More Marshall Wierdness-John, Guys Help Please

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Dickie Fredericks, Feb 7, 2006.


  1. Dickie Fredericks

    Dickie Fredericks Member

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    To recover some ground here I have a Marshall 900 series combo 4102. It was having Volume loss issues. I changed the tubes to YJ's/EL84 and all seemed OK. The volume wasnt where it used to be and I attributed that to the YJ's.

    Last week I cleaned the chassis with Deoxit and Preservit. Nothing happened. As noted in the other thread on this I had to run the Treble and Presence at like 3 and the Bass on 10 with the Mids at 5 to get a good sound and I thought the sound was OK.

    Today I turned the amp on and started playing. Master Vol at 4 and the Gain on 20 (is there any other setting? LOL). I hear 2 thumps which were kinda low endy. Then boom the amp is 2 times as loud as it was and is running like it was when I bought it. Sounds Great!!!

    Any ideas on this? Some further info. I never have noticed a difference when swithing the LOW/HIGH power switch. Also when the amp is on Standby there is still sound coming out. Am I shutting down wrong? I always put it on Standby first then turn it off. When powering on I hit the Power then the Standby.

    Id love to figure out what is causing this since I love the tone now that it is working as it should. BTW I now have T/M/B/P on 10 with plenty of bottom end running to a 4 x 12 cab. This is a great sound now !!!:JAM

    Could this be caused by the High/Low switch being bad or something? Did the deoxit finally clean it? Could the Standy switch cause this?


    In the past I have noticed this happen on a gig. The power would cut and the thing would be all high endy again. I used to think it was the tubes now Im not so sure. Once time it did this and I took the tubes out and removed the chassis and then reinstalled the chassis and tubes and it worked fine again. Then another loss which I thought was the tubes.

    Any info is appreciated guys. You're the best...

    Richard
     
  2. Mark Robinson

    Mark Robinson Member

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    My JCM 900 had cutting in and out, which turned out to be partially broken and intermittent contact between the circuit board and the potentiometers. The pots are at right angles to the board and are attached. So if you were to try to pull the board, you first have to remove all the knobs, keeper nuts and washers. The amp is pretty heavy and my rough handling, stressed the chassis, which turned out to be enough to fracture the connections. This happened twice in about 12 years of use. I also have a pretty imaginative tech, who put a little bypass cap accross the clean channel volume, to allow more highs in the clean channel. I preferred the dirty channel with the shared treble nearly off, so this greatly improved that amp for my use. Good luck!
     
  3. amoebas

    amoebas Guest

    http://www.geofex.com just thought i'd cite my sources.. R.G.'s tube amp FAQ: (a very informative article)

     
  4. John Phillips

    John Phillips Member

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    Sorry, I forgot to follow up on the original thread...

    That High/Low Power (actually it's a pentode/triode) switch is well-known source of trouble with these amps. It tends to arc, due to being operated at way beyond its designed voltage rating. In really bad cases it can arc underneath, burn the board and make the amp very difficult to repair.

    The first thing to remember is not to operate the switch without putting the amp on standby first. This will remove the potential for arcing since it takes the high voltage off the switch.

    If the switch had gone partially bad - ie just making a bad connection, rather than arcing, it might have done something like you describe - it would have stopped the power tubes on one side working, which would have made the amp sound thin, more distorted than it should be (you'd hear rough, asymetric distortion, but if you were running the preamp flat-out all the time you might not notice) and a lot quieter. I didn't suspect this from your original post since usually it's fairly obvious that the amp is sounding oddly distorted rather than just too bright... and I could still be wrong, it might be a bad contact somewhere else. I also wonder whether running with no speakers might have been significant, since that could have caused the switch to arc too...

    It's odd that you should not have noticed a tone difference using the switch, since the low setting is definitely 'thicker' as well as quieter - you don't so much notice the volume change unless the amp is cranked up, but you should hear the difference in tone.

    To check it out thoroughly you'll need to pull the rear circuit board and check for any damage to the solder joints under the switch as well - not too difficult, you just have to undo the power cord socket and all the jacks. Make sure the amp has been on standby for at least five minutes, then turned off, before you do this. You're doing it right BTW - turning on: power on first, wait a minute or two, standby on. Turning off: standby off, wait a minute, power off. (With some amps it's different, but on these amp the standby switch disconnects the HT at the transformer, so leaving the amp powered up but on standby drains the filter caps via the tubes.)
     
  5. Dickie Fredericks

    Dickie Fredericks Member

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    Yes I do notice the tone change when I use the high/low switch. Yeah not volume really since I now cant turn the master past 2 in the house. Remember before I was running it at 4 before this thread.

    There I was noodling along and I hear the 2 thumps and boom The volume jumps and the bottom end it through the roof as the bass was on 10 and the Treble and Presense were on 2-3.

    Right now the thing is sounding like it is supposed too. I guess it was a different distortion before.

    Ill check the board and let you know what I find. And, many, many thanks for your reply. You too amoebas...

    Richard
     
  6. Dickie Fredericks

    Dickie Fredericks Member

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    Just to update. I took the chassis out of the cab today to check it. As soo as I set it up on top of the cab and plugged it in, tunred it on, I had the same thing. Horrible distortion and just no fullness at all.

    So, I disconnected everything and took all the knobs etc off to check the boards. They were fine with no burn marks or anything else unusual.

    I did notice howver that a few of the pots were bent ever so slightly due possibly to getting bumped around in the car over the years. Nothing major here either to the best of my knowledge.

    When I put the boards back in and tightened all the nuts I turned the amp back on. Same thing. I turned the amp on standby and then turned it back on again and boom, the sound is fine again. Is it possible guys that the standby switch is causing this? I only wonder cause I can still hear the amp even with the switch on stanby. Maybe this is normal?

    Your thoughts of course are always welcome. FWIW the amp is running as it should be right now after all of this...


    Richard
     
  7. John Phillips

    John Phillips Member

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    You shouldn't be able to hear anything after the standby has been switched off and the caps have drained (at least a few seconds). It's possible that the standby switch is faulty, and also making a poor contact when it's on - that might restrict the current enough to change the volume and tone.

    If I remember rightly, this amp uses a two-pole standby switch, but only one side is used. If so, try swapping the connections to the unused side and see if it cures it (as before, make sure the amp is off and the caps drained before you touch anything!).

    The other possibility is that something in the HT circuit, most likely a bad or burned connection (in the high/low switch, or possibly the OT :(, or maybe somewhere else) is the cause, and the surge from flipping the standby switch is making or breaking the contact. The history of running with no speakers attached could definitely have caused something like this, unfortunately. These kinds of faults are sometimes a pain to trace, but I'm pretty sure it's in the HT circuit not the preamp, or it would probably not come and go with using the standby switch.

    What you really need to do is to meter round the HT circuit when the amp is in 'fault' mode and see where the wrong voltage is - that should pinpoint the problem fairly quickly.
     
  8. Dickie Fredericks

    Dickie Fredericks Member

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    Agreed. BTW, This problem was happening before I ever ran the thing without a load. So again Im not sure its the OT. However, you are correct about the standby switch, one side is not used. Im not sure if its where the solder was cooked on or what but it looks like I might be seeing rust on the temonals of the switch.

    Also, though I can hear the amp on standby, I hear its volume decrease after switched to standby though the sound is still there only quieter

    Thanks again, It'll get figured out.

    Richard
     
  9. Dickie Fredericks

    Dickie Fredericks Member

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    Yes I can hear it on Standby but John... I just figured it out. Its a short in the effects loop. Heres how I found it.

    Last night I was at amptone.com reading up on some stuff. It was suggested to get tuners out of the signal chain. I usually run my guitar right into the tuner.

    But I digress. Last night I plugged the tuner into the effects loop (still in the chain, I know) using 2 small 6" cables for send and return. Before I went to bed I unplugged the input of the tuner (send) and left the cables plugged into the back of the amp in the loop.

    This morning I get up and turn the amp on and strap on the Strat just to make sure its working fine as it has been. I hit the Standby switch and its back to the same ole volume loss all trebly sounding nasty distortion.

    Thinking its the standby switch I flip it on and off a few times to see if it kicks in. I look at the tuner and notice it is unplugged from its input.

    I think to myself "No way, Cant be"? So I plug the send of the amp back into the tuner and there it is. Everything working as it should.

    So there must be a short in the loop somewhere as the problem happens whether there is anything hooked to the send/return or not.

    This is great. Didnt someone else mention they had this problem?

    Thanks again

    Richard
     
  10. Dickie Fredericks

    Dickie Fredericks Member

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    Or maybe I simply isolated that the distortion Im hearing is preamp distortion? The power side is not engaged so all Im hearing is preamp. The master volume still works though.

    I am inclined to believe that its the loop though as it has happened during gigs when the loop was active with effects.
     

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