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Fender deLuxe , PR772, 2009, Mexico

Discussion in 'Electronic Repair' started by N_Cook, Jun 20, 2013.

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  1. N_Cook

    N_Cook Guest

    Like the previous time quoted below, thought it was a PbF problem at the
    input socket, as a definite problem in that area causing about 1/10th volume
    but not mains noise intrusion, when testing on receipt
    PbF solder used inside but nowhere stated , as per usual. The same symptoms
    appeared before and as used with a footpedal at that point, under warranty
    Fender replaced the foot pedal.
    Is the schematic for this digital+valve amp around ?
    I cannot see how loss of ground contact and ring contact to ground would
    cause a consistent (in failure) 1/10 or so volume drop. PbF solder points at
    the socket (now managed to remove the preA pcb )
    look good (for PbF)
    No WD40 this time, input socket bush nut , not loose on receipt, same off
    centre type , not desoldered yet but probably another junk Neutrik. The 2
    parts of the barrel turn relative to one another ,easily , without the bush
    nut in place braced against the chassis

    previous experience
    Fender Bassman 250 , PR597, 2005
    Bad input socket, is there a generic problem with these?
    Of course I thought it was a PbF problem but it escaped that , leaded solder
    inside. Solder joints seem fine and no broken tag. Socket make not seen ,
    not desoldered yet, maybe make underneath.
    The jack centre line is off-centre of the socket centre line , the sleeve
    contact is a barrel which is chromed and only makes contact with the now
    tarnished ground contact if the metal bush nut is tight and the 4 pointed
    grounding tangs are not bent and are making good contact with chassis metal.
    So 2 ways of loosing ground contact. With no bush nut the chromed barrel
    will rotate a few degrees and then just touching contact to the socket
    ground terminal .
    Of course I have the preamp out of the chassis now and so do not know what
    the initial state was other than the bush nut was not loose when received.
    All compounded by gallons of WD40 squirted everywhere that has certainly
    lost the tip and ring bypass contacts to ground with no jack inserted.
    I can see repeated tightening/ overtightening? of the metal bush nut
    plenty of spanner/wrench force can be applied as metal nut and metal bush)
    will deform the 4 tangs/ dig deeper pits into the front panel metal, until
    eventually no contact.
    I expected some no-name Chinese make but no , it is Neutrik with this crappy
    Reassembled preamp with the socket exposed and it was then obvious there is
    a third way to loose ground contact. There is no sprung contact to the
    sleeve of the jack , just rests on the chromed barrel , relying on the
    spring contacts of the tip and ring to supply pressure to keep the sleeve in
    contact with the inside and innermost small segment of arc of the chromed
    barrel . Even without WD40 , corrossion and general crud , because of the
    lever action, it takes very little upward force on the jack, at the cable
    end , to lift the sleeve and break contact.
    So failure mechanism probably went like this.
    Bad ground contact due to slack nut. Instead of tightening, owner squirts
    WD40 in there which
    pools in the lower surface of the barrel , collects crud, congeals etc.
    Eventually he tightens the nut , but by then its too late.
    The input was always bad from new, according to the owner, but got worse
    over the years.
    AFAICS no way could this design be of merchantable quality
    A get by , on the
    road, solution to this would probably be some sort of reaming action inside
    the bush barrel
    Replaced with 3 sprung contact Cliff with plastic barrel and added star
    washer wired-in. You can then have
    something like an interference fit between jack and barrel , so little or no
    play in any off-axis direction. Because the Neutrik is metal and metal there
    has to be a clearance fit and so its easy to break the sleeve contact.
    Neutrik barrel internal about 6.30mm, 6.20 jack sleeve, Cliff So 6.68mm "
  2. N_Cook

    N_Cook Guest

    Seems to have cured the problem again, just wish I knew exactly how and why.
    Will have to wait until a PR772 circuit emerges out there, the input line
    goes off simply to a TL072 but there is compexity around the ground lines
    and chassis
  3. N_Cook

    N_Cook Guest

    KISS, replaced the "Dyson" over-designed Neutrik with a plain and simple
    Cliff .25 inch socket of which the design and materials have been around for
    40 years or so.
    The previous Fender Bassman never bounced back so this cure must have worked
    on that occassion.
    No one else come across problems with these Neutrik sockets?
  4. N_Cook

    N_Cook Guest

    I'll try that.
    There is a ground/ quasi-ground?/switching? line that goes off via a ribbon
    to the PA somewhere, removing the PreA board was enough to be getting on
    with , without added intellectual exercise removing the PA board just to
    see where that line went
  5. N_Cook

    N_Cook Guest

    I wasted 1/4 hour wading through the bloatware on trying to find
    the em address.
    Then 10 seconds googling with search-term

    technical support email

    found the em.
    A Matt this time has forwarded me the schema
  6. N_Cook

    N_Cook Guest

    The only mechanism I can see for this 1/10 volume throughput.
    The Neutrik design flaw inside the ground/barrel section, whereby tight bush
    nut but the 4 sharp tangs have produced pits that then get aluminium oxide
    coatings from the chassis metal or tinpest build up at the loose part inside
    the barrel section. Say it builds up to 5K ohm resistance , then loss of
    ring contact and so instead of say 500 ohm of a pickup it is now 5K5
    presented to the input of the amp and attenuated signal current
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