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Thread: Replace caps?

  1. #1
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    Replace caps?

    I've been reading about replacing caps in Soundcraftsmen vintage amps.
    .
    I've a ~22 year old PM860 that, after a modest warm up period, simultaneously goes up and down in power (volume) level on both channels simultaneously.
    .
    DC offsets are modest @ output. In the mv range.
    .
    I've not tested the amp at non-idle conditions (need a non-speaker load; thinking about some light bulbs?)
    • some 50-100 watt bulbs seem to be about four ohms at thermally stable conditions
      .
      • Thoughts? If you please?
        .
      • They are not inductive or capacitive, so they seem to be a good load?


    .
    At no load, p/s rails are stable at ~70VDC.
    .

    • I presume that this is either a problem with the regulation circuit or, perhaps, the output filter caps?

    .

    I've posted a schematic at http://www.mediafire.com/?fw5oikud3d1az54.
    .
    The original is here: http://www.audioheritage.org/vbullet...4&d=1296216160
    .

    • I presume the regulation circuit is the upper left hand corner of the schematic?
      .

      • If not, please advise. .


    .
    I'm an engineer, an ME, and am gasping for air here.

    .
    And, when I read of replacing caps, does this include ALL caps, or just the electrolytics?

    .
    AND, btw, I have ALL manner of test gear at my disposal. LOTS AND LOTS OF STUFF, including:
    .

    • dmms -- some pretty awesome ones
    • o'scopes
    • power supplies; all manner of such
    • low frequency network analyzers -> 50 KHz dual channel, 100 KHz single
    • transient capture devices, lots of 'em
    • counters
      • etc etc etc.


    .
    Thanks for you help. WE DO appreciate your inputs...
    .
    mac
    Last edited by pcmacd; 09-24-2012 at 11:05 PM. Reason: clarity

  2. #2
    Senior Member lgvenable's Avatar
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    my experience in replacing caps in BGW amps

    I replaced the two electrolytic caps in each of the amp modules in my 13 BGW 250D or 250E amps, and got better bass response . that meant 4 electrolytic caps per amp>>there were two caps in channel 1 and two caps in channel 2. I have not done this yet to my 750B's; but wouldn't be squeamish after my first experience.

    They were all in the signal path; and really wasn't too expensive (3.00 /channel). Just remember there is a + and - on electrolytic caps.

    I didn't replace the two much larger caps in the center of the amps...as an FYI. Pretty easy to do, just use a good electronics grade silver-tin eutectic solder, and while you're at it reflow any suspicious solder joints while you're in there.

    Oh and invest in a good soldering station, I got an industrial station for ~ 75.00 in great shape off eBay.
    Integra DHC80.1,3x 4636LF, 2360-2446J 2404H,12 x 8340 Surrounds, 2 x4645B, BGW 250D's,250E's,& 750B's 16 amps...7600 watts

  3. #3
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    It is better to replace them all.

  4. #4
    Senior Member lgvenable's Avatar
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    Quote Originally Posted by rickedd46 View Post
    It is better to replace them all.
    Agreed...Just more expensive
    Integra DHC80.1,3x 4636LF, 2360-2446J 2404H,12 x 8340 Surrounds, 2 x4645B, BGW 250D's,250E's,& 750B's 16 amps...7600 watts

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