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

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  1. N Cook

    N Cook Guest

    All valve, no solid state devices, probably made 2002 from barcode date? and
    2001 date on Sovtek 5AR4.
    Powered on for about 5 seconds with cracks and bangs then HT light went out
    and dead.
    Inside looks and smells fine but "gently" blown HT fuse. I've not tested the
    valves yet.
    1 of the 22K 1/3W HT R/C dropper resistors slightly discoloured and all 4
    EL84 1/2W, 100 ohm grid resistors slightly discolured. Someone has crudely
    cut one of the 2 grounds to the pcb, presumably trying to reduce hum when it
    was working.
    All DVM DC resistance measurements of the transformers seem ok.
    Any pointers to known problems with this version?
  2. DaveM

    DaveM Guest

    First question back to you is.. is the HT fuse after the rectifier or before it?
    Or, are you really talking about the AC mains fuse?

    If the fuse is really in the AC mains line, then almost all my bets are going to
    be on a shorted power transformer or a shorted MOV or line bypass capacitor.
    If the fuse is after the rectifier, then you should pull the rectifier tube,
    replace the fuse, and see if the fuse blows again. If it blows again, then
    check to see if maybe one side of the power line is shorted to ground...
    possibly caused by a shorted MOV or line bypass capacitor. Or a stray piece of
    wire, or other malady.
    If the fuse doesn't blow again, then you have a bit of troubleshooting to do.
    Most likely a shorted B+ filter capacitor or shorted tube. Be sure to keep your
    eyes peeled for stray wire clippings.


    Dave M
    MasonDG44 at comcast dot net (Just substitute the appropriate characters in the

    Life is like a roll of toilet paper; the closer to the end, the faster it goes.
  3. N Cook

    N Cook Guest

    MOV= metal oxide varistor ?
    Probably the most diagnostic feature is the 500mA HT fuse, on the DC side of
    the rectifier tube, is a small gap in the fuse filament with a glob on one
    broken end, both remnants axially present in the barrel, no staining inside
    barrel at all.
    Last gasp rather than catastrophic failure.
    I'm struggling with sticky needle on my valve tester , again.
    I know if i poke around in the meter I will totally bugger up the movement ,
    so no use rather than problematic use with a lot of case tapping.

    As an aside anyone know when barcodes started appearing on electronic kit or
    components? for age determination assistance. Mid 1980s is my recollection
    but googling has not helped
  4. N Cook

    N Cook Guest

    the third EL84 tested , started out normally then rapidly accelerated off
    the scale. I've never had the protection relay on my Avo CT 160 dropping out
    I had to get inside to manually reset it and took the opportunity to sort
    out the meter needle sticking problem, much easier to get to and into this
    meter movement than I'd thought.
  5. Arfa Daily

    Arfa Daily Guest

    The schematics for most of the AC30 derivatives are on the VOX website for
    free download

  6. Meat Plow

    Meat Plow Guest

    Why not pull the tubes, put a new fuse in it and see if it blows?
    Then replace them one by one. I know it ain't a pretty way to diagnose but
    I've worked on many tube guitar amps and never caused additional damage
    doing this as long as I was careful. And it did help to have a variable ac
    supply with a current meter but it can be done using a 100 watt light bulb.

    #1 Offishul Ruiner of Usenet, March 2007
    #1 Usenet Asshole, March 2007
    #1 Bartlo Pset, March 13-24 2007
    #10 Most hated Usenetizen of all time
    Pierre Salinger Memorial Hook, Line & Sinker, June 2004
  7. N Cook

    N Cook Guest

    only the older amps

    Laney did the same but pulled them from open access a few months ago. I'd
    managed to download about 2/3 of them, over a few months, before they were
    all pulled. Just for my personal use at some future date. I would suggest
    doing the same for any site that has such technical details. I assume some
    accountant or intelectual property rights lawyer comes along and says
    "remove that content" from open access that some well meaning technician had
    put up there.
  8. N Cook

    N Cook Guest

    or quite likely being removed from site/s because of some health and safety
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