12v amp question

Discussion in 'General Electronics Chat' started by tacoma_kyle, Apr 30, 2009.

  1. tacoma_kyle

    Thread Starter New Member

    Apr 30, 2009
    Hey there folks.

    So I bought a amp used and it was partially dead. I would like to see if I can fix it---if possible. I have a soldering iron and electrical citciut mathematic knowledge. I filed a claim against the person too.

    I tested it back and forth with a known working speaker and consistent power, my truck. It is a 4 channel.

    Problem: Only 1 of the 4 RCA inputs works. Actually I should say only 1 input gives sound through the respective out-channel. Like mentioned, I tested it thoroughly. It doesnt blow fuses or give any static that I have noticed.

    Questions: Common problems that may make that occur? Seeing as the one input that works seems to work perfect, it leaves me thinking it -could- be a more minor problem. Could... I would expect the amplification is done in one 'process' and the power is divied up 4 ways, rather than 4 individual low power amplifier. So thats why I am wondering it if it a minor short or something like. Toasted connection...:confused:

    NOTE: WHen I received it there was something loose inside it. Knowing that metal conducts electricity, lol, I remove the bottom and found two screws. Before I did anything... No visible arcing from a glance at the circuit board.

    If anyone has iny input it woudl be great. I have a few friends who are senior in electrical engineering so I may pick their brains this weekend. :) It is presently disassembled on my table.
  2. PRS

    Well-Known Member

    Aug 24, 2008
    I hope you have some troubleshooting equipment. A DMM may be all you need, but if you have an oscilloscope you can trace the signal from the input to the output. At the spot where it does not appear that component is suspect. Remove it, test it, replace it if necessary. I should not have to say that when testing like this turn on your amplifier and apply a signal to the input. And don't touch any high voltage with your fingers! How's everything in Tacoma? ;)
    Last edited: Apr 30, 2009
  3. tacoma_kyle

    Thread Starter New Member

    Apr 30, 2009
    I dont have a oscilloscope but I may get one. My friends may have one.

    THanks for the input, I was expecting the process would be like you described.

    And yeah I actually live in OR. I drive a Tacoma lol.
  4. Mike33

    AAC Fanatic!

    Feb 4, 2005
    I agree, you'll need to trace the signal. Plugging in a sound source and tracking it with an audio probe would work well, using another amp to hear the signal. If it's a micro-board type amp you could be out of luck, but if it uses discrete components you might be able to diagnose it. The screws floating around could have once connected a ground somewhere (check for that, I have had that happen with a digital reverb, free 10 minute fix!), or may have bridged something and caused a short.

    There could be protection diodes in there on the power supply, which may have blown, so make sure to measure all active device voltages, see if they're getting power (IC's, transistors...). Often these things are an easy fix - once you find the problem - or a total disaster causing said unit to be stomped on many times until the user feels better, ha ha.
  5. tacoma_kyle

    Thread Starter New Member

    Apr 30, 2009
    Excellent, thanks for the assistance.

    Yeah the amp is officially mine too lol. The guy wanted me to pay 20 bucks to send it back and I said f-that. Filed a claim and won. Got my money back.

    Also got a project because he was an idiot.

    I will be sure to test the diodes, it does have a protection system. I havent been able to dedicate really any time yet but I will soon.

    Thank, KYle.
  6. DonQ

    AAC Fanatic!

    May 6, 2009
    Having spent some time doing A/V repair, this might help.

    I don't know if I can remember anything but a very few amps that had anything wrong with them other than either fuses (easy), or the output drivers were blown.

    Shorting the output wires while the amps are trying to drive a speaker will blow things easy. People install their own speaker, run their own wires, and crimp their own connections, often without much care. Mistakes, bare wires, sharp corners, and a variety of other things make it easy to blow the outputs. Other things can blow, but it takes a lot more effort, to the point of almost being impossible, whereas blowing the output is so easy any idiot can do it (and often does).

    A lot of the repairs I did consisted of a visual inspection of the output drivers. Sometimes you could see a chunk blown out of the side of a transistor, or a discolored area from getting way hot! Other times it would take a ohm-meter to test a BJT or FET. Only about 1 in 10 actually needed to have a signal traced, usually because the final driver stage was in an IC. This was the hardest because even if it had a number you could look up, it may be something hard to find.

    So anyway, look for output fuses, then physically blown transistors, then do a P-N junction check on Bipolar Drivers, or a Conduct/Insulated check on MosFets. This will find 90% of what's likely wrong. If that don't work, then you have to start digging.