The Motorola 33 cm Linear Amplifiers
By Dave – W6OAL
Throughout our ham careers we clamber for just a bit more power in order to be heard in the next grid or the next city, state, country. Commercial HF and VHF amplifiers are not cheap and home brewing takes a lot of time and in the end, with the rising cost of components plus the frustration of even finding the needed components, may wind up costing as much as a commercial amplifier.
Today we find HF amplifiers fairly reasonably priced but those of us that have ventured up into the “rare ether” as it is referred to, above the 70 cm band, power becomes expensive. The surplus markets have afforded a few of us some good deals and with a bit of ingenuity, perseverance and sweat we have been able to make some things usable from old relics that have been cast off by the cable companies and the cellular organizations, etc. Pyro Joseph (on E-bay) has been quite instrumental in the acquisition of an abundance of various amplifiers for the different microwave bands. The nicest catch that I was made aware of and was able to capitalize on was the acquisition of the commercial 20 and 40 watt amplifiers that are operational in our 9 cm band (3456 MHz). The most recent prize was the acquisition of the 33 cm Motorola linear amplifiers (150 and 300 watt units) by RDR Electronics in Centennial, CO. These units originally sold for probably hundreds if not a couple of thousand dollars each and are now going for $25 to $65 a unit (depending on power and model) on E-bay in the buy now category. They ship within the U.S. only
These units exist in three models; the low power input of -20 dBm input for 150 watts output, another low power unit that requires about 1.5 to 2 watts input for 150 watts output and the high power unit that for 3 to 5 watts input will output 300 watts. Now, being very linear they don’t have to be run at their maximum input or output rating. I have bench tested the intermediate range amp and found that for 0.5 watt input it yields a healthy 50 watts output. That is a 20 dB gain when you apply the math. They are 50 ohm input and output devices so there are no impedance matching problems. They are equipped with internal isolators so if your antenna looks like a bit more or less than 50 ohms the finals won’t be hurt.
The heat sink they sport seems very adequate when used in the SSB or CW mode (~50% duty cycle). If used on FM I would recommend a muffin fan or two on top of the heat sink. The sinks are 12 inch X 7 inch made up of 36 5/8 inch fins providing a surface area of ~378 square inches. If this were a flat surface it would occupy an area of ~20 X 20 square inches. The unit could be used as is if the proper mating connector(s) could be found but then either the connectors would have to be modified or jumpers made to accommodate some standard coaxial connectors for input/output RF power and the DC voltage.
Here is a description of what‘s inside the amplifier unit. The bottom cover of the units requires a #2 Torx to remove the six screws. Once removed there is an RF barrier/blanket of sorts that is sandwiched between the outer cover and the unit frame. Once removed the amplifier control and RF power boards are visible. As per model; low power: “control board”, preamp and two transistor power amplifier boards, intermediate: “control board”, single two transistor power amplifier board, high power: “control board”, and a pair of two transistor power amplifier boards plus input and out combiner boards. It appears that no expense was spared in the design, construction and manufacture of these amplifiers. The “control board” is a solder masked circuit board of FR-4 material very nicely laid out – but fairly useless for ham application. The RF boards are either etched or vapor deposited copper on Alunina and mounted on a 3/16 inch thick copper plate which is bolted to the heat sink. The RF, power, interface connector panel floats in a slot and can be used with considerable modification or I found that removing it and building my own was considerably easier and more sanitary. There is a sub connection panel of feed thru capacitors that inputs power to the RF boards and instrumentation signals back to the control board. It also floats in a baffle that separates the “RF” and the “control board” compartments. This can be left in tact or discarded and your own constructed which I chose to do – more on this later.
My first attempt at modification was to use the original floating I/O panel (highly modified) that contains the I/O RF and power connectors and keep the “control board” in tact. As some problems developed in my testing phase of the project I found it best to remove the “control board”, sub panel and I/O connector panel. Then I built my own from some sheet aluminum scrap. The several instrumentation lines going through the sub panel and into a connector on the control board. I removed the green and brown wire combo w/connector, the gray and blue wires w/connectors leaving the yellow wire (bias line), red/white and red wires (28 volts) attached to the RF board(s).
A new sub panel was created of similar dimension and shape of the original and made of thin aluminum and fitted it with three feed thru bypass capacitors (value is relatively unimportant – probably .01 or .001 is fine) but of heavy enough construction to carry the full output current (around 30 amps in the case of the high power unit). Two of these three units will be handling on the order of 20 amps so I was not conservative in my selection of Vcc feed thru‘s. Since the original I/O panel was discarded and a new one created it was fitted with an female SMA connector for input and a female N connector for output. Also, a power connector (I used an Anderson 30 amp rated pair) was provided, a female RCA connector for PTT or initiation of bias and a female TP connector for T/R relay operation. More on these later.
Since 12 to 17 VDC is required for bias to place the RF board(s) in operation and with the control board gone a I added a 7812 regulator which was employed to provide the required bias from the 28 VDC line. The 12 VDC from the regulator is also used to supply 12 V to an added DPDT relay which will apply bias on transmit and 28 V to the TP connector for T/R relay control. The low side of the relay is connected to the PNP emitter, the collector to ground and the base via a 2.2K ohm resistor to the PTT jack, the RCA connector on the new I/O panel. The hot side of the Anderson power connector is attached to one of the feed thru caps and jumpered to the second of three feed thru caps on the control board cavity side of the unit. The return side of the Anderson connector is brought to chassis ground. The RF side of the new sub panel will have the red wire(s) go to one feed thru and the red/white wire go to the other. Break the red wire(s) and add a 0.5 to 3 ohm chassis mount resistor to provide a little less voltage (Vcc) to the driver transistor stage(s).The third feed thru connects the yellow wire from the RF board(s) to the new sub panel to the 12 V relay NO pin on one side of the DPDT line up. Jumper the hot side of the coil of the relay to the common (C) contact on the same half of the relay so that when activated, 12 V will go to the RF board(s). On the second side of the relay contacts; connect a wire from the main 28 V feed thru to the common (C) pin and run a wire from the NO pin of this side of the DPDT line up to the TP connector on the new I/O panel. This is just about all I did for the modification. For T/R I used a Transco “Y” relay and a DowKey Microwave SMA relay, made the bypass line between relays of UT-141 hard line.
Here is a list of components that have been added to the amplifier for this modification:
RF Input connector, SMA (f) or BNC (f),
RF Output connector, N (f)
Power connector (30 amp Anderson pair or similar)
PTT connector, RCA (f) Phono
Coaxial Relay Control Connector (TP jack)
Voltage Regulator (7812)
Relay, 12 VDC coil DPDT (Omron G5V-12 or equivalent)
Resistor, 0.5 to 3 ohm, 5 to10 W (chassis mount)
PNP transistor (to handle 28 V at 0.5 A)
Resistor (2.2 to 2.7K ohm ½ watt)
Optional bells and whistles:
LED (red, DC indicator)
LED or two (green, RF output indicator)
SPDT small toggle switch to switch between RF boards (HP amp only).
I’ll supply an amp and perform the modification for LP $125 and HP for $150. Email Dave