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Eden "The Metro" Bass Amp intermittent problem

Discussion in 'Electronic Repair' started by N Cook, Mar 19, 2007.

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  1. N Cook

    N Cook Guest

    Cut out in use then came back.
    Next time failed to give output but owner noticed clip light in pre-amp
    functioned as normal
    Now I have it, I cannot induce it to fail.
    Anyone know of generic/specific Eden problems?
    Likely suspects the discrete wire IDC connectors for power and signal
    interconnects, unsupported wire-wounds on end, mains thermal switch.
    Anyone know the identity of the valve/tube ?
     
  2. Lord Valve

    Lord Valve Guest

    Of course not. ;-)
    12AX7.

    If it's got an effects loop (or pre-in pwr-out jacks)
    check for dirty normalling contacts.

    Lord Valve
    Expert
     
  3. N Cook

    N Cook Guest

    --
    --
    Thanks for the info.
    Do they remove the valve labelling ? no trace of anything on this one.
    Send/return switches are the first things I look at with this sort of
    problem along with speaker plug/lead
    ..
    I wondered if there was any Eden specific problem areas. I've not checked it
    out but is that a thermal cutout in the mains feed , rather than filter, to
    monitor general chassis temperature, not the output N/O(cold) one for the
    fan
     
  4. Elvis Kabong

    Elvis Kabong Guest

    Their power supply boards have been known to become intermittent even though
    they are double-sided PCBs. You may really have to pound on the filter caps to get the
    bad one or the bad connection to act up. Also look underneath the supply PCB for high
    watt
    ceramic resistors sitting on the chassis with heat sink compound on the chassis
    while they are soldered to the board but a lead may be broken and only touching
    where it's supposed to be soldered.

    Sometimes the zeners and their current dropping resistors (more ceramic ones)
    for the plus/minus low voltage supply become overheated and melt solder or get
    bounced out of whack resulting in a loose lead.

    I would suggest to run a signal thru it directly into the scope bypassing the dummy load
    while tapping and wiggling things around or get it heated up with a load and tap around.
    Try it both ways, but do it the cool way first, right?
    Checked the pots for loose cases?
    Customer already ruled out his bass, instrument cable, speaker cable and cab or
    did you?
    Retighten the tube socket terminals and clean them?
    Detachable AC cord plugged in firmly by the customer? Have him/her demonstrate without
    saying anything first. Then again, you just may have to have the customer come in with
    their entire rig and demonstrate the problem.
    Whatever it is, good luck. I know the feeling.

    Ed
    Sonic Surgery
    New Orleans
    Earth
     
  5. Guest

    re solder all connections on pc board but we have seen
    the speakers in these amps fail frequently.
    check the pigtail leads on the speaker basket, damaged
    leads from loud playing can cause what you describe,
    intermittent output.
    play the amp really loud to trigger the symptom, with a bass.
    or tap on the pc board with a plastic handle to find evidence
    of the bad solder connections.
    eden sells the replacement basket which bolts to the
    magnet. easy field replacements.
    the valve is ECC83.
     
  6. N Cook

    N Cook Guest

    I did not like the front panel switch in line with the speaker line.
    In standby instead of cutting/shorting the main amp input you switch out the
    speaker. IDC interconnects on the speaker lines and power lines seem a bit
    irregular to me but as cut out rather than distortion those power lines
    presumably ok.
    As 2 main bass speakers plus 1 in the cab, as sound died unlikely a problem
    with a speaker but will check the wiring in there as well as the other
    likely suspects.
     
  7. Arfa Daily

    Arfa Daily Guest

    I haven't come across one of these, but I can't help thinking that a front
    panel switch in line with the speakers, hooked via *any* kind of 'pin'
    connector, let alone IDC ones, has got to be one of the most ludicrously
    crazy pieces of design work I have ever heard of - especially in a bass
    combo with two drivers in it. Peak currents of many amps will be (attempting
    ! ) to flow through this combination ...

    Arfa
     
  8. N Cook

    N Cook Guest

    My thoughts exactly, someone fumbles around the front panel with full load
    going through amp and breaks the line to the speaker at this switch.

    Anyone know what the function of the 12V, 10 W "festoon " bulb in the
    variable crossover in the cab is, fuse? variable impedance element?. Working
    order but just curious
     
  9. Elvis Kabong

    Elvis Kabong Guest

    Tweeter fuse. However, half the time the tweeter coil blows and half the time the light
    bulb fuse blows. One needs to stock both items.
     
  10. ....
    That's probably your culprit... going below
    the minimum impedance. What is it rated for,
    what are you running it at? SS amps overheat
    when run below their rated min imp.
    __
    Steve
    ..
     
  11. N Cook

    N Cook Guest

    I think I'll bypass this front speaker switch and wiring.
    Its only rated 3amp and thats for AC, could easily be the problem, let alone
    potentially "fatal" to the amp. The cab wiring and pair of 4 ohm speakers in
    series seem fine.
    I may trace the circuit around and between preamp o/p and amp i/p and change
    that switch to a line level cut function.
     
  12. DGDevin

    DGDevin Guest

    Eden used to have such a good name, yet here are all these posts about goofy
    design issues and recurring problems, I wonder if this dates from them being
    borged by the dreaded big fat corporation?
     
  13. bk

    bk Guest

    How could the switch be "fatal to the amp"?
    Designed appropriately, it's there to disable the speakers
    in the event that one elects to operate things
    'silently' as in use of stereo phones or in certain
    recording situations. It isn't a common failure mode.
    Yes, an intermittent switch will shut down the speakers but
    as far as the power amp finals are concerned,
    "fatal" is impossible as no current would flow when
    imposing an open at the load connections.
    Here's a tip: Instead of poking around blindly, give Eden
    a call. They'll not only send *any* schematic you
    desire, they'll also be the most helpful resource out there...
    since they spawned the thing.

    The output employs thermal protection via use
    of an agc circuit which also operates the cooling fan.
    Have you cranked the thing up against some dummys
    in order to test for thermal related shut down? You should.

    Any chance that the owner/operator is using too low of a load
    impedance?
    IIRC, the agc circuit can shut the power amp completely off. Barring
    any
    "connection-based" problem (previously mentioned),
    a guess would be that something in the agc circuit is failing
    or theres a problem with the impedance at the load.

    Contact Eden.

    bk
     
  14. N Cook

    N Cook Guest

    Here's a simple experiment unless you have a heart problem.
    Find a small 12V relay , connect to a 12V dc source, hold the relay contacts
    between thumb and index finger and disconnect the wire to the 12V supply.
    This simply demonstrates the power of back emf and V=L* dI/dT, speaker
    inductance about 1mH and peak current through a speaker of maybe 20 to 50
    amps.
    If those few hundred volts (at least) speaker induced back emf exceed the
    breakdown voltage of whatever partial contact gap, not the final gap, soon
    (microseconds) after the switch break/ poor IDC contact/poor pcb header pin
    contact then goodbye pa.

    It would not take much fumbling in poor light, drunkeness, confussion or
    whatever to flip that switch in full use , its not recessed and in the back
    of the cab. The owner does not use the switch so I will block off the
    relevant outlet with a nylon bolt pushed in the 1/4 inch socket as a
    reminder to him, not to use. The other 2 paralleled outlets are not
    switched, but I may hardwire/solder back to the pa.
     
  15. N Cook

    N Cook Guest

    Found another nasty under the ps board but don't think it is the main
    problem.
    This pair of wires to the speaker switch had one of the wires squashed
    between a large W/W ceramic cased dropper under the ps and the chassis.
    Melted through but as it cuts the earth to the speaker presumably no problem
    as such and wire not broken. But there is smoke blackening or something
    grimy in that area but it could be a small electrolytic parallel to the fan
    cooked as it is directly over the large droppers and leaked electrolyte but
    all rather nasty. The aluminium of the chassis directly under this
    particular dropper has a strange flecked grey corrosion or something that
    probably more likely due to electrolyte rather than vapours off the charred
    PVC insulation. I don't see why this insulation has charred rather than just
    melted as the owner never switched this speaker switch, I assume somewhere
    maybe ages ago it was open at some point

    Anyone know the function of the big triac on the mains, crowbar operation to
    blow the mains fuse if too much mains power drawn ?, I've not traced any
    triac associated line back into the amp that could trigger it as more active
    amp protection.
     
  16. N Cook

    N Cook Guest

    The corrosion i've photoed here
    http://home.graffiti.net/diverse:graffiti.net/ali2.jpg
    The overexposed T area is the mains transformer and the groove marked "<"
    was created by the squashed/trapped cable or exposed wire core probably
    vibrating.
    From "<" to T only about 10mm so just 5 mm or so free gap to run these 2
    speaker wires between mains torroid and high temp wire-wound dropper with no
    added high temperature sleeving.
    The grey flecking is what I'm assuming is electrolyte corrosion effect
    rather than burnt PVC product.
    If this fan lytic over the fan supply dropper leaked to short in use then
    dropper would generate more heat as well so compounding as well as shorting
    out the fan to the main amp heatsink. The owner had used the speaker switch
    in the past, during sound checks. What a tangled web we weave.
     
  17. N Cook

    N Cook Guest

    Before hard-wiring, the 2 lines from amp to speaker outlet , I've just
    realised there are 12 chances of a break in that route, to and return.
    3 pairs of IDC and pin connections in each line.
     
  18. N Cook

    N Cook Guest

    The thermal protection circuit is activating before the 60 degree centigrade
    fan switch is cutting in, under high load.
    The fan switch temperature , testing in isolation , is about right.
    I'd rather change the switch to a 40 degree one rather than tracing the
    drift in the thermal protection circuit.
    Any advice? or even if there is a recognised cure in the thermal matching
    circuit ,eg replacing/matching Vbe of the sense transistors.
     
  19. N Cook

    N Cook Guest

    I've never had an LED causing failure of an amp.
    Intermittently o/c losing the fan supply ok signal that goes through this
    LED to the pa causing it to shut down . Also not passing current to the
    power-up hold-off circuit so pa failing to come on, thinking the fan supply
    was absent. Like the fan lytic, both perched right on top of two large
    droppers on the ps board.
     
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