Series LA Communications Amplifier Technical Support – Amplifier Deck



This section describes typical failure conditions, troubleshooting and repairs

In all cases, please (1st) review the Owner’s manuals for your model as it includes detailed operation and troubleshooting guides.

High Voltage Power Supply

  • No high voltage output
    • Blown open series resistor
    • Blow fuses / circuit breakers

RF Deck / Amplifier

  • LA70B
    • Amplifier will not key up – No high voltage indicated on the Plate Voltage meter:
      • Solution: possible open resistor in the series resistor string. The amplifier will be inhibited from going into transmit AND there will not be any indication.
  • LA22A
    • The RF output power decreases to zero during digital (key down) operation. All RF decks include a resettable “fuse” in the cathode return circuit and located on the main circuit board. Some early revision models use a 900mA version and the newer models use an 1100mA part. There have been some failures of the 1100mA parts. To correct the problem, replace the fuse. In all cases, the part should be upgraded to the 1100mA part. 
  • HV Arcing around the chassis mounted feed through/filter
    • There have been some instances of high voltage arcing between one or both ends of the feed through to the chassis (ground). If the component is not internally shorted (failure mode), clean up the carbon tracks and arcing residue with alcohol and/or acetone. A non-metallic brush may help the clean-up process. Once dry, apply a coating of red glyptol to the exposed connections.
  • HV Arcing under the plate line hold down buttons
    • We have repaired amplifiers that have a failed (arced through) sheet insulator or hold down button. The 10 mil Ultem plastic insulator sheet AND the buttons should be replaced as the arc will leave a residue or cause microscopic holes that will continue the problem even after clean up and re-assembly.
  • The amplifier will not complete its warm-up cycle – review the owners manual for your model and reference the simplified schematics (download links below).
  • What is the voltage and current at the T-R relay connector (located on the rear of the RF deck)?  
    • 26VDC @ 750mA (maximum) is available to switch one or more coaxial an/or control relays. To use a 12VDC relay, add a series resistor (same as relay coil) to drop the voltage. Use a resistor with the proper power rating to ensure reliable service, add a quench diode across the coil, AND, always pre-test and measure the voltage drop using a separate bench type power supply. 

Troubleshooting Documents