Car stereo amplifiers

Discussion in 'General Electronics Chat' started by bipin, Sep 10, 2004.

  1. bipin

    Thread Starter Well-Known Member

    Mar 21, 2004
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    0
    Hi all,
    Its quite some time since i perused forum, very happy to c the forum is highly active with lots of peoples and so many threads.

    I am facing a problem with my car stereo amplifier. without having any catastrophic failure of any of the component, my car amplifier is going mute gradually in two or three hours. This especially happens if I dont use the carstereo for some few days. I can bring it back to working by connecting it to AC mains using a 12V adaptor and just by touching the input termingal with bare hand (a crude way of injecting the line noise and making a 50Hz hum) or by injecting a external signal of a little higher amplitude than the pre-amp out.
    This will again go gradually mute after few days or weeks.
    This I have observed with four to five other amplifiers also.
    I find this happens most often after a rainy season and I doubt whether the humidity plays a role?(Mine is a coastal area)
    I had changed all the capacitors of an amplifier ciruit once when I faced this problem, still did not help.

    Any once has encountered such a trouble? Kindly share.

    Thanks in advance.
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    Sounds wonderfully frustrating. I'd suggest caps if you hadn't already changed out a number. Something like one accumulating a charge and biasing a transistor off.

    That done, look into the quality of your chassis ground. Old car amps used the case as the de-facto ground (earth) connection. Modern stuff usually connects through a dedicated wire. Check for corrosion, grease, or a poor connection where this wire is supposed to bond to the chassis. As a check, route the wire directly to the battery terminal for a bit. If that keeps it working, then the chassis connection would be the culprit.
     
  3. mozikluv

    AAC Fanatic!

    Jan 22, 2004
    1,437
    1
    hi,

    long time no hear from you, i guess you have been very busy with your job, welcome back.

    am not sure i understand what you meant when you said your amp goes mute after two or three hours of use. when we say the system has a mute control, we normally just turn it on or off. does your systerm have this?

    on another paragraph you said "This will again go gradually mute after few days or weeks.
    This I have observed with four to five other amplifiers also." your problem lies either at the preamp section or the output section.

    try disconnecting the preamp output and use another signal source. you will know where your problem lies.

    another possible source of trouble is the thermal protection system. does the problem occur most of the time during hot weather?
    :D

    dont' worry, that can be cured. once cured you can now charged the other guys who has the same problem as yours :D :p :D :p :D :p :D :p :rolleyes:
     
  4. bipin

    Thread Starter Well-Known Member

    Mar 21, 2004
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    0
    Hi Beenthere,Mozikluv,
    The problem lies with the power amp only, as both the radio part and the tape part would not work when this problem persists.
    There is no-mute option for this amp. And thermal protection may be there in the power-amp-IC,(stereo is a sony make) but i think thats not the culprit, because it happens mostly when I keep it idle for some days and try turning it on in the cold condition, problem occurs totally in a random way, some times it does not work at all, or it might work for some time and sound goes feeble and distorted and gets muted off.
    There is no power supply problem or gound problem, thats for sure.

    and another electronics device which i have observed similar kind of problem where no component break down but still it will not work is with a Casio Piano (SA-150). This was giving erratic noises when keys were pressed, I did nothing but just removed it off expossed it in the sun for one hour and assembled, the instrument was fine.
    Are you aware of telephone instruments(not the land line connectivity problem) going dead during humid climatic conditions, if its happening the easiest remedy is to expose the circuit boards of the phone to sun, it will be fine.
    I know it sounds very funny, but this happens.
    This made me to think whether my car amp also has some thing to do with humidity.
    waiting for all of your comments
    bipin
     
  5. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    Humidity affecting the power amp is possible, but I don't like it as a reason. Mfr's know that car equipment gets exposed to lots of challenges, and the last one I looked into (unsuccessfully) had a coating on the pcb to keep moisture off.

    Most amps use ic's these days. Can you get at the amps with a can of chiller to see if it's heat-related? I had an old FM radio with a class-A biased output transistor that went into thermal runaway. A spare 2N3055 got it going again. Local heat, possibly because of a poor thermal bond to a sink might be the thing to check for.
     
  6. mozikluv

    AAC Fanatic!

    Jan 22, 2004
    1,437
    1
    hi

    what you have is what we call intermittent operation. try resoldering the audio power ic and the input and feedback components. :blink:
     
  7. bipin

    Thread Starter Well-Known Member

    Mar 21, 2004
    80
    0
    :) Hi Sirs,
    The humiseal conformal coating of the pcb has been done on this PBC as I can see the glossy PBC solder part. And the next step which I planned was to give a solder touch up(resoldering) for all the possible points, most of this is SMDs except the power amp side and some at the AM reception side.
    "Can you get at the amps with a can of chiller to see if it's heat-related?" this line is not so clear to me, could you elaborate?

    I will try solder touch up and let you know by this weekend, whether it cured the disease. Thanks :)
     
  8. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    The chiller I referred to is a spray can of freon, made to spot cool components to see if there is a thermal sensitivity. It's a bit pricey, though, but think of the ozone you get to kill off.

    Alternatively, one can use the back of a knuckle to feel for hot components and heat sinks. The coating makes it very unlikely that moisture is a factor in your problem.
     
  9. bipin

    Thread Starter Well-Known Member

    Mar 21, 2004
    80
    0
    :) Hi

    Finally got the culprit, it was the radio of the car amp.
    As I could not fix up the problem, I gave it to an authorized sony dealer, the technician in a glance told that radio board is the problem and it is a common problem for those car stereos. He has disconnected the radio, (not sure whether the signal-out or the power of radio board) Now the tape part is fine.

    I think the radio output and pre-amp output switches over through an analog switch which is enbled by a mechanical contact switch, via the cassette mechanism.

    The radio board might have been loading and pulling the power input to ground in some ways, other wise how could the power amp go mute?!!

    Still I dont understand how it used to start working when I inject some signals externally!!! huh... :rolleyes:

    Any way Thank you all :p :)
     
  10. Perion

    Active Member

    Oct 12, 2004
    43
    1
    I think you said it was a Sony? Anyway, if you know the model number browse around the folders and files at http://fileshare.eshop.bg/browse.php?id=26 and download the schematics. If they don't have the exact model number, try a few that seem close. I've found that Sony uses a lot of the same (or very similar) circuit design across a multitude of models and platforms. I just fixed a Sony stereo reciever by comparing and using 3 or 4 schematic sets for vastly different units - but the audio driver and final output stages were nearly identical and proved close enough for my purposes.

    Perion
     
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