Microwave Modules MMT 50/28S Conversion to Four Metres
Having recently purchased a Microwave Modules MMT 50/28S Six Metre transverter from the local radio rally I decided to convert it to four metres. This is the later version of these transverters which is built in a much larger box with a silver front panel and bar graph meter. It should not be confused with the earlier incarnations which were built inside black die cast boxes.
New Basic Specifications.
1) Power requirements 13.8V @ 2.5A.
2) Transverter input 28.000 to 28.500MHz @ 250mW.
3) Transverter output 70.000 to 70.500MHz @ 5W.
4) Single socket (SO239) Transceiver connector.
5) PTT line socket (3.5mm jack).
6) Independent receiver input (SO239) internally switchable.
7) Receiver post mixer gain control (internally adjustable).
8) Front panel bar graph meter which reads Power Output and SWR
which is switchable from the front panel.
9) Front panel Power On and TX indicators.
10) Front panel Power Ouput Level control (0 to 5W).
11) Rear panel Drive Level control.
12) Rear panel DC line fuse (supply is reverse polarity protected).
The transverter is mounted in an aluminium case measuring
245x150x80mm. The main pcb is fitted above chassis and pa below.
The Front and Rear panels and the two printed boards are shown
below after the modifications were carried out.
This is what the front panel looks like. The control at the bottom left is the power power output pot, the bar graph meter on the right reads power and swr controllable by the switch in the middle.
The rear panel shows the aerial socket on the left and the transceiver socket on the right. The Din socket in the middle is for power and ptt line, the ptt line is also available on the jack socket to the left of the fuse holder.
This is the main pcb. The 42MHz crystal can be seen in bottom left corner. The band pass filter is the 3 Toko coils adjacent to each other at the bottom right.
The PA pcb is located under the chassis. The pre PA low pass filter is the 2 toko coils at the bottom and the new post PA low pass filter can be seen at the top.
Main pcb circuit part 1
Main pcb circuit part 2
PA pcb circuit
The above circuits can be used as a guide only as they are for the 144MHz to 50MHz version as I have been unable to trace the correct information for this version. Many thanks to Denis G3UVR for the supply of the info.
The basic changes to the transverter are.........
a) Change local oscillator from 22MHz fundamental to 42MHz overtone.
b) Change the band pass filter from 50 to 70MHz.
c) Change both low pass filters to 70MHz.
d) Re tune all stages.
Because I do not have the correct circuit and the fact that Microwave Modules did not use circuit references for their components makes it rather difficult to describe the changes but here goes.
All of these changes can be done above the printed circuits so there is no need to remove them from the chassis.
Firstly the local oscillator at 22MHz is running in fundamental mode, this needs to be changed to overtone mode when using the 42MHz crystal. Remove to two original crystals (22 and 24MHz), connect the new crystal between the collector of the oscillator transistor (2N2369) and the emitter, a 10pF miniature trimmer can be fitted across the 220ohm resistor to allow fine tuning. NB, do not be tempted to run this oscillator in fundamental mode because although it will still work on four metres it will also produce some nice sprogs!
There is a low pass filter after the local oscillator buffer tansistors consisting of two coils and three capacitors, unsolder the three capacitors at the earthy ends (2x39pF and 1x68pF).
The band pass filter can be re tuned to 70MHz by de soldering the three 15pF capacitors at the earthy ends which are across each of the tuned circuits. This band pass filter is used on both transmit and receive and is easily identified by the relay shown at its input and output on the circuit.
The low pass filter before the PA stage is very different to that shown in the circuit above and can be re tuned to by de soldering the two 15pFcapacitors at the earthy ends across the tuned circuits, this is on a sub board mounted vertically at the front of the PA pcb assembly.
The post PA low pass filter was removed completely and replaced by a simple PI filter consisting of two 65pF trimmers and a 5 turn coil using 18swg enam wire about 10mm diameter.
It should now just be a case of re aligning all the stages being careful to use the correct trim tools so that the cores are not damaged.
The receiver post mixer amplifier stage (2N5109) was found to exhibit far to much gain and this was remedied by simply fitting a 5kohm miniature pre set pot in series with its emitter resistor (56ohm) and adjusting to the required level having regard to the front end gain of the 28MHz receiver being used. The front panel power output level control is simply a 5kohm linear pot wired between the output of the driver and the input of the pa.
There is a directional coupler after the pa which is used to feed the bar graph relative output meter but there is also an unused swr output. A small relay was fitted here controlled by the front panel output/swr switch (formerly range switch) to enable the meter to read swr as well as power output, an additional 100kohm pre set pot was fitted in series with the swr output for calibration purposes.
It would be quite simple to make this transverter dual band to cover both six and four metres but I did not require this.
The 5 watts output is ideal for feeding my mosfet linear amplifier to provide the UK legal output power.
It should be noted that an attenuator will be required if a 10watt 28MHz drive source is used.
The basic changes to the transverter are.........
a) Change local oscillator from 22MHz fundamental to 42MHz overtone.
b) Change the band pass filter from 50 to 70MHz.
c) Change both low pass filters to 70MHz.
d) Re tune all stages.
Because I do not have the correct circuit and the fact that Microwave Modules did not use circuit references for their components makes it rather difficult to describe the changes but here goes.
All of these changes can be done above the printed circuits so there is no need to remove them from the chassis.
Firstly the local oscillator at 22MHz is running in fundamental mode, this needs to be changed to overtone mode when using the 42MHz crystal. Remove to two original crystals (22 and 24MHz), connect the new crystal between the collector of the oscillator transistor (2N2369) and the emitter, a 10pF miniature trimmer can be fitted across the 220ohm resistor to allow fine tuning. NB, do not be tempted to run this oscillator in fundamental mode because although it will still work on four metres it will also produce some nice sprogs!
There is a low pass filter after the local oscillator buffer tansistors consisting of two coils and three capacitors, unsolder the three capacitors at the earthy ends (2x39pF and 1x68pF).
The band pass filter can be re tuned to 70MHz by de soldering the three 15pF capacitors at the earthy ends which are across each of the tuned circuits. This band pass filter is used on both transmit and receive and is easily identified by the relay shown at its input and output on the circuit.
The low pass filter before the PA stage is very different to that shown in the circuit above and can be re tuned to by de soldering the two 15pFcapacitors at the earthy ends across the tuned circuits, this is on a sub board mounted vertically at the front of the PA pcb assembly.
The post PA low pass filter was removed completely and replaced by a simple PI filter consisting of two 65pF trimmers and a 5 turn coil using 18swg enam wire about 10mm diameter.
It should now just be a case of re aligning all the stages being careful to use the correct trim tools so that the cores are not damaged.
The receiver post mixer amplifier stage (2N5109) was found to exhibit far to much gain and this was remedied by simply fitting a 5kohm miniature pre set pot in series with its emitter resistor (56ohm) and adjusting to the required level having regard to the front end gain of the 28MHz receiver being used. The front panel power output level control is simply a 5kohm linear pot wired between the output of the driver and the input of the pa.
There is a directional coupler after the pa which is used to feed the bar graph relative output meter but there is also an unused swr output. A small relay was fitted here controlled by the front panel output/swr switch (formerly range switch) to enable the meter to read swr as well as power output, an additional 100kohm pre set pot was fitted in series with the swr output for calibration purposes.
It would be quite simple to make this transverter dual band to cover both six and four metres but I did not require this.
The 5 watts output is ideal for feeding my mosfet linear amplifier to provide the UK legal output power.
It should be noted that an attenuator will be required if a 10watt 28MHz drive source is used.
