Modifying 450W Motorola 900 MHz Amp
Mats Bengtsson (KD5FZX)

Mats takes us through the process of modifying a 400W 900 MHz amplifier for service on 902 MHz, for 500W output. This is similar to his article on modifying the 150W and 300W versions.


This amplifier is a Feed-Forward type amplifier used in the cell-phone service.

Feed forward is a way to greatly improve the linearity of an amplifier and achieve extremely low distortion needed for some digital modulation systems.

This is achieved by measuring the relationship between drive power and output power. When the output doesn’t follow the drive because of compression, then the difference is added to the output in phase from an error amplifier (or several amplifiers) to achieve almost zero compression.

The result is excellent linearity, but the amplifier has to work at fairly low level and with high losses. Excellent for AM ATV, but not so good if we try to achieve high power.

By removing all the feed forward parts and use the amplifier in a very basic configuration, we can achieve 400W+ at about 1dB compression. This compression level is very acceptable for amateur radio use, and probably better than most amateur radio driver amplifiers.

Click on small pictures for zoom.

Original amplifier

After modification

You will need 26VDC - 45Amps power supply for 400W+ power level.

Modification Steps to a Basic 400W+ Amplifier

The steps are given in general sequence.

  1. Remove the controller board with the ribbon cables in top of the picture.

  2. Remove the cover on the lower board and disconnect everything from the board. Remove the bottom box that was housing the lower board.

  3. Disconnect all cables going to the final cover parts, so the cover can be removed.

  4. Remove the final cover plate, and remove all parts from the plate.

  5. Remove the rubber-foil shield that covers all compartments.

  6. Remove the four shielding covers over the error-amplifiers.

  7. Disconnect all cables and remove the error amplifiers in the bottom of the picture.

  8. Error amplifiers removed.

  9. Remove the big black output filter and any ribbon cables connected to the controller board.

  10. Remove the input board to the right in the picture, and also remove the board next to the driver in the left part of the picture.


Now it is time to decide drive level 5-10mW or 100-150mW drive. The 100-150mW drive version will be able to provide 20-30W more output at 1dB compression. The reason is that we are removing one amplifier board that contributes to the total compression level.

Use one of the steps below, resuming at step 13 for all remaining modifications.

  1. 5-10mW drive version

    By connecting drive power to J101 as indicated in the picture, you will need 5-10mW drive for 400W output after remaining modifications. This picture also shows all the remaining coax inter-connects after low-drive modifications.

  2. 100-150mW drive version

    1. Remove the board with J101 connection (picture at step 10).
    2. Connect drive power as shown for 100-150mW drive level.
    3. Coax inter-connects after medium-drive mod also shown.
  3. To prevent plated-thru fole failure:

    Remove the output coax. Install a short piece of tinned solid copper wire through the plated hole connecting the center conductor pad on top of the PCB to the power divider on the bottom side.

    Push the wire through the plated hole from top until it bottoms out, then pull it up about 1/16" to clear the heatsink. Now fold the wire on top of the PCB to rest on the center-conductor pad. Add flux and solder in place; make sure the solder flows down the wire through the plated-through hole.

  4. Replace the output coax with ¼” rigid or flexible TEFLON coax. Anything less will melt at 400W level.

  5. Route coax inside the amp as shown.

  6. Install SMA-N chassis connector. Connect the ¼” output coax.

  7. To turn the front LED into a TX indicator light:

    Find the empty error amplifier compartment next to the PA.

    Connect the yellow LED wire to ground.

    Connect the green LED wire through a 1KW resistor to the 5th pin from top in the picture.

To re-install the shielding covers over the removed error amplifier compartments:

  1. Increase the notch in the rubber-foil shielding material to clear the newly installed output coax.

  2. Increase the notch in the final-amp cover-plate to clear the new output coax. Install the plate.

  3. To defeat Q3 collector current sensing that would limit power to about 200-250W out:

    Look at the regulator-sensing board from the fan-side.

    Locate the Q3 transistor on the left side.

  4. Put a jumper across the sensing resistor next to Q3 on left side of the picture, just below the screw.

  5. Connect a small wire to the center pin of Q7 for 5V TX-relay power.

  6. Fabricate a new plate for power supply and TX switch connections. Install two feed-through capacitors. The power supply feed-through must be capable of 45 amps. Ground all the negative leads for the cooling fans.

  7. Connect the 5V wire to one side of the coil on a small NO relay with a 5V coil. Look at the upper center of picture for the glued in place 5V coil PCB relay. Connect the other side of the 5V coil to the TX feed-through capacitor.

    This relay is NOT part of the amp; it must be installed by the user.

    1. Remove the multi-wire connection between the regulator-board and the amp main-board.
    2. Remember how it was installed, because it will be used again.

  8. Cut the wire to the left of the plugged hole on the connector in the picture.

    Cut in the middle, so extensions can be soldered to the cut wire.

  9. The two green wires in the middle of the picture are connected to the wire cut in previous step.


  10. Connect the two newly installed wires to a normally open contact on the user-installed relay.

  11. Another picture of the relay coil connections.

    Connect the heavy red wire to the power-supply feed-through capacitor.

    Connect the heavy black wire to a solder tab on the back of the power-plate.

  1. Connect a power meter capable of 500W to the output.
  2. Connect a 26V - 45A power supply to + and - input.
  3. Ground the TX relay wire to enable the amp.
  4. Supply enough drive to reach 250-300W output.
  5. Adjust the trimmer capacitors on the driver boards for max output. Start with the capacitor closest to the input and work toward the final. Repeat until max power is reached.
  6. Turn off the power supply and install all cover plates.
  7. Turn on the power supply and increase the drive until 400W is reached.

Modification is complete

  1. Test with 125mW drive and 27V - 44A power consumption.
  2. Bird 1000W - 400-1000MHz slug indicating 450W output on a dummy load.
  3. My HP-432 power meter with a calibrated directional-coupler indicates 420W.
  4. Output is 400W+ with 30W difference in reading between 2 power-meters.


Good luck with conversion.


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