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Thread: Amplifier Unusual Loud "Shots"

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    Amplifier Unusual Loud "Shots"

    Hey all once again ,

    A day after jamming with my Marshall Valvestate VS100 (That was working like a charm) , I noticed a constant sooshy-windy sound that gradually (2-3 hours) mutated into a very loud crackle-rumbly sound (I still get guitar sound & the noise is independant from the volume),it literally makes your guts jump , I started 'chopsticking' the amp as I usually do , noticed that nothing seemed to trigger this sound

    These loud rumbles are 100% random , whether the volume is on 0 or the guitar unplugged - also noticed that it makes a brutal loud pop when switching to the clean channel. (it never did this until today)


    Can these random loud crackly/rumbly/poppy noises be caused by failing capacitors? I wouldn't say that it's a resistor...

    Any thoughts?

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    Supporting Member nevetslab's Avatar
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    VS100 Manual & Schematics attached

    Quote Originally Posted by GasMask52 View Post
    Hey all once again ,

    A day after jamming with my Marshall Valvestate VS100 (That was working like a charm) , I noticed a constant sooshy-windy sound that gradually (2-3 hours) mutated into a very loud crackle-rumbly sound (I still get guitar sound & the noise is independant from the volume),it literally makes your guts jump , I started 'chopsticking' the amp as I usually do , noticed that nothing seemed to trigger this sound

    These loud rumbles are 100% random , whether the volume is on 0 or the guitar unplugged - also noticed that it makes a brutal loud pop when switching to the clean channel. (it never did this until today)


    Can these random loud crackly/rumbly/poppy noises be caused by failing capacitors? I wouldn't say that it's a resistor...

    Any thoughts?
    After downloading the Schematics and User Manual (which I've attached here), I'd start by first unplugging the one Preamp tube (power down first, of course), then, with the tube removed, power back up to see if the problem is still occurring. If so, we know it's not the tube, and so plug that back in. I didn't see the Line Out jack could serve as a Power Amp Input jack, disconnecting the preamp, so you can't isolate the preamp circuit from the power amp circuit. If the Effects Send/Return jacks are Always in circuit, you could try plugging a shorting plug or a short patch cable into the Return jack to see if that changes anything. We're trying to isolate the preamp section from the power amp section to find WHERE the problem is occurring.

    Assuming no success there either, power down, and unplug ribbon cable Conn 2. That WILL isolate the Power Amp circuit from the Preamp Circuit, and only contains the rear panel Effects Send/Return jacks, footswitch jack and Power Amp input circuit. Power back up with it disconnected to see if the problem continues or not. If not, then unplug/reconnect both the Conn 1 and Conn 2 ribbon cables. Two main cables.....Conn 1 and Conn 2. Exercise those connections a few times to see if after doing so, anything has changed. If not, then I normally remove the rear panel PCB for close inspection for solder joint fractures. I haven't seen the insides of this amp, so I don't physically know how difficult it is to pull the amp apart for inspection of all the PCB's. You've already been probing with chopsticks without success, which often finds the problem components having intermittent solder joint connections.

    Pounding on the chassis to see if the problem is activated by vibration....usually caused by solder joint connections within. It looks like the Reverb circuit is a traditional 'tank', instead of a solid state reverb circuit. I'd disconnect the tank Output from the amp's return circuit to see if it's involved.

    If none of these reveal where the problem is, then you're into measurements with test gear to find where the fault is. I'd begin with the power supply circuits. We don't know your skill level in testing/service, as at this point, you're dealing with some high voltages and shock hazards.

    Marshall-VS100R-100W-Schematic.pdf

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    Last edited by nevetslab; 05-16-2020 at 08:47 PM.
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    Thank you nevetslab

    No problemo , I'm 400% familiar with this amp , inside out, from the very first component to the last
    Before searching for help on this forum I already did a few tests that I forgot to mention :

    *I previously repaired this amp - it used to turn on if hitted with no power source available - it was the power filter caps , they were loose , re-soldered them

    *Tested it without the ECC83 , noise is still there but at a very low volume (obviously)

    *Reverb tank was always disconnected during tests

    *Weirdly (noticed this on all 4 VS100 that I own) disconnecting one of the CON molex connectors will disengage the power, like if ALL CONs must be connected for the amp to power up

    *Noticed a static sound (different and independent from the main crackly noise) while poking C38 & IC2 (on PreAmp) , re-soldered & replaced , but no difference, it still makes static noise when touched, I guess it's "normal" , no cold solder joints on them

    *I read that many people that had my same issue, solved it by replacing all caps - this sounds fishy, it doesn't sound like a cap problem to me - after all , while I was 'chopsticking' the caps , nothing got triggered , the crackly swarm of bees was random and nearly constant

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    Supporting Member nevetslab's Avatar
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    OK....you're a lot further along with these than I am. I've only been inside a couple of their Valve State amps, though not this one. I mostly service JCM 800's, 900's, 2000's and TSL heads and combo's, with several others that's in our rental inventory at CenterStaging in Burbank, CA.

    I've certainly come across similar symptoms on lots of gear. Some are component based, others (seemingly the majority) have been from solder joint fractures or tubes. It does seem like a power supply related problem. You could selectively short out different circuits, beginning with shorting the input to the power amp stage, such as shorting R4 input bias resistor to T1 input side of the diff amp stage. Then, I'd work backwards to preceeding stages to see if you can isolate where the noise source is (assuming it's NOT power supply sourced). With no signal applied, this shouldn't cause any problems in the analog signal path by shorting the IC outputs. Or, short with a 10 ohm resistor to ground.

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    Rather than suspecting capacitors or resistors, I would first suspect the active components (transistors and IC's).
    Freeze spray is a big help for faults like this.

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    Supporting Member nevetslab's Avatar
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    I have another thought. Since you own four of these VS100 amps, that gives you modules that you can swap to see if you can make the problem move or stop.

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    Yep, that exactly what I did an hour ago, I swapped out a couple of boards , and I came to the conclusion that the issue is in the PreAmp - the Power Module is 100% fine

    Though I noticed that today , I literally get no output from the guitar , I just hear very light crackling , as if there's no tube, when there is

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    Quote Originally Posted by g1 View Post
    Rather than suspecting capacitors or resistors, I would first suspect the active components (transistors and IC's).
    Freeze spray is a big help for faults like this.
    Makes sense, I already replaced IC1 & 2 on the PreAmp , but no luck - I would start targeting specific components but I'm not that good at reading the vs100 diagram to the point where I know which components are vital for guitar output

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    Click image for larger version. 

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    These are the only visual "oddities" that I spotted till now, R43 looks weird , wouldn't say its dead/opened but it's the only resistor that looks like this

    Under the Daughter board you can easily see the naked copper fom the cables BN1 & BU1 that connect themselves to the positive & negative spots on the power supply - would these create output problems if not soldered decently? I'll add some solder to find out

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    "Thermionic Apocalypse" -JT nickb's Avatar
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    You really need to approach this with a divide and conquer strategy, which is what nevetslab was trying to get you to do There is no reason why simply disconnecting CON1 on the power amp board will do anything other than disconnect the the preamp that I can see. So pull it and listen. If noisy then it's on the power amp board and I'd start with the freezer spray test on the semiconductors and especially suspect T1 and T2. When freezer spray works it's great but in my experience it only works well about 25% of the time. So, if you get a negative result on T1 and T2 I'd replace them anyway. If you get a positive result, well you still replace them If silent then it's not the power amp. That said I think you already have a clue that it's in the preamp so read on...

    You already pulled the tube and said the noise was still there but a at low volume, 'obviously'. Dunno, but that's isn't obvious to me at all. If it's before the tube it should be silent and if after then it should still still noisy. What you have is something inbetween so that has to be worth looking into. With the clean channel selected and the tube pulled, try shorting out R49 (on the input to IC3), does that stop the remaining noise? If it does I'd suspect R23 100k on the cathode of V1. If not then I'd suspect IC3 next, again freezer spray might confirm.

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    Quote Originally Posted by nickb View Post
    You really need to approach this with a divide and conquer strategy, which is what nevetslab was trying to get you to do There is no reason why simply disconnecting CON1 on the power amp board will do anything other than disconnect the the preamp that I can see. So pull it and listen. If noisy then it's on the power amp board and I'd start with the freezer spray test on the semiconductors and especially suspect T1 and T2. When freezer spray works it's great but in my experience it only works well about 25% of the time. So, if you get a negative result on T1 and T2 I'd replace them anyway. If you get a positive result, well you still replace them If silent then it's not the power amp. That said I think you already have a clue that it's in the preamp so read on...

    You already pulled the tube and said the noise was still there but a at low volume, 'obviously'. Dunno, but that's isn't obvious to me at all. If it's before the tube it should be silent and if after then it should still still noisy. What you have is something inbetween so that has to be worth looking into. With the clean channel selected and the tube pulled, try shorting out R49 (on the input to IC3), does that stop the remaining noise? If it does I'd suspect R23 100k on the cathode of V1. If not then I'd suspect IC3 next, again freezer spray might confirm.

    ok nickb thanks so much, here's the deal!

    **Disconnected CON1 from the power module - NO noise , so it's in the PreAmp

    **Shorted out R49 , removed the ecc83 and as soon as I turned the switch on it made a loud POP sound , but NO noise/crackles after, turned it off and it made another loud pop sound. (guess that happens when R49 is removed??)

    You reckon I should replace R23 with the coupling cap? I'll do it now

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    Ok I mounted R49 back, replaced R23, now i get NO noise WITHOUT the tube, but WITH the tube i get crackling noise and no guitar output, (replaced IC3 aswell) what now?

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    Last edited by GasMask52; 05-17-2020 at 05:03 PM.

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    The test was to SHORT OUT R49, not remove it. Not exactly sure what you did as you say both shorted and removed. With it shorted out there's no reason for a loud pop if it didn't do it before. Let's have a sanity check. Are R49 and R23 100k on your board? If yes, what is the voltage across R23, R49 and on pin 5 of IC3?

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    My bad sorry - first I removed R49 to test it with a multimeter (was curious) , then I resoldered it and disconnected the lead that connects to pin 2 of IC3 (+input), removed the tube and the crackling was gone - though when I tested it with the tube in , the crackling came back - also no POP sound when I engage the power.

    I don't feel confident enough to mesure the voltages on a live amp, I would prefer not to

    So now we solved the issue that it crackles with no tube - but it still crackles with the tube

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    I had one of these amps with the same symptoms. The solder job from factory was horrible. After fixing all connections that were obviously bad, symptoms occurred much less but would still pop up.

    In the end I went through the entire amp and re-soldered every connection, and that cleared it up for good.

    Also double check the speaker quick disconnects AND the 1/4" cable/plug that connects them to the amp.

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    Last edited by garytoosweet; 05-18-2020 at 04:15 AM.

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    Quote Originally Posted by GasMask52 View Post
    My bad sorry - first I removed R49 to test it with a multimeter (was curious) , then I resoldered it and disconnected the lead that connects to pin 2 of IC3 (+input), removed the tube and the crackling was gone - though when I tested it with the tube in , the crackling came back - also no POP sound when I engage the power.

    I don't feel confident enough to mesure the voltages on a live amp, I would prefer not to

    So now we solved the issue that it crackles with no tube - but it still crackles with the tube
    None of that sounds like shorting out R49. I'm not at all clear what you did. Trying to move on.... Is the situation that the pops are gone, it still crackles and otherwise you can hear your guitar normally?

    If yes, then is it also true that the clean volume has no effect of the crackles? If so, then replace the ECC83 / 12AX7 tube. Chinese ones work well in this position.

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    Quote Originally Posted by nickb View Post
    None of that sounds like shorting out R49. I'm not at all clear what you did. Trying to move on.... Is the situation that the pops are gone, it still crackles and otherwise you can hear your guitar normally?

    If yes, then is it also true that the clean volume has no effect of the crackles? If so, then replace the ECC83 / 12AX7 tube. Chinese ones work well in this position.
    There's still no guitar output, just very loud crackling even with the volume all the way down, I've replaced the tubes many times with new ones, that's the first thing i do - This is gotta be some dead cap/resistor/transistor - you wonder why I keep shotgunning? Imagine yourself living in a desert island with a guitar Amp, a guitar, some few components, a soldered and wanting to play really badly - that's me , how would you fix the amp in that situation

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    Supporting Member The Dude's Avatar
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    If there's no guitar signal getting through the amp, that actually makes it easier. Run a test signal through the amp and trace to the spot where you lose it. It's likely going to be the same spot that's causing the noise.

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    Quote Originally Posted by GasMask52 View Post
    , how would you fix the amp in that situation
    I'd get measuring. Seriously in order to make sensible decisions you need data and that means getting a meter or whatever tool it takes to get the info you need.

    With sensible precautions, there is no need to be afraid to make measurements on a live amp. On this particular amp, most of the voltages are low and pose no risk. The problem seems to be in around the tube which does have higher voltages so if you use one hand (other in pocket) the risk of electrocution ( if that is your fear) is really quite tiny. You need a decent about of current to flow through your heart to kill you and that means both hands have to be in the circuit. Consider latex gloves as an added precaution.

    If you have a meter check the DC voltages around the tube with respect to ground. That means you have clip the negative lead of your meter to 0V and use the positive lead to probe with. Only one hand needed.

    To avoid scary loud pops unplug the speaker.

    Measure the DCV at
    Junction of R16/R16 (low voltage)
    Both ends of R22 (high voltage)
    R18 ( one end is 0v discard that, low voltage)
    R23 ( one end is 0v discard that, medium voltage)
    Check the tube is glowing.

    If you have a second amplifier you can use that as a signal tracer ( see The Dude's post). The idea is you inject a signal, a signal generator or audio source is best as no hands required, you then probe along the signal path and listen to find out where the problem starts. You'll have to make a little probe to protect your amplifier input.

    Something like this should do the job.
    Click image for larger version. 

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