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Hiwatt Hi-Gain 100, 2007

Discussion in 'Electronic Repair' started by N_Cook, Aug 13, 2013.

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  1. N_Cook

    N_Cook Guest

    Blown mains fuse, cracked through the ceramic and sand and blackened
    internal surface. Someone had replaced the 1A HT fuse with 3.1A at some
    point, which was ok. Someone had managed to strip the internal
    "screening" card from the staples holding it the cab base. Metal side to
    the inside, only the vaguest of plastic sheet insulation over it and
    could have been touching anything. No tag for an earth point/wire that I
    can see anyway, so that will not be going back inside. All logo/printing
    of each valve looks balanced thermal degradation (slight) but these
    Electro Harmonix all dated 0611 have flattened domes at the getter end.
    There is a recessed rough dimple in each of these , all
    off centre so unlikely at manufacture, signs of overheating? I,ve not
    seen these rather raggedy dimples before but I cannot believe the
    dimples would be so remarkably similar if the flat part of the tube was
    heated enough to start collapsing inwards (valves with base at bottom ,
    ie not inverted orientation in use). Valve bases look normal, no
    colourations or discoloured valve Rs inside etc .
    No EL34 tested yet but the first valve I remove, the forked contact for
    grid pin 5 takes a 2.5mm drill shank inserted but touches a 2.55mm one.
    Previous Hiwatt, different model, stopped bouncing back after I replaced
    all the p5 forks with proper 30 yearold NOS ones . That one had forks
    with about 2.2/2.3mm forks that were suspect. That Hiwatt had wired in
    sockets but this is pcb type, so how to remove individual pins without
    removing whole sockets? Replacing complete 2007 sockets with 2013 ones
    seems a hiding to nothing. For good measure RoHS/PbF stickers aplenty.
    Owner queries whether it is due to being borrowed by a wannabe bass
    guitarist who uses extra mass per unit length strings on ordinary
    electric guitar, for some musical style purpose.
     
  2. N_Cook

    N_Cook Guest

    Replace the fuse and the amp works fine, which doesn't help of course.
    The insertion / pull force of the EL34s is far too low, from experience,
    what force in KN that is I don't know
     
  3. N_Cook

    N_Cook Guest

    Extraction only fingers of one hand , no wiggling required. Insertion
    force 3 to 4Kg far lower than usual. C frame around and under amp
    connected to a spring balance and a block of expanded polystyrene , off
    amp deformed with impressions to take top of C and top of valve.
    All 8 laminations bolts for the 2 Tx are loose to the point of nuts soon
    to be dropping off for 2 of the mains TX bolts.
    One of the seriously cruddy output fuses, with LED indicators, has
    highly suspect contact where the end cap quarter-turns in, so yet again
    having to replace these pcb mount fuse holders
     
  4. N_Cook

    N_Cook Guest

    Looks like a matter of desoldering the heater chain wires and then hot
    air gun to the pcb under each socket while pulling the socket , to
    remove them.
    Seems odd to have a fuse in the AC to the bias supply
     
  5. N_Cook

    N_Cook Guest

    Next time I have to desolder double sided large-mass PbF soldered valve
    bases , I will attack as the only time I've done lead-pipe soldering and
    use 2 hot air guns, one heating the base and one on the other usual side
    of the pcb.
    These white no name porcelain bases have a form of pin I've not seen
    before. If you view into the holes that take the valve pins , you can
    see the pins have joggles to them in a circumferential sense. Not the
    fairly common pip joggle inwards towards the valve pin at the point of
    contact but off the flat face of the pin , like mini humps. What that
    means is that instead of the valve pin forcing the forks apart, the 2
    parts of the fork can slightly rotate out of diametrical alignment and
    so just a touching contact, not full mechanical contact
    I think the pin material is thick enough and the forks are not splaying
    apart , the usual dodgey contact failure mechanism but a slight
    torsional twist instead. Perhaps removal of p5 at least and squashing
    out the 2 humps is all that is required.
    The flimsy no name pcb 20mm fuseholders have a type number
    PTF
    50
    on them
     
  6. N_Cook

    N_Cook Guest

    These joggled pins would probably have worked if the slots in the
    ceramic bases were a mm thinner so the metal forks could not twist/splay
    along their length. So an in-situ workaround would probably be formed
    metal forks pushed into the p2,5,7 slots to close up the freedom to move
     
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