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Laney CD850S, 2002

Discussion in 'Electronic Repair' started by N Cook, Aug 22, 2007.

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  1. N Cook

    N Cook Guest

    2x 200W mixer amp
    I have the schematics for this amp but anyone familiar with them ?

    Problem in protection (falsely triggering occassionally), both amps dropping
    Of course I have not been able to induce activation.
    2 separate power amps otherwise apparently the same , both carying output DC
    protect and overheat protect output relay drive ccts. But only one normally
    open thermal switch going to one amp only so apparently dropping out that
    relay only with no apparent interconnect to the other amp.
    When it was up together I did not think to try shorting the thermal switch
    but I assume it would drop out both relays and both amp outputs.
    Anyone know how they interconnect. Also what I assume to be o/p fault DC
    protect uses 2 470uF back to back electrolytics off the output sense line.
    I never like to see this form of "non polarised" high C cap , is that just
    me or anyone else find it iffy, questionable or perfectly valid design-wise.
    More likely a problem with the thermal sw or 12V supply to both protection
    cct and the fan , rather than a fan problem as it did occur once at sound
    check and once at switch on. The fan does stop when this happens so unlikely
    a send/return bypass sw problem, the monitor outputs still continue to work
    so not a mains failure problem.

    There is also a thin low current , signal level earth wire from the ps that
    has come adrift loose in the case , broken at some pcb solder point
    presumably, somewhere indeterminate from the schema, anyone happen to know
    Yet again bits of hardware, 2 bolts that hold the heatsink to chassis loose
    and wandering around inside the case, but probably just incidental to the
    main problem.
  2. Ron(UK)

    Ron(UK) Guest

    You say you found an earth wire off, is it possible that there`s some
    parasitic hf oscillation going on which is tripping the protection
    circuits? Does that earth wire look like it was added as an afterthought
    by any chance?

  3. N Cook

    N Cook Guest

    Proper jobby, it is on the same connector as one of the main +/-56V +ground
    connectors but is long enough to go to somewhere on the preamps/ graphics/
    effects area of the mixer.
    Much smaller gauge wire than those power feeds.

    I've just emailed Laney on the interconnect and loose wire prob and temp of
    thermal switch marked
    Logo - C followed by 2 square wave pulses
    K85 probably (smudged)
  4. N Cook

    N Cook Guest

    I just checked the switch in a can of water and it goes s/c at 86 degrees C
    and reverts to normaly open at 71 deg C and no amount of knocking will
    disrupt it.
    So the 85 means 85 deg C and perhaps K means Normally Open
    Bit low for a 2x 200W amp even if fan assisted cooling but as ' no'
    operation means its immune to false triggering from contact corrosion I will
    leave that as is.
  5. Meat Plow

    Meat Plow Guest

    Have you checked this amp out with the owner's speakers and cabling?
  6. N Cook

    N Cook Guest

    No wonder I did not get a reply from Laney/Technical
    Emailing a schematic (not necessary as it was on the www )
    is one thing but implied technical criticism is another.
    The single thermal switch, as i suspected now it is back together, only cuts
    out the upper (left) channel, I will add a slightly higher th sw to the
    empty socket of the other channel.
    Just a signal diode connecting the existing thermal sw to the
    same pin on the other amp , empty socket, would have coupled both together
    but I will add a higher temp switch instead to allow half-cock continued but
    still protected use on one amp should the other fail.
    The rest of the protection is to protect from DC at the output, functional
    in both amps.
    schematic (page 8 of 8) on useful Polish site

    The stray low current wire would seem to bridge back one side of the ps to
    the other , on the same board, when they are both connected to main
    reservvoir cap common via large traces anyway - there was a lone socket pin
    in the housing with matching frayed wire taking power to one amp , the other
    end was with the power connector for the other amp.
    I've also replaced the 12V fan as worn and wobbly and more to the point,
    fully cut the casing open at the fan vent area and covered with a wire
    grill. There was just 14 small slots with a combined area of only 1.5 sq
    inches, let alone vortexing, so overloading the 3 inch diameter fan and
    losing cooling efficiency.
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