Troubleshooting a Guitar Amp

Discussion in 'General Electronics Chat' started by elec_mech, Dec 8, 2011.

  1. elec_mech

    Thread Starter Senior Member

    Nov 12, 2008
    I'm attempting to repair a Spider IV 15 guitar amp for someone. Unfortunately, I'm not an audio circuit enthusiast and have zero background in musical instruments.

    Here is what I'm attempting to fix:

    I was told this experienced a power surge and blew the slow-blow fuse in the AC receptacle. I've replaced it and turned it on with no problem. I also examined the board, SMT with a couple of high pin count ICs and several electrolytic capacitors, but did not "see" any obvious damage in the way of puffed or blown caps, burnt traces, etc. although I have not removed the board and examined the back of it yet.

    This has an MP3 input, so I hooked up my Ipod and it sounded fine. I then added a stereo 1/4" adapter to plug the Ipod into the guitar input. As the owner described, the sound had a loud buzzing going over the music. If your turn the volume up, the buzz was louder, moreso than the music. The controls like tremble and bass appeared to change the sound a little, but the dominant buzz sound was always there. The buzz went away as soon as I unplugged the Ipod.

    I'm not sure if the ICs are cooked and beyond repair or if I should look at something else. Seems the MP3 input is fed directly into the speaker or through another set of circuitry since the sound is fine through that input. I also assume the speaker is good since the MP3 input sounds fine.

    Does anyone have experience with guitar amps? I don't know if I shouldn't be using an MP3 player to test the guitar input (different signal types or impedences?), but I don't own a guitar either. I can post pictures or a video if anyone would like. Failing everything else, this would still make a decent speaker for an MP3 player or similar so I won't have to scrap it, but I'd really like to get it working and back to the owner if possible. Thank you in advance for your assistance and advice.
  2. jimkeith

    Well-Known Member

    Oct 26, 2011
    My guess is that you have a broken shield wire on your guitar pickup that is causing the amp to pick up 60hZ noise--this loud noise can cause the input power to the amp to be perhaps high enough to blow the fuse--the broken wire is most likely at the plug end, so disassemble the plug if you can to check the connection to the shield--if it is molded, take a sharp knife and split the cable insulation somewhere just enough to expose the shield braid so you can measure continuity between that point and the barrel of the plug. If the circuit is open, replace plug. Common problem.
  3. Adjuster

    Late Member

    Dec 26, 2010
    Unless the 3.5mm to 1/4 inch adaptor is coincidentally faulty, I would doubt that the problem is outside of the amplifier.
    There may well be a broken shield (or screen) connection within the amp, perhaps most likely in the socket or the connections to it, e.g. a broken wire or damaged track on a printed board.

    These parts are subject to mechanical wear and fatigue, especially if items are plugged in or unplugged carelessly, or the wire is pulled on, or perhaps something strikes against the plug. Any of these things are likely to happen to a guitar amplifier.
  4. sheldons

    Well-Known Member

    Oct 26, 2011
    first things here to check is the quality of the soldering around your input jack ground wise....if the amp was working ok sound wise with your mp3 the amp cant be at fault .try another ip source....also check soldering around the level controls and check for noisy controls.
    with no signal present you should be able to hear a slight hiss from the speakeras you turn the gain up which should change as you adjust the tone controls(base,treble etc)
    Last edited: Dec 9, 2011