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Diagnosing an amp

Discussion in 'Amps and Cabs [BG]' started by ryguystye, Dec 21, 2013.

  1. I picked up a broken amp at a garage for dirt cheap and wondering hard would be fix it it and how much it would cost. It's a Peavey basic 112 and it makes no noise when nothing is plugged into the input. when i plug my bass in, the amp detects some sort it'd signal from touching the strings and makes a popping sounds whenever the sound or other metal parts (even the cable's plug connector) are touched and the bass sound isn't played. Any ideas?
  2. This is the earlier version, probably? Does it have a Preamp Out and Poweramp In?

    First, try plugging a patch cord (or instrument cable) from the Pre out to Poweramp In. Perhaps the switching jack there is corroded.

    Still nothing? Try plugging your bass (or a guitar) into the Poweramp In. Hear anything at all? If so, what does it sound like?

    Still nothing? Do you have a second amp (guitar amp, mixer, anything?) Go from the Preamp out into the second amp. Plug your bass into the Basic 112. Hear anything from the second amp? If so, what does it sound like?
  3. I'm not sure which version I have, but it's the one with the red accents on the head and a little triangle badge at the bottom of the speaker cover. I don't see anything labeled Preamp or Poweramp Out/In, but I do have a "Post EQ Effects Loop" Send/Receive. I plugged some headphones into the headphone jack and there's still the same popping and buzzing. No sound when powered on with no input plugged in, but once plugged in, touching anything metal makes a popping sound, then buzzing until I let go. If I don't touch anything with the amp powered on, there's no loud buzzing (at least nothing unordinary). Anything else I can try since there's no Preamp Out?
  4. Substitute Send/Receive in place of Pre Out/Power Amp In in the above comment.
  5. Preamp out= Effects send, Power amp in = Effects return (receive).
  6. I only have one instrument cable, so I tried plugging my bass directly to the Effects Return and the bass sounds normal. I can only make adjustments with the controls on the bass itself and none of the other knobs on the head do anything, which is to be expected, right? So if the speaker works fine, how do I figure out which part isn't working? I know some basic soldering and have looked at the signal diagram in the manual. Is this something I can fix myself? If I open up the amp head, could I tell what's wrong by looking at it?

  7. Looking at the inside of the amp head usually does not find anything. Plugging into the effects return and getting sound is a good sign the expensive parts of the unit is okay. First thing to do is borrow a second instrument cable (or buy a spare you should have anyway) and try the looping. Maybe work the passive/active switch a few times just incase there is corrosion in there. Does the input jack feel pretty much like the effects return jack when you plug a cable into it? You now know the trouble is in the preamp or connection to the power amp in the amp head.
  8. Hmm, I opened the amp head, looked at the internals and did a few things. I noticed a screw connecting the ground wire to the back of the head enclosure was really loose, so I tightened it. I also cleaned some of the pcb traces with isopropyl alcohol, and then resoldered the input jack since it felt a little wobbly.

    When I put it back together, everything seemed to work! I don't know what did the trick, but I guess I'm happy as long I don't run into any more problems. Is there anything else I should be aware of? What do you guys think was the problem originally?
  9. Probably the input jack. Lead required to be removed from solder makes solder joints more brittle among other things.