New member, new JCM900 SLX! Need help!

Discussion in 'Marshall Amps' started by Patastrophe, Apr 20, 2015.

  1. Patastrophe

    Patastrophe Active Member

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    Howdy,

    Shameful first post...but lurked for about a year

    I recently purchased a JCM900 SLX (2100) in a private sale. The thing ripped at the dudes house, gobs of distortion. A passable clean, and to boot, it had a pretty good price tag ($400). I did a ton of research before my purchase, and got one with EL34(G)s and ECC83 tubes. From what I've read, it's the "one to have" in this model

    ...but after playing it for the second time at my buddy's place - it's acting strange, and I'm starting to get a bit nervous. I'm hoping I didn't just buy somebody else's mistakes.

    This is my first tube amp - so bear with me. My descriptions might be a little...off.

    While I'm playing, sometimes the volume will fade, and return to normal, or just cut out entirely. Or, it will go from being loud and distorted to very low volume and clean. Usually if I wait a few seconds, it comes right back, or if I strum hard, it "fixes" itself.

    From what I've read, this can be caused by jacks / etc getting crud in them and shorting out. This thing is FILTHY inside. Is there something I can use to (safely) clean it?

    Is there anything else I can do to troubleshoot? I bought a head first, and planned on buying a cab - but now that the head I bought isn't working, I can't test cabs to buy one! (I also don't have one to troubleshoot problems with, so I need lots of suggestions!)

    I'm located in the Westlake, Ohio area, if you guys recommend sending it to a tech to get a once over, please suggest one in that area.

    I've stuck some pictures in, hoping that can provide more help and get me more information.

    Thanks in advance

    IMG_0438.jpg

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  2. Dogs of Doom

    Dogs of Doom Moderator Staff Member

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    Re: New member, new amp! Need help!

    well, for one thing, it's missing a tube!

    [​IMG]

    :wave:

    JK! I'd take a venture to say that it's the power tubes need replacing...
     
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  3. Patastrophe

    Patastrophe Active Member

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    Re: New member, new amp! Need help!

    :lol:
    Hah, that's the one I'm holding in my hand in one of the other pictures. It's got all the tubes.

    It was an awesome sounding amp when I tested it at the guys house for about an hour. Now, I've just got this nasty queasy feeling in the bottom of my stomach. I'm hoping I get some help here!
     
  4. Dogs of Doom

    Dogs of Doom Moderator Staff Member

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    Re: New member, new amp! Need help!

    I'd still say that it's probably the power tubes going out. I'd get a set anyway, just to make sure.

    You might have a tech give it a good look over. If you don't know how to bias your amp, you'll have to have a tech pull the chassis to swap the tubes anyway.

    Welcome to the forum...

    :cheers:
     
  5. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    Plug a short guitar cable from effects send jack to effects return jack.
    If the problem goes away, you know the jack switches are corroded.

    You can just play the amp with the cable installed, if you don't want to change the jacks.

    Try the cable first before you change the tubes.

    Get one new 12AX7 tube, swap out the small preamp tubes one at a time. Retest each time.
    if there's a bad preamp tube, you will find which one.

    There's no reason to go hog wild and change all the tubes...unless you are wealthy...
     
  6. tresmarshallz

    tresmarshallz Well-Known Member

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    My JCM900 was doing that the last two times I was using it, drove me bananas trying to track it down. It was the effects loop jacks needed cleaning.
    Hope that fixes it for you.
     
  7. peterichardz

    peterichardz Well-Known Member

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  8. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    Cleaning the jacks is temporary. Replacing the jacks is priceless.

    It's almost always the jacks or the preamp tubes. (whoops, one preamp tube usually)

    No, it's probably not the power tubes.
    Pretty rare if it is...it's rare but not impossible.

    Another thing that could act up is the speaker cable.
    The market is flooded with crap cables these days.
     
  9. marshall1967

    marshall1967 Active Member

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    ..you should just take it to a tech..they'll get it going correctly.
     
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  10. Patastrophe

    Patastrophe Active Member

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    I've had a couple recommendations so far -

    1. Wild Dutch's Guitar Hospital - guy is supposedly super knowledgeable based on posts I've read on here before. Anyone have experience?
    2. Beamish - I've heard bad things about their prices / turnaround
    3. Stagecraft - nobody has said anything about experiences with them so far
    4. Guitar Riot - they show up on Google. Anyone have experience with them?
     
  11. Ken

    Ken Well-Known Member

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    Yes, it could be lots of things. So let's eliminate the easy ones like the guitar, guitar cable, and as AMS suggests the speaker cable first.

    I would then jump the FX loop as suggested. I would get some contact cleaner and clean the jacks and the pots if they're scratchy.

    If it's still wonky (that's a technical term) I'd check the preamp tubes, as suggested. I would retube the power tubes myself even if they're okay, as everyone should have a spare set and on this amp people tend to cook the tubes anyway, so a new set might really wake up your tone.

    If none of this works, it's tech time. You'll need a tech to bias the amp anyway when you get the new power tubes. Don't worry, $400 is a great price and your amp's symptoms don't sound like it's serious, like it is when smoke is coming out of the vent.

    Ken
     
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  12. V-Type

    V-Type Active Member

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    I noticed the factory sticker was removed from the chassis in the 2nd pic.
    Generally that larger sticker tells if its 5881 factory or El34 factory wired.
    The removal of the sticker makes me wonder if someone put 34,s in a 5881 series and also wonder if the proper changes were done too do so.
    Also my SL-X had intermittent crackling but was solved with a good cleaning out of the loop and speaker jacks.
     
  13. Patastrophe

    Patastrophe Active Member

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    Done.

    My first reaction was that the cable was bad. Checked with a brand new speaker cable. Tried using both my Les Paul and my buddy's strat. No dice. Guitar cable wasn't it either, we swapped in known working ones.

    Checked those first cause they were "free" fixes. Unfortunately, didn't get anywhere.


    Much appreciated Ken (and all!). I'm gonna swing by Advance Auto and pick up some CRC QD Electronics Cleaner. I know it's not as great as DeOxit for long term protection - but I can get it TONIGHT and eliminate issues ASAP. I'll have to order the DeOxit.

    The DeOxit can be reapplied later to the jacks / pots / fuses / tube sockets to prevent future corrosion, but I'm trying to get all the crap off of everything for the time being. The amount of dust/dirt in this thing is...incredible. My hands were BLACK just in removing the backplate and two of the tubes.

    ****.

    I'll pull the chassis open tonight and snap pics. It should just be the resistor on the bias pot, correct?
     
  14. MarshallDog

    MarshallDog Well-Known Member Premium Member

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    If its that filthy then you should either take it to a Tech and have the jacks and pots cleaned along with all new tubes and biased up. If you can do it yourself great. I a great with the others, it could be a bad tube either the pre's or the power. Even if they are new, I have had them go bad hours out of the box.
     
  15. dreyn77

    dreyn77 Well-Known Member

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    nice amp!

    anything S/H needs to be looked at internally.
    You'll find all sorts of strange things happening inside sometimes.

    I just looked inside 2 guitars and one had grease squashed into the pickup selector switch and the other was wired any which way. It never made a real sound.
    SO the guys who had these things sold them cause they never bothered to take a look with expert eyes.
     
  16. indeedido

    indeedido Active Member

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    My money is on tubes. You don't know how many miles are on those tubes. Used amp always makes sense to change tubes first thing. I've had this before and was just old tubes. Not a big deal just routine maintenance.
     
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  17. Patastrophe

    Patastrophe Active Member

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    Well, by the looks of things, I'm going to have to take it to a tech. I cleaned up all the contacts today and when I plugged it in - the amp was REALLY quiet for a 50W/100W amp and I couldn't get hardly any distortion out of it. I'm talking - not far above conversation level.

    I made sure to put all the tubes back in the same slots. I don't know enough about tube amps to do anymore. I tried reading some schematics but - it's been a while.

    By the looks of it, R31 - R34 are all 5W 1kΩ resistors. This is consistent with an EL34 setup. Is there anything else I should look for to see if it's been converted from a 5881? Schematics below -

    JCM 900 SLX EL34 POWER TUBE SECTION

    JCM 900 SLX 5881 POWER TUBE SECTION

    Attached are pictures of the board, because I'm too much of a newb to know if something is glaringly wrong.
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  18. Patastrophe

    Patastrophe Active Member

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    Double tap for the last of the pics
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  19. Adrian R

    Adrian R Well-Known Member

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    I had some similar issues with a 92' MK3 (900) I just bought several weeks ago...

    The amp was in mint condition, but it was dirty and dusty as hell..

    Spent several days cleaning all the control pots...over and over again...and all the tube sockets..AND all the jacks...ESPECIALLY the FX loop level pot. That pot along with one of the MV controls and presence control was completely clogged. It took allot of electronics contact cleaner and working it in to stop the problems. If you are using the FX loop ensure that the FX level pot is clean..it will completely kill it if it's dirty...(no sound)...

    Oh, check to be sure you have the power switch set to full power.

    I also replaced the filter capacitors...and added a choke..

    **was looking at the pictures of the board you posted...looks very similar to my amp...In one of the pics I see the FX loop pot...if you look closely at it...there is a hole in the side of it in where you can spray the cleaner in...you have to rotate the pot to find a point in where you can actually get the cleaner inside of the pot...

    The amp sounds awesome now...1000x better then it did when I got it!
     
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  20. marshallmellowed

    marshallmellowed Well-Known Member

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    Sounds like the next step is to try a known good ECC83 in each preamp slot (and especially the PI slot). It's easy, and at very little cost. All you need is one good tube to test with.
     

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