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Troubleshooting guide

Discussion in 'Electronic Basics' started by phaeton, Apr 3, 2007.

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  1. phaeton

    phaeton Guest

    Hello!

    Thanks to everyone who gave me help a few weeks ago regarding the bass
    amplifier that was doused in soda. I ended up running the circuit
    board under warm water in the sink, and rinsing it off with distilled
    water afterwards. It cleaned right up with no hard water deposits.

    However, while it seems to work better than its description, it's
    still got some issues (surprise, i know). It still has a problem
    where the volume level will fluctuate upon its own whim. There seems
    to be two different symptoms, which could be two different situations:

    1) Usually when it is first started up, the volume level is very weak,
    and distorted. It almost sounds as if there is a transistor or opamp
    misbiased. Then, it will suddenly correct itself. The sound level
    comes up to where it is supposed to be, and it plays/sounds fine.
    Sometimes it will be alright for half an hour, sometimes it will act
    up again.

    2) Sometimes the volume level just drops, with no distortion. It
    doesn't act like a dirty pot, either, as I can turn the volume up and
    down, and it follows it perfectly, it's just as if it is running a
    huge powerbrake. In my experience, dirty pots can make the volume
    fluctuate, but usually a small turn of the pot will move the wiper off
    of whatever piece of crud it is on and it restores to its proper
    volume level. This is different.

    Looking at the schematic, I don't see any bias adjustment points.
    Things are pretty much fixed, and that still wouldn't solve whatever
    condition is changing things up. My first thought is then maybe the
    power supply could be acting up, but I'm not sure what sort of event
    would cause that, or how to measure it (aside from hooking my DVOM up
    to +57V and -57V and seeing if it fluctuates from ~114V over time).
    My next thought is to probe the pins of all the opamps and transistors
    to see if their levels are reasonable, and/or change when it acts up.

    If anyone could throw me a bone, that would be appreciated. I won't
    expect someone to walk me through testing a live circuit they've never
    seen before, but I'd appreciate any help or pointers. I know that
    this post qualifies for the canonical "if you have to ask these
    questions, you have no business messing around with that circuit", and
    on Tuesdays and Thursdays I might agree. However, how else do you
    learn?


    I left a lot of stuff out for brevity, so I hope I don't sound as
    crazy as I think I do.
    Thanks.

    -ph
     
  2. Chris

    Chris Guest

    Hi. Haven't worked on that one, but I'll bet if you post this message
    to sci.electronics.repair, there's a better chance someone has.

    Good luck
    Chris
     
  3. phaeton

    phaeton Guest


    Thanks Chris. I'll do that.

    -phaeton
     
  4. ehsjr

    ehsjr Guest

    You most certainly *should* be messing around with that circuit!

    Even if the pots don't seem bad, they might benefit from
    cleaning. Also drrrrrrrrrrrrrrrrrrrrrryyyyyyyyyyyyyyyyyyyyy
    it *really* well. (You probably did both already but it bears
    repeating.)

    For a shotgun approach, you can try heating and/or cooling
    things, rapping (gently) on parts, flexing (gently) the
    board. You've got some respectable voltage in there, so
    caution is mandatory.

    Next, if you have no test equipment other than the DMM,
    you can use a cheap audio amp and speaker as a
    signal tracer. You'll need to couple the signal into the
    amp through a capacitor, and may need to attenuate the
    signal so you don't overdrive the cheap amp. Since the
    thing plays correctly sometimes, you can fiddle with the
    tracer so you know how it should be set up at the various
    input/output points. When the sound is good at the input
    to a stage and bad at the output, you've found the bad
    area. Your idea of monitoring the voltage couldn't hurt,
    either. When you find the bad electrolytic cap that
    is causing the problem, let us know. (That's my way of
    expressing what has to be a predjudice - I always suspect
    electrolytics.)

    Ed
     
  5. Lionel

    Lionel Guest

    Hello again. Glad we could help.
    Most likely a corroded or dry solder joint, component leg. Another
    likely cause is a bad/dirty pot. It could easily be a combination of
    those things.
    Given the history of the amp, you should give all the pots a squirt
    of contact cleaner just on general principles. If there are any
    trimpots on the board, mark their settings with a permanent marker
    *before* applying the contact cleaner, turn them a few times
    afterwards, & return them to their original settings (*important!*).

    Next step:

    Put a continuous tone into the amp, & gently poke around the board
    with a non-conductive (wood or plastic) stick. Start with the
    physically large & heavy components like the electros, power resistors
    & heatsinked transistors, & work your way down in size. Don't forget
    to check the pots & connectors! If you hit one that affects the sound,
    label or mark it so that you can find it again.

    After that, you do a close physical inspection with a magnifier under
    the best light source you have. (The back porch on a bright day is
    perfect.), again, marking/labelling any corroded/burnt spots, or
    'grainy' looking solder joints.

    You should generally replace any parts that are burnt, pitted or
    bulging. Grainy, dull, or cooked-looking solder joints need to have
    the solder removed & replaced with new solder. For a typical amp, a $5
    RatShack solder sucker is fine for this.
    All the above diagnostic techniques are SOP for any piece of
    electronic equipment, & will solve most problems without requiring any
    electronics expertise. I've trained many junior tech's with zero
    electronics background to do all the above very successfully. All they
    need is good eyesight, good hand/eye coordination, & a methodical
    attitude.
    How indeed? I started learning electronics in the exact same way when
    I was about eight, & I'd recommend the same approach to anyone. ;^)
    Not at all. Your symptoms are all typical of a device that's been
    through all that.
    My pleasure. Good to see that you're sticking with it.
     
  6. Lionel

    Lionel Guest

    [...]
    <grin>
    IME, the most reliable way of determining the experience level of a
    tech is to see how long he takes to blame the electros. You can spot a
    /really/ experienced tech because he keeps the spare parts drawers
    with the electros right on the workbench, & automatically assumes that
    any faulty device will have /some/ sort of cable/connector problem.
     
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