedure is also available as a downloadable PDF available in the Download section at the end of this webpage. 

PLEASE (PLEASE!!) read all the relevant pages on the website and the various documents BEFORE you begin the conversion so that you better appreciate the overall procedure and all of the steps involved in the conversion process.
 

Construction of the Front Panel

1. The new front panel is constructed from a front panel PCB and a panel spacer PCB. The latter eases mounting and alignment of the OLED display. Bolt the two PCBs together temporarily and use some hot glue to mount the OLED. The OLED connector should be adjacent to the encoder and Band switch.

2. Remove the front panel knobs from the original Formica panel and clean them before reuse. Record where the microphone cable connections are wired and then disconnect the cable. One of the prototypes had the wiring shown in Figure 9 but this may not hold for all microphones and transceivers. Remove the microphone cable and its grommet from the front panel. Remount the grommet into the new front panel assembly.


Figure 9 : The microphone cable wiring probably follows one of these two diagrams. Carefully check your transceiver before disconnecting the microphone cable.

3. Carefully feed the microphone cable back through the grommet again allowing enough cable length to reconnect it to the transceiver. DON’T reconnect those microphone wires just yet.

4. Loosen the rest of the front panel hardware including the volume potentiometer, channel switch, antenna connectors and the Antenna and Battery LEDs.

5. Remove the original Formica front panel and complete the removal of the panel components but do not disconnect them from the radio yet. Disconnect the antenna connector wiring at the panel connectors to allow the transfer of the connectors to the new panel. Be VERY careful not to damage the antenna LED or its associated resistor (R6 – 220 ohms) or the toroid (L5)!!!!

6. Remove the channel switch and associated wiring and set aside.

7. Mount the new panel using the three original screws. Replace any corroded screws.

8. Refit the volume potentiometer, the two LEDs, and the antenna connectors into the new panel.

9. Resolder the antenna connector wires back in place. Each of the two LEDs can be held in place with a drop of hot glue.

10. Mount the new Lock pushbutton using hot glue. Check that the pushbutton is not rubbing against the sides of the hole.

11. Loosely mount the new Band switch in the panel. This makes it easier to wire later.

12. Fit the ground wire and the two resistors to the rear of the encoder and mount it loosely on the new panel assembly. This makes it easier to attach the two encoder wires later. Note that the encoder used is one of the (cheaper) pulse type encoders, the type that give a momentary closed contact as they rotate. If your encoder outputs remain closed to the ground at the detent, it will not function correctly with this software.

13. Using four thin insulated stranded copper wires, make a cable for the 4-wire cable going to the front panel OLED display. Terminate the OLED end with a four pin 0.1” DuPont socket to match the pins normally supplied with the OLED display. The wires in this cable in the prototypes measured 75mm before soldering.

The front panel wiring will be completed once the SC+M module is installed in the transceiver. For clarity, the wiring details are shown in Figure 10, 11 and 12.


Figure 10 : The additional front panel wiring is shown in this drawing. The antenna current toroid wiring is shown for clarity due to the relocation of the blue antenna wire which now goes to the Band switch.

Note the addition of two green wires and two grey wires between the antenna low pass filters and the Band switch. 

The SC+M module requires additional wiring to the MRS-1 PCB which is detailed later. (Right-click to see it full size) 


Figure 11 : The two antenna lowpass filters can be seen to the left of the SC+M module along with  the rotary encoder, the upper part of the Band switch, and the OLED wiring.



Figure 12 : The lower section of the Band switch wiring as well as the Lock switch and OLED mounting can be seen here.


Preparation for Mounting the SC+M

This is done in two stages, the first being to remove the unwanted crystal oscillator components.

14. Remove the existing Channel Crystal Oscillator components. See Figure 13 for details. The red dashed line highlights the ground track surrounding the oscillator which quite precisely identifies the location of the parts to be removed.

Remove the following parts adjacent to the double balanced mixer (SBL-1):

 
Figure 13 : View of the underside of the MRS-1 PCB showing areas to be cleared of components


15. Install a 100n disc ceramic capacitor, 100 ohm resistor, and 47 ohm resistor as shown in Figure 14.


Figure 14 : Location of new components near the SBL-1 mixer. This drawing also shows the new location of the VFO coax cable tail from the SC+M which is fitted later.

16. Remove the following parts from the Carrier Oscillator (Figure 8):


17. Install a 100nF disc ceramic capacitor and a 100 ohm and 4k7 resistor as shown in Figure 15.


Figure 15 : Mounting location of Carrier Oscillator interface components.

Fitting the SC+M Module

18. Install three thin wires in the corners of the SC+C. Cutoff capacitor leads are one source of suitable wires.
Mount the SC+M in the area previously used by the channel oscillator. The three wires should be fitted into the holes indicated in Figure 16. These connect the SC+M PCB ground securely to the MRS-1 PCB ground. J1 on the SC+M board should be adjacent to the antenna low pass filters. You can see the placement of the SC+M clearly in Figure 11 (above).
 

Figure 16 : Three points are used to secure the SC+M module via short cutoff component leads.

19. Complete the wiring required for the SC+M module as shown in Figure 17. This should also include the coaxial cable from J4 (SC+M) to L17. Use the thin coax removed earlier. The wiring shown in Figure 17 is in addition to the wiring which goes to the front panel described earlier and shown in Figure 10.


Figure 17 : The additional SC+M wiring shown here includes connections to the regulated 6VDC supply, the AGC voltage for the S-meter, and for the VFO and CIO/BFO oscillator outputs.

Modifying the Antenna Low Pass Filters

The operation of the MRS-1 on 80m and 40m requires modification of the antenna low pass filter(s) as shown in the schematic in Figure 18.


Figure 18 : The low pass filters were redesigned for 80m and 40m operation. Some original LPF components are reused in the new filters (See text).

20. Using Figure 18 and 19 as a guide, begin by carefully desoldering and setting aside all the antenna low pass filter parts fitted in the original MRS-1 including any short input and output wires. Take care removing the delicate inductors!

 
Figure 19 : Layout of the MRS-1 antenna low pass filter components

21. Reinstall the larger of the two variable inductors as L1B.

22. Remove the wire used on the smaller of the two variable inductors. Rewind this coil with 12 turns of 0.25mm enameled copper wire. Fit this inductor as L1C.

23. Install the LPF capacitors shown in Figure 16 into locations identified in Figure 17. Some of the components set aside earlier may be used in this step.


Alignment and Testing

1.    Calibrate the SC+M output frequency. Refer to the calibration details in the SC+M build details.
2.    Confirm the programmed band edges and starting frequencies for both bands are correct.
3.    Confirm the CIO frequency is 10.69850 MHz (LSB)
4.    Align the 80m and 40m antenna lowpass filters using a 1kHz test tone into the transmitter. Some
       adjustment of L7, L8 and L9 may be required. The latter requires specialized test equipment and
       only undertaken if absolutely necessary.
5.    Verify the MRS-1 output power is 3W PEP or more across both bands.
6.    Verify the receiver sensitivity (Better than 10dB SINAD for 0.2uV) on both bands


Final Comments

With the alignment and testing completed, the modified MRS-1 transceiver should now be operating correctly.

Despite what may appear to be extensive modification instructions and procedures, this modification is actually a quite straight-forward conversion. The most time-consuming task is probably removing the unwanted crystal oscillator parts.

If you take your time and carefully follow each step, the end result will be a compact portable HF SSB transceiver for use on 80m and 40m.

 

Appendix A : Part List for the SC+M and MRS-1 Conversion



Notes: Multistrand Hookup Wire Length Estimation Calculation




Appendix B :  Modification Details for Jenel Microphone

Some modifications may be required if your MRS-1 has a Jenel microphone. These remove the undesirable tone call functionality which is not required in amateur radio.

These modifications should be carried out BEFORE testing the modified transceiver. If these are not completed, the MRS-1 will probably not get microphone audio and the PTT signal will not work!

1.    Open the back of the Jenel microphone and set aside the three screws.

2.    Remove the Jenel tone control board from the microphone leaving the keypad and its LED PCB in place.

To do this, desolder the ribbon cable from the Jenel PCB and fold the ribbon cable down against the keypad. Also, desolder the red and black wires going to the keypad LED PCB.

3.    Rewire the microphone audio, PTT and ground wires as shown in Figure 1. Use heatshrink to cover the cable joints.

4.    Use a cable tie to retain the spare microphone cable wires.

5.    Carefully place the wires inside the microphone so that the back of the microphone can be fitted and the three screws tightened.

------



Appendix C : MRS-1 AA-cell Battery Modification

The MRS-1 contains a dual battery holder inside the transceiver case. This was originally fitted with eight 1.5V alkaline AA batteries which were inserted into the holder from a screw-on cover on the cover’s base. These batteries provide a nominal power supply to the radio of 12V.

These batteries could continue to be used in the modified MRS-1 transceiver but at a relatively high operational cost.

Alternative rechargeable options include using three (3S1P) or six (3S2P) AA-size LiPo cells. These deliver a cell voltage ranging from 4.2V/cell (charged), 4.0V/cell during operation, and a minimum voltage and load disconnect terminal cell voltage of 3.5V/cell. In this option, the battery system terminal voltage ranges from 12.6V (charged) to 10.5V (recommended minimum/disconnect voltage). Using an arrangement such as a 4S1PS or 4S2P LiPo configuration places excessive voltage on the radio and is NOT recommended.

A battery system using a 3S1P set of the popular 18650 LiPo cells offers a better capacity within the available battery holder space and a reduced weight over the original eight AA alkaline batteries. This is the recommended solution.

Another option is to use four or eight LiFePo4 AA cells. a cell voltage ranging from 3.65V/cell (charged), 3.2V/cell during operation, and a minimum voltage and load disconnect terminal cell voltage of 2.5V/cell. A 4S1P or 4S2P arrangement delivers between 10 and 14.4V for the 100mA (Rx) and 300-700mA (Tx) current load of the transceiver.

All of these options (aside from the AA alkaline batteries) require a charging system as well as a cell-based battery management system and a low voltage disconnect to ensure balanced charging and protection from accidental discharge below minimum recommended cell voltages.

If this is installed, it is also likely the original battery holder hardware inside the transceiver case will have to be removed or extensively modified in some way and the new battery, mounting, and battery management system fitted. This work has not yet been undertaken.



Finally...

First, let me extend my sincere thanks to Ian Gardiner and the team at the (now closed) Canterbury Mountain Radio Service for supplying the Christchurch branch (Branch 05) of the NZART with the MRS-1 transceivers described here along with a number of other items of supporting equipment.

These MRS-1 transceivers which I modified were intensively used during their life in the field. When I opened them up for the modification process, they were very clean and tidy inside, and there were signs that they had been carefully maintained during all those years. Both transceivers worked after a couple of minor repairs. In part, I chose them for that reason. When bench-tested, they met all of their specifications. They are a credit to the designer, builders, and maintainers of these radios.

I’m now quite keen to use one of these bright yellow modified MRS-1 transceivers on some of my trips away to the mountains. They are just one or two hours drive from my home.

I hope to hear you on the air soon!


Downloads

arrowMRS-1 Modification Procedure (A multipage illustrated PDF document)

arrow Gerber files for the front panel and panel spacer PCB (MRS1_Panel_PCB.zip)

arrow MRS-1 ORIGINAL Schematic (PDF file)

arrow MODIFIED MRS-1_Schematic_with SC+M (PDF file)




Want to go back to the main page? Click here to return directly.