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Marshall JCM 2000, DSL of 2003

Discussion in 'Electronic Repair' started by N_Cook, Mar 15, 2012.

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  1. N_Cook

    N_Cook Guest

    Hum particularly on clean channel builds up from nothing ,over half an hour,
    and very intrusive 0.3V rms of hum over 8R speaker load and still rising.
    Putting a signal in Return, for PA only, is fine but opening guitar input
    hum returns.
    Owner had replaced all the valves and exactly the same hum.
    Hum is negligible on the downstream HTs from HT1. What sort of grounding
    problem increases with warmth?
    I've not started exploring the low voltage electros around V1 yet, I'm
    letting the amp cool down
  2. N_Cook

    N_Cook Guest

    C13 100V 1uF at V1(A) cathode , of all things, is highly sensitive to hot
    Monitoring the hum over the output load. Once force warmed and hum level
    risen it is reluctant to go down again from natural cooling or from freezer
    spray - what process is going on with in it?
    Now to replace by fudge fitting to in situ component leads or take the whole
    thing apart to replace properly?
    Marshall saw fit to bodge 3 resistors by cutting off the originals and
    flying replacement ones to the cut wires - so whats good for the goose ....
  3. N_Cook

    N_Cook Guest

    Any ideas about the origin of the "heat ramping" effect?
    Allowing the whole amp to cool for 1/4 hour obviously resets the effect
  4. N_Cook

    N_Cook Guest

    I will check that, I could understand with that old lino or whale hide they
    used to use on Fenders.
    Marshall fault , some chemical getting capilliary fashion down the glass
    fibres of the composite?
  5. N_Cook

    N_Cook Guest

    It would be nice to find a schematic for this specific variant , but only
    one Google Ref for the number on the end face label

    I'm wondering if there is any significance in the bodged 1/3 W R
    R70 of 5K6
    R7 of 5K6
    R48 of 100K

    unfortunately none of these numbers agree with any JCM2000/ DSL schemas that
    I have.
    Similarly I cannot find the 2K2 HT line dropper for (a) half of V1 mentioned
    on these schema
  6. Phil Allison

    Phil Allison Guest

    "Gareth Magennis"

    ** The PCB material is definitely the cause of the problem.

    It is made from UTTER CRAP !!

    However - uber clever fixes involving a Dremel or mounting resistors in
    mid air or any similar hare brained ideas are all just re-arranging the deck
    chairs on the SS Titanic.

    If no new PCB is available from Marshall at a reasonable price - just add
    a bloody fan !!!!!!!!!!!!!!!!!

    A single 120mm, 240VAC fan blowing directly on the OP tubes from below cures
    the overheating / bias runaway problem completely.


    Marshall are a total bunch of arseholes.

    The Pox of the audio industry.

    Just like their shitty amps.

    .... Phil
  7. JW

    JW Guest


    Gotta remember that one!
  8. N_Cook

    N_Cook Guest

    I should be able to get back to it in a couple of hours. Only just got
    around to building a Dexion support frame so I can toss these sorts of amps
    around with gay abandon, and not risk the bottles.

    The valve set that are in here are all 2009 , he took out the new ones and
    replaced these used ones. They look brand new except for simple dragon's
    teeth marks, ie not repeatedly moved around.
    The Russian markings are quite visible and there is no sign of overheating
    of the bases , still light brown , and the pcb around looks as new.
    But the first thing I will do is check for o/p bias drift, then heating the
    pcb with hot air and a 2Gohm megger and then remove V1 and see if hum
    returns then replace and monitor DCs around V1 on heating ,

    No mention of that Marshall Forum page about hum but it could be these pcb
    problems around V1 the immediate problem
  9. N_Cook

    N_Cook Guest

    main bias results
    monitoring every 5 minutes for the first 15 minutes
    rate of increase is falling
    passing 10 to 15 minutes in
    one side rising 1.0mV per 5 minutes and the other side 1.5mV per 5 minutes
    starting from that 15 min reading of 76.9mV and 68.2mV should be about 90mV
    over an hour .
    locally heating with hot air (low setting) for 20 seconds the voltages shot
    5 or so mV but soon dropped back to where they were , unlike the problem
    around V1 it would seem.

    now for the other preliminary tests
  10. N_Cook

    N_Cook Guest

    No E number of board supplier found but I would say, by trying to press a
    needle into the board, it is epoxy rather than polyester composite.
    Megger showed nothing untoward
    With no V1 in place , the grid socket pins measure about 10 or 20mV DC, wave
    hot air over the valve base and the readings shoot up to 100 to 200mV, just
    like applying a magic wand.
    The cathode lines have 2/3 orders of magnitude lower resistance to ground so
    any such effect not so obvious there

    These are the small bottles not the hotter big bottles, something to do with
    that metal shield plate for them? I'm assuming the effect is at the valve
    base through board , hot pins rather than where the anode dropper leads pass
    through the pcb, and passing through rather than along the surface . I will
    explore this, as hard wiring the HTs to isolated valve bases ,only, is quite
    different to hard-wiring all HT traces.
    Now what is the physics /chemistry of all this ? I assume something
    hygroscopic is grabbing moisture then forming a conductive salt that stays
    within the micropores of the surface of the glass fibres, but why
    temperature increasing the conductivity , what salt has highly temp
    dependent conductivity, we're only talking 50 degree C or so
  11. N_Cook

    N_Cook Guest

    For those thru-board valve bases it looks as though you would have to
    replace with standoff types to retain structural integrity but with
    electrical isolation , as well as all that hard wiring and intermediary
    isolation/ mounting points.
    Looks like too much of a work up in comparison to paying Mr Marshall for a
    replacement board.
    I'm just wondering if this particular board , only 1 google ref, that they
    will not have a replacement
  12. N_Cook

    N_Cook Guest

    Why did the mod-bods choose to isolate the grid pins rather than the anode

    Anyone know if they use mineral filler (cheaper) in the epoxy of epoxy pcb
    manufacture. Like the use of calcium carbonate in the epoxy bulking/fixing
    of toroidal transformers or even car body repair filler.
    This morning I made a test cell of some calcium carbonate and water to thick
    paste consistency in an inch wide plastic bottle cap. Resistance across a
    diameter about 60K. Waft a low-setting hot air gun over it and resistance
    drops to about 2K, now rising again.
    So moisture/condensation can get into the edges and component holes of such
    a pcb and the glass dutifully conduct it capilliary fashion.
    Perhaps a cure might be a low oven bake of a day at 105 deg C of a populated
    board , assuming nothing comes to grief at that temp and then some sprayed
    on conformal coatinf along all edges and component leads, but that would not
    get to the prime source of problems , inside /under the valve bases and
    those pcb holes
  13. N_Cook

    N_Cook Guest

    I can see run away bias , less neagative grids with temperature rise being
    caused by conductive epoxy. But as I say (for the moment) this amp PA seems
    ok, but where is the hum coming from around V1? some conductive path from
    the heaters?
  14. N_Cook

    N_Cook Guest

    For an owner who knows he has such an amp, this would be worth doing.
    Obtaining or making an amp sized heavy duty polythene bag that can be
    resealed easily and some sachets of activated silica gel crystals and a
    large jam/pickled gherkin jar to keep the oven acivated ones in, until use.
    After each use of the amp , place amp while still warm prefereably ,in the
    bag with a fresh sachet from the storage jar.
  15. N_Cook

    N_Cook Guest

    A day on of drying out and my test cell showed 150K, warming easily brought
    it down to 10K.
    Tried a sample of calcium carbonate as it is , powder straight from the open
    bag a decade old at least , and no response to a megger.

    My chemistry failed. 30 percent HCl showed no fizzing with the powder.
    Warming up and a lighted taper extinguished in the tube.
    Ground off a sample from this Marshall board and the same.
    But then tried a tube of HCl on its own and same extinguishing of taper.
    I'll stick with the day job.
  16. N_Cook

    N_Cook Guest

    My test cell now measures >500M cold or warm , so perhaps not calcium
  17. N_Cook

    N_Cook Guest

    Now trying a test cell mix of NaCl and calcium carbonate, as no convenient
    source of calcium chloride to see if a mix of those , via deliquescence,
    will go thermally conductive without addition of liquid water. Until anyone
    gets more info I will go with chloride contaminated calcium carbonate as the
    suspect pcb filler

    As nothing is learnt by handing over money to Mr Marshall, modded this main
    board. As this 06 variant is almost a google-whack I doubt an exact
    replacement board is available ,off the shelf, anyways. Missing opto devices
    Now I've worked out how to make a 5mm diameter hollow end face cutter , to
    go in a Dremmel. Once the main board is disconnected ( how many
    connections?) it is now a simple matter of cutting away a neat hole of pcb
    around the grid socket pins. And while at it, did so to all 12 grids ECC83
    and EL34 and hard wired with silicone sleeving , so all grids are now
    isolated from the pcb.
    Now no trace of any hum over normal background even over an hour and also
    no sensitivity at all from blowing hot air around V1 or the other valves.
    Just leaves the mystery of what the exact hum intrusion mechanism was ,
    something to do with DC+AC to V1 and V2 heaters?
    Now all that is left is to get more history from the owner, why and what
    Marshall had farted about with when returned to them before, other than the
    all too obvious cut and fly 1/3 W resistor replacements.
  18. N_Cook

    N_Cook Guest

    Nothing so grand as that, I will look out for those, but I was unaware of

    Hollow cutting tool
    Particularly for Marshall conductive epoxy pcb boards
    To isolate valve base pins by cutting into the board around a pin.
    A steel sleeve , this one dimensions internal 4mm and external 5.3mm . Cut
    eight small
    cuts on one end to make a castellated form with Dremmel and .6mm disc.
    Grind a rake angle behind each cutting edge. Place a rod inside the other
    end to mount in
    a drill chuck. Start at an off axis angle and bring up to axial after first
    Leave soldered joint in place.
    As this was actually perhaps a roll pin rathe rthan a sleeve, not a solid
    ring in plan,
    an axial join line along its length. Found a use for a Dremmel mandrel where
    the screw is
    sheared off inside the stem. With free hand grinding with the mandrel
    rotating in a sleeve
    and grinding against a disc brought the diameter of the mandrel support down
    to a tight
    fit inside this sleeve. Then a plastic filler for the other end and fitted
    in a Dremmel.
    Perhaps more reliable , repeat with a good mandrel , find a longer screw,
    some compressible silicone sleeving to grip the inside of the sleeve like
    Dremmel sanding cylinder holders.
    Not necessarily punch through as will be ragged anyway. With a dart point
    around while desoldering the pin .

    Owner had previoiusly returned the amp to Marshall for changing those
    resistors and general maintainence only. He has exactly the same amp and
    components back , only difference is a few holes in the pcb and wiring path
    a bit different.
  19. N_Cook

    N_Cook Guest

    I just realised the ideal starting form for such a tool would be one of
    thise jeweller's screwdriver nut spinners. Grind back the internal hex form,
    castellation cuts and cut through the stem to mount in a chuck
  20. N_Cook

    N_Cook Guest

    20 percent NaCl with calcium carbonate seems to show the undesired affect
    over a few days so maybe the Marshall contaminated boards have 1 or 2
    percent contamination.
    Pitty no Chinese? E number on their boards so we could tell if they turn up
    with other makers products, but perhaps not with valve voltages.

    So after a few days the dry test cell showed about 500M and over a few hours
    dropped to 80K or so ,maybe the Megger high V across the probes accelerating
    this effect , and light heating with hot air , drops to 40K or so, returning
    to 80K or so soon after. Much the same today as yesterday
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