How to Repair Transistor Power Amplifiers

Written by simon foden Google
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How to Repair Transistor Power Amplifiers
Transistors behave like valves; they allow current to pass through a circuit. (transistor image by Alex from

There are two types of amplifier: Tube-powered amplifiers and transistor-powered amplifiers. Vacuum tubes were superseded by transistors in the late 1950s for use in telephones and computers, but many amplifiers are still made using tubes due to the superior tone. Transistor amplifiers are considered to be more reliable because the transistors, unlike tubes don't burn out. Transistor-power amplifiers still require maintenance and transistors can still blow. Like tubes, transistors control the flow of voltage. So if a transistor is damaged, your amplifier will stop working.

Skill level:

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Things you need

  • Screwdriver
  • Multimeter
  • Soldering iron

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  1. 1

    Remove the printed circuit board from the power amplifier. Unscrew the top of the amplifier to expose the interior components. The majority of internal components will be mounted on the printed circuit board. These are called "surface mounted components." The jacks and power supplies are typically mounted to the amplifier chassis or cabinet. Input jacks on a guitar amplifier are mounted on the preamp section.

    How to Repair Transistor Power Amplifiers
    This Park amplifier is transistor powered. (la guitare image by richard villalon from
  2. 2

    Troubleshoot your amplifier. The transistors are located in the power section of the amplifier. This is typically the section closest to the power supply. Disconnect all output connections and turn the master volume dial to zero. Power up the amplifier. If the amplifier powers up but there is no sound, this typically indicates that the problem is located between the power section and the audio section. If the amplifier fails to power up or cuts out intermittently, the problem is most likely located in the power section.

    How to Repair Transistor Power Amplifiers
    Loose surface-mounted components are easier to identify than blown transistors. (circuit board image by dwags from
  3. 3

    Examine the amplifier to identify the faulty component. Loose fitting surface-mounted parts are easy to spot. For example, the job of a resistor is to regulate the flow of current. A loose fitting resistor may cause the amplifier to blow a fuse. Resistors are typically cylindrical and brightly coloured. Melt the loose connection with a soldering iron and make a new connection. Blown out transistors usually have a black colouration, similar to a blown light bulb. In some cases, soot may gather on the circuit board if the transistor has blown.

  4. 4

    Test the transistors. Set your multimeter to "ohms" and connect the test probes to any part of transistor between the legs. If the meter gives a "zero" reading, it means there is no power in that part of the circuit. This identifies a shorted transistor.

    How to Repair Transistor Power Amplifiers
    A zero reading indicates a shorted transistor. (multimeter image by Aleksey Bakaleev from
  5. 5

    Replace the faulty component. Transistors are soldered onto the circuit board. To remove a faulty one, melt the solder connection with a soldering iron. Wait for the transistor to cool, remove it and connect a new one of the same value. Brush away any solder residue with a dry cloth.

  6. 6

    Test the amplifier again. Test the new transistor with the multimeter. The new transistor should now be generating current. Enclose the amplifier.

Tips and warnings

  • Test a functioning piece of equipment to ensure that your multimeter is accurate.
  • If you are unsure, always consult an electrician. Mistakes can lead to serious injury.

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