Motorola HPN3000A power supply modification

Used and decommissioned Motorola gear is a common sight at swap & shops and ham flea markets.  And hams are always in need of one more 12 volt power supply to run station equipment or accessories.

At the intersection of those two truths is the Motorola HPA3000A power supply. It’s a rugged, brute-force linear regulated power supply designed to power Motorola’s Spectra base stations. It’s specified at 14.5 volts at up to 16 amps – not quite enough for a typical 100-watt HF transceiver, but plenty for a mobile VHF-UHF rig or miscellaneous devices that you don’t want to hang on your HF rig’s supply. I acquired one about twenty years ago and had been using it in my shack to power my 2-meter rig, but set it aside when I picked up a small switching supply.

I decided to bring it out of retirement to power the 6 meter beacon but thought the output voltage (measured at 14.93 volts no-load) was a bit too high. Looking around the unit, I couldn’t find any trace of a voltage adjustment pot anywhere. A search of the web took me to the Repeater Builder website’s Motorola Desktop Power Supply Information page where they have posted a link to the schematic for the HPA3000A.  

A little investigation led to the regulator circuit and its reference amplifier, and some experimentation determined that the network of R8 and R10 were used to establish the output voltage. R10, a 10k ohm resistor, was noted as an optional component, so I removed it and noted that the output voltage dropped to the specified 14.5 volts. But I needed it to be lower still – aiming for 13.5 volts or so.

Checking the junkbox, I found a nice 500 ohm potentiometer which when placed in series with the R8 (1k ohm) allowed adjustment of the output voltage from 14.5 down to 13.0 volts. Perfect! I drilled a small hole in the front panel, mounted the pot and wired it in to the circuit board, and I now have a well-regulated, adjustable (but heavy) power supply. It’s powering the beacon now.

  1. Making sure the unit isn’t plugged in, remove the top screws on either side of the unit. Loosen the four lower screws and pull the top cover off.
  2. You’ll see a small circuit board mounted upside-down in the top of the frame. (Mine is labeled with Motorola part number TLN5274B.) Remove the two mounting screws and swing the board out.
  3. If R10 is installed (10k), remove it. (I left mine hanging at one end.)
  4. Unsolder one lead of R8 (1k) and pull it free from the circuit board.
  5. Solder two wires to the top and wiper of a 500-ohm pot. (A 1k pot would allow you to adjust the voltage lower; I didn’t test it but 1k should take the output to below 12 volts.)
  6. Mount the pot in a convenient place on the chassis.
  7. Solder one of the wires from the  pot to the circuit board where you removed the 1k resistor. Solder the other wire to the free end of the 1k resistor. (Use a piece of sleeving for insulation.)
  8. Remount the circuit board, plug in and turn on the supply, and check the output voltage while you turn the pot.
  9. Replace the top cover. Recheck for proper operation, adjust the output voltage to your requirement, and enjoy!