How to turn an old radio into an RF power amp for less than $20
I received a partially working Kenwood TM-2550A mobile rig as part of an estate sale last year. The transceiver did not come with a mic, mount, power cable, or accessories. I was able to get the unit powered up and transmitting, but the receiver was blown on this unit and the microcontroller was damaged from over-voltage. The cost to repair and restore this radio far outweighed the cost to convert it into something more useful. So the best option was to turn this unit into an amplifier to be used with HT's and other low power devices for analog and digital modes such as FM, WinLink, APRS, Packet, DSTAR, DMR, and Fusion.
I already had a great starting point as the RF module was in good shape. The one I scavenged from the Kenwood is a 43 Watt module which appeared to output full power. I reused the heatsink and original mounting holes for the module and the SO-239 connector from the Kenwood. The next step was to either inject a low powered signal in the final stage of the PA amp circuit on the existing radio with a relay, or simply install a new PCB with the T/R circuitry built in. I chose to go with the PCB and use the rest of the radio for spare parts.
This PCB is compatible with RF power modules such as the Toshiba RA/SA series and Mitsubishi M series to provide an amplification interface for low power radios under 5 watts. There are two LEDs onboard which indicate TX and Power In. The low power TX signal is detected at the first relay (J1) through Q2? via a current sensing circuit which enables power to the RF module pin 2, then to the second relay (J2) with the amplified signal. The relays are switched off in RX mode to pass the receive signal back to the input (radio). There is a bias voltage adjustment pot for pin 2 of the power module at the bottom of the board to set voltage for gain of the final amplifier.
The RF Power module I used from the Kenwood is a two transistor, class C design which has filtering onboard for 144-148 MHz to reduce harmonic distortion. The module is broad-banded to support a frequency range of 140-152 MHz with a typical input of 400 milliwatts to produce up to 45 watts at the output. As you can see in the photo below, the module is a Mitsubishi M57726.
Here are the results on a nearly discharged FT-70. I was able to pull over 30 watts into a dummy load. I noticed the bias pot did not have enough adjustment to supply 12.5 volts to pin 2, so I had to remove the series resistor R6 to increase voltage to get more power output. The next step is to reuse sheet metal and screws from the Kenwood to make a shield and print a 3D case for the front. I will also figure out cable management and upgrade the wiring for a more durable and permanent solution.