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Vibration testing of combo amps

Discussion in 'Electronic Repair' started by N Cook, Jun 18, 2007.

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  1. N Cook

    N Cook Guest

    Because of neighbours and respect for my own hearing I can only check out
    combo amps, after repair, in the thermal and electrical power sense with a
    dummy load.
    Any thoughts on making a vibrational platform driven in range 1/5 to 5 Hz
    range say via fractional HP AC or DC motor or large solenoid?
    Or are mechanical component failures due more to hours and hours of various
    vibration rather than excessive but short duration resonant vibrations, so
    it would be a waste of time.
  2. Meat Plow

    Meat Plow Guest

    Waste of time. Vibration failure in those amps can be from overtones and
    resonates along with main frequencies that can hone in on weak spots.
  3. N Cook

    N Cook Guest

    Sounds a likely reason.

    When fixing replacement resistors to ones that have fractured, always at or
    in the board, these days I add a ceramic bead insulator near the resistor
    body and then slightly compressed block of that orange silicone rubber found
    on photocopier pressure rollers that abutt the fuser roller, between bead
    and board to , in theory, damp any vibration.
    Given a free choice of the material for such resistor leads, what would you
    Obviously the copper rich leads must work harden and then fail. What about
    brass pins or multistrand copper wire swathed in solder to bulk up to rigid,
    any other suggestions ?
    I imagine if you placed a small solderable collar on the component side of
    the pcb then the lead would fail at the top of the collar instead.
    Where heat is the problem , so failed joint , rather than failed lead, I run
    the lead through the pcb and along the trace for a more distributed solder
  4. Ron(UK)

    Ron(UK) Guest

    Do what musicians do.

    Bounce it up a flight of cellar stairs then roll it across some block
    paving on a dolly with rigid wheels, then hurl it into the back of a
    Transit van. Be sure to pack it so that it`s regularly bumping against
    some other item of gear, then drive it at least a couple of hundred
    miles - be sure to get in some inner city potholed roads.
    After carelessly unloading the van in the rain, roll the amp across some
    cobbles before dragging it up three flights of the fire escape making
    sure to clip a door frame or two as you enter the venue, finally balance
    said amp precariously on a couple of wobbly beer crates.

    you`ll naturally have to miss out the next stage out of respect for your
    hearing, but normally the amp is subjected to a prolonged thrashing at
    volume 11 for a good two hours with an old bar towel over any
    ventilation slots - you should be sure that plenty of clipping and
    distortion occurs for this to be a proper scientific test. It`s also
    best if the amp is connected to a dodgy power source via a pound shop
    four way extension socket. To avoid any unexpected quiet moments,
    replace the fuse with silver paper from fag packet.

    Repeat the above in reverse order (apart from the actual playing of
    course) making sure that the amp doesn`t have time to cool down before
    being left outside in the rain for a while before it`s bumpy journey
    home and subsequent deposit back in the damp cellar.

    Repeat three to four times a week and if the amp can withstand that,
    it`ll survive anything you can throw at it in your workshop.

    Hope this helps

  5. N Cook

    N Cook Guest

    Didn't HH amps run ads where there were testimonials from users where one
    amp had been thrown out of an upstairs window onto concrete and another
    whose amp had been run over by a delivery truck and no structural damge,
    only cosmetic damage sustained.
  6. Ron(UK)

    Ron(UK) Guest

    I don't remember that, but I can well believe it They were very well
    put together.

  7. Meat Plow

    Meat Plow Guest

    The softer the lead the less it's likely to fracture correct? Personally I
    am a vintage amp tech. I really won't work on point to point wiring unless
    it's one of mine or a friend's in desperation. I have 4 high power
    tube and one SS combo guitar amps. 3 Peaveys (2 Artist 1 Special) a newer
    digital Waller DSP 100, and a 70's Fender Twin Reverb that was a basket
    case that I traded a cheapo acoustic guitar for. Also have two 50 watt
    heads, a Laney and Fender Bassman. I've had problems with the SS Peavey
    and intermittent connections that seem to come and go. The amp I used the
    most back when I was gigging was the Twin. Smaller gigs with limited stage
    space I used the Waller because it had some built in effects. I repaired
    the Waller under warranty myself after it developed a digital noise when
    idle. Talked to Waller tech support and it was a known problem. Gave them
    my credentials, they said go ahead and do the modification.

    As far as stress related failures in combo amps goes, Peavey seems to lead
    the way as far as what I've worked on. I can only imagine some of these
    newer amps like Berringer and the likes made in China crap. Good luck to
    you, you must be a younger man as i haven't the patience to do anything
    but the oldies made in the USA and the UK. (love my AOR Laney Pro Tube 50
  8. Meat Plow

    Meat Plow Guest

    BWaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaahahah you hit the nail on the head.
  9. Ron(UK)

    Ron(UK) Guest

    We never used to get many stress related failures in those old point to
    point hand wired valve amps despite - or maybe because of - long
    component lead lengths. Even the old AC30`s despite looking as tho they
    had been assembled by an enthusiastic schoolboy, were pretty reliable in
    that respect. Of course in those days, we used the old technique of
    making a sound mechanical joint first, then soldering it! The problems
    started when PCB`s were introduced.

    I cant really understand why anyone would want to vibration test a
    customers amplifier, unulss it`s looking for dry joints, when a rubber
    mallet or chopstick will suffice. Surely any piece of kit has a
    'vibration lifetime' (MTBF thro V?) and I cant think why anyone should
    want to shorten it by deliberately shaking it. Every minute spent on a
    vibration table is one minute closer to failure in the field (IMO)

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