Simple VHF RF Power Amplifier |
Overview
This is a quick project to utiltize the final RF power amplifier section from an old Motorola MOCOM 70 radio as a stand-alone 50 watt RF power amplifier.
These MOCOM four channel (crystal) radios were heavily used in the late 1970s up through the 1990s and can often be found at ham radio swapfests for a few dollars - or even for free!
There were several models of the Motorola MOCOM 70 made, with most covering the 25-50 MHz range (50/100 watts), the 136-174 MHz range (30/50/100 watts), and the 450-512 range (25/50/100 watts).
The most common of these radios were tuned for the 150-162 MHz range and so is the model we'll be using in this amplifier project.

Just the stock rear amplifier section (and low-pass filter) from the MOCOM 70 will be needed. The MOCOM amplifier module just needs a +13.8 VDC @ 10 amp power source and around 500 mW of RF input power to reach around 50 watts RF output. When properly tuned for a narrow frequency range, the amplifier can reach 70 watts peak RF output.
The MOCOM 70 amplifier section shown here had the Motorola part number TLD1622A stamped on the heatsink and the low-pass filter is Motorola part number TFD6122A.
With an approximate RF input power from 250-500 mW, the amplifier used for this project output the following RF power:
Frequency (MHz) RF Output Power (Watts) 140 15 (+42 dBm) 150 50 (+47 dBm) 160 40 (+46 dBm) 170 31 (+45 dBm)
Pictures & Construction Notes

Stock VHF RF power amplifier module from an old Motorola MOCOM 70 radio.
This amplifier module only requires a few wire connections.
There are only three +13.8 VDC power lines, one each for the high-current final RF transistors and another for the lower-current driver stages.
There is also a single ground wire which should be soldered directly to an emitter tab on one of the final RF transistors.
The RF input of approximately 500 mW enters via the coaxial cable on the lower-right of the circuit board.
The RF output from the amplifier passes through the cylindrical directional coupler (for SWR protection) and onto an external low-pass filter.

Closeup view of the two final RF output transistors.
There are two variable capacitors which can be used to peak the amplifier response over a narrow frequency range.
Each of the two final RF output transistors has its own +13.8 VDC power line which passes through a series 0.01 ohm / 3 watt resistor for voltage equalization.
The ground wire is soldered directly to the emitter on one of the RF transistors.

Cut the rear bracket off the Motorola MOCOM 70 radio using a cut-off wheel.
Grind the edges to get them as smooth as possible.
Heat the bracket using a blow torch to remove the stock feed-through capacitors and other soldered-on bits.
Clean the whole thing up and add your own feed-through capacitors and standoffs.
Here we can see the two series 0.01 ohm equalization resistors for the final RF output transistors and their own high-current 1000 pF feed-through capacitors.
The small feed-through capacitor is for the other +13.8 VDC power line.
The large box is the stock low-pass filter.
TNC connectors were added for the RF input/output to the amplifier from the TX/RX relay circuit.

Internal wiring overview of the completed amplifier module.
The low-pass filter connections are labeled IN and OUT.

The RF output is passing through the low-pass filter, then onto a TNC connector on the left side.
The yellow wire is the lower-current +13.8 VDC power line.
The two striped wires are for the high-current +13.8 VDC used by the final RF output transistors.
Each of the +13.8 VDC power lines has a ferrite bead slipped over the wire.
The large-gauge brownish wire is the ground.

Overview of the RF-switched TX/RX relay circuit.
The relay circuit's RF input is via the BNC connector. The RF output is via the SMA connector.
A capacitive tap is taken via a series 1 pF capacitor / 100 ohm resistor on the RF input line.
Two MBD301 hot-carrier diodes rectify the RF input signal and generate a DC voltage which is then applied to the base of the TIP141 Darlington transistor. The TIP141 controls the coil in the DPDT RF relay.
Those DPDT RF relays can be salvaged from RF amplifier in certain older Motorola two-way radios.

Bottom view of the RF-switched TX/RX relay circuit.
Try to keep any traces carrying RF as short as possible.
Surface-mount components should be used in the RF detect circuit.

Finished amplifier block with the TX/RX relay mounted.
The RF input to the amplifier is via the TNC connector on the lower-left, the RF output is via the TNC connector on the lower-right.
The main RF input is via the BNC connector and the RF output via the SMA connector. These will go to front panel-mounted RF connectors.

Amplifier case internal overview.
The amplifier's case will be an old ammo box.
The RF input (BNC) and RF output (N with a SMA adapter) connectors are along the left-side.
The +13.8 VDC power input is via the banana jacks. Single-point connections and 14 gauge (or larger) wire should be used for the amplifier's DC power wiring. The amplifier can draw up to around 10 amps.
The 4,700 µF / 50V capacitor mounted on the right-side helps to smooth out the power supply.

Finished amplifier internal overview.
It's a little cramped, but it works.
Add a smear of heatsink thermal compund along the amplifier's edge to make better thermal contact with the ammo box case.
A SMA jumper connects the RF output from the amplifier to the panel-mounted N connector.

Finished amplifier front-panel overview.
The BNC connector is for the RF input and the N connector is for the RF output. Dustcaps are on both the connectors.
The banana jacks are for the +13.8 VDC @ 10 amp power input.
The power switch has an integrated 15 amp circuit breaker.
The red LED is for DC power indication.
