Mobira MD59 modifications for radio amateur use (draft version)

NOTE: This is a translation from a finnish document made by OH3JGV. I haven't modifed MD59 myself so I can't be sure if I had understod everything right.

1. Buy a MD59

2. Open the radio. Note the right connection of power connector

3. You will need these:
- Ham software (from
- New antenna switch which replaces the original diplexer
- Crystal 21.401 MHz (original specification: SA 11B)
- Some ceramic capasitors (about 10 nF)

4. Replace the original software with ham software. You can find the righ EPROM like this: Open that side of radio where there is handset connector. Open the screws of the audio board where that handset connector is. Pull the PCB carefully out. Under that board you can find the CPU board. The EPROM that you should replace is the centermost IC of those big IC:s.

5. The repeater beep tone (1750 Hz). Take the audio board. Put it in the table so that handset connector points away from you. In the left upper corner there is an IC. Next to that there is a 4066 IC. Cut the pin number 10 of that IC. Solder pins 11 and 12 together. Locate the "0-dev"-trimmer. Find a piece of wire and connect that pin of trimmer which is closest to you to the ground. That's all.

6. Remove the duplexer (the big metal can). Take those SMB connectors from that board. It is easier to desolder those if you cut the brass and warm only a small piece of brass instead of the whole diplexer. Take that coaxial cable too.

7. Turn the rig up-side down.

8. Locate and replace the crystal. (I don't know where it is...)

9. Detach the centermost PCB (VCO board). Locate the pipe trimmer condensator in the left border of PCB. Turn the PCB so that the solder side is up. Add about 10 pF capasitance from the center pin of pipe trimmer to the ground. Attach the board again and connect the handset. Power the radio. Select the frequency of 433.000. Measure the voltage against ground from the pin of (ligh square shaped) condensator which is next to pipe trimmer. It should be between 0.7 and 9V. Try to align pipe trimmer. It should change the voltage. If it effects to the voltage everything is OK. If not then you have to change the capasitance value of that capasitor which you just added. You can find out if the capasitance value is too small if you increase the frequency the voltage should change. If it does not you propably have too much capasitance. You should end up to situation where the control voltage is in tha range when the frequency is in the area of 425-435 MHz and the timmer is in about the middle position.

9a. Squelch modification: In the left side of rig there is an RX board. Solder 0.47 uF electrolytic capasitor between pins 11 and 13. Minus to that pin that is minus in the board too. Plus to that pin which is the second from left.

10. Build a new antenna switch board. The idea of the board is to switch antenna between RX and TX. You can use relay but we have used mainly PIN-diodes. Design some kind of band pass filtering to antenna branch to improve RX blocking and to decrease spurious emissions.

11. Control voltage for antenna switch cam be build in this way. Find the pin number 1 of power aplifier boards IC TAA765. This is the place where you can take a coltrol voltage. Build this circuit:

From pin 1 <---RFchocke---+---Resistor---+---RFchocke---> to TX connector
                          I              I
                         Cap            Cap
                          I              I
                         gnd            gnd

12. Now we try to get some power out from the radio. Connect a power meter and dummy load to antenna connector or preferably directly to TX output connector in diplexer pit. Measure the VCO voltage as peviously. Set the low output power by pressing CS until in the display you see x0. x is the volume setting. Set the frequency of 434.575 (the highest frequency where you want to transmit). Press M-key which is PTT. Check the VCO control voltage. Turn the trimmer so that voltage is just below 9V. Release the PTT. Check if the VCO gets locked in the lowest wanted frequency for example 433.400 MHz.

12a. When the PLL stays in lock both in TX and RX we can move on. Set the power level 1. Set the frequency to the middle channel. Turn every helix can one turn in. Then you can try to transmit. If you are lucky you can have some milliwatts out now. If not try to adjust the uppermost capasitor in TX board. Adjust the rest of capasitors to maximum power.

13. That's all folks.