Marshall Dsl100 Head... Sound/no Sound

Discussion in 'The Workbench' started by Publius, Jul 4, 2018.

  1. Publius

    Publius Member

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    Hi, everyone, it's been a while since I posted anything.

    It all started several weeks ago and I have been trying to pinpoint the root of all evil myself with no luck yet.

    The Marshall DSL 100H powers up perfectly and I can play, the sound is great... well it's a Marshall. After five to ten minutes of rocking, the sound disappears and after 2-5minutes it comes back. The past few weeks this was happening only once, unfortunately, yesterday that kept happening throughout the entire rehearsal. The symptoms are:

    1. No-sound after a while of playing, the sound goes off abruptly,
    2. Sound comes back after 2-5minutes with a fade-in effect, (either waiting with standby switch on or off, it doesn't matter for that specific problem)
    3. If amp switched off, leave to cool down and then switch-on, it does the same thing all over,
    4. All wires, pedals and guitars have been triple checked and are good-to-go,
    5. While at "no sound" state, I connected a preamp pedal board directly to the power amp through the FX loop and still no sound,
    6. The HT fuse is NOT blown but that is just inadequate to call power tubes as working good,
    7. The tubes have been there for several years but have not played that much to even think that they have gone bad... YESSSS these are SED-C,
    8. The amp is well away from a wall and is within range to an air-condition... it is pretty well ventilated,
    9. Have not isolated nor tested the pre-amp alone... I' ll do it today and post feedback.
    10. From the looks of it, all caps seem OK i.e. they're top is flat.

    Even though the caps are OK, visually, I believe the problem is with some of the caps and their charging/discharging capabilities. Probably due to local heat, the plates may abruptly dilate and they lose the capability of collecting the necessary amount of charge. Once the heat is decreased the plates contract and the caps charge nicely pushing power to the tubes.

    I will check the bridge rectifier BR1 (BR62) but if that goes bad then it remains bad it doesn't go on and off and on again, once gone rogue then it stays rogue.

    What's your opinion?
    Do make a suggestion, even technical ones... trust me I am an engineer :)
     
    Last edited: Jul 4, 2018
  2. DSL401isHot

    DSL401isHot New Member

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    Hi Publius,
    with the old DSL401, the bridge rectifier was the main culprit. It was close to the board and the resulting heat lead to bad solder joints on the board and the sound was gone. When it cooled down, the joints went back to normal and the amplifier worked again. There are several threads on this forum about this problem in this specific amp.
    So, it might be a heat related problem on your solder joints. You should have a look for suspicious looking joints and traces around parts with a lot of heat stress.
    good luck!
    Edit:
    Do your tubes (preamp/poweramp) still glow when your amp looses the sound?
     
  3. Publius

    Publius Member

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    Hi DSL401isHot.

    I have that DSL 100 for the past 10-12 years and this is the first time it's giving those symptoms, well it is never too late for something new but let me first reply to your question above and then I will give more feedback on the works so far.

    Yes, it may be heat related but I am not so sure because other things occur as well. After all, I am not cooling it down I just stop playing (as nothing can be heard) without switching off either the power button or the standby. It just comes back on its own pace.

    Yes, the tubes (all tubes) still glow but... watch out below.



    What I have done so far.
    1. Power up with standby off and let it stabilise for 15minutes. Also removed back grill for measuring.
    2. Switched standby and I measured the voltage of power tubes. They were a bit off to 86mV each.
    3. I rebiased to almost 90mV (as specs) with no problem of shifted bias.
    4. Plugged in the guitar directly and played with no problem.
    5. After half hour I plugged the guitar to the pre-amp pedals and rocked like crazy for another half hour... everything was smooth. The rating of the tubes was almost 90mV as expected.
    6. I connected the FX loop pedals and continued to rock until I thought that all previous incidents belonged to the past... BUUUUUT!!!! (still, the rating was almost 90mV for most of the time).
    7. I placed the guitar to its stand and then measured the voltage of the power tubes which dropped down to 86mV... well it's not that much but I recorded it, watch out there's more.
    8. While measuring I noticed a tiny hum, so I went and measured again. The right side as we look was giving a measurement of 92.5mV but the left was rated at 74mV which is quite a difference. I strummed the guitar but nothing happened... no sound again the evil has returned! It also produced a sheering hum which pointed out that something was terribly wrong.
    9. I waited while turning on the air-condition in order to cool it down pushing cold air but nothing happened. After 10minutes the sound returned with a fade-in effect...BUUUT the ratings as I measured in order to see if they came back up; the right side dropped to 88mV and the left has dropped even more to 72mV.
    10. I kept on measuring while the amp had its sound. The right side fell down to 86mV and left went down to 69,4mV

    I then just stopped and will repeat the process in a few hours to inspect the repeatability of the symptoms.

    So I don't think that its the bridge rectifier even though the additional hum suggests so. The power tubes and their ratings are shouting that something is wrong over there. I will try to change the middle ones with each other to see if the circuit or the tubes reproduce the symptoms. If changing the tubes gives the same error, then its something in the circuit if the ratings are reversed then something goes wrong with the tubes.

    Any other suggestions?

    Please do indulge my geekiness!
     
  4. charveldan

    charveldan Well-Known Member

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    I wouldn't buy a Vietnam Marshall, i had a JCM-2000 DSL and it was good.
    Id just put it in the shop and be done with it.

    The fact as an "Engineer" you're here asking questions pretty much sums it up.

    Not sure i buy your Theory about the electrolytics. Unless they're bulging or leaking id be surprised if they're bad.
     
  5. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    1. Take the power tubes out
    2. Turn amp on but leave standby off
    3. Measure voltage on pin 5 of all power tube sockets (note the voltage)
    you can measure this from the socket and do not need to be inside the amp.
    4. Make sure there is negative voltage steady on pin 5 of all output tube sockets.
    Test this and monitor for at least 30 minutes.
    5. If not , the bias circuit is defective. You cannot play the amp until the negative voltage remains constant on pin 5 of each socket. Take corrective action.

    6. As above continue to monitor pin 5 voltage.
    7. Turn the standby ON.
    8. Monitor negative voltage on pin 5.
    9. If there is a substantial change in the voltage of pin 5, between standby off and standby on,
    (for example, bias negative voltage becomes positive when standby is ON)
    the bias is drifting because of:
    A. defective circuit board
    B. defective coupling caps between phase inverter output and power tube grid (pin 5) input.

    Do not randomly change any parts until you have actual test results. Narrow it down to the board or the coupling caps first.

    Do not assume any part is bad until you have actual confirmation in a real test. Avoid above all: guessing and random parts replacement.

    Circuit board: is easily damaged by soldering. Unless you are an expert with tools, hire a professional.
    The board can be destroyed by a soldering iron. (and frequently IS).
    People who try to repair PC boards without the knowledge and tools often wind up with much worse problems than when they started.

    Bridge rectifier:
    If the rectifier is actually bad you can often get a loud hum, or the fuse will blow.
    Just because somebody tells you "rectifier is bad,: doesn't mean it is.
    Take the rectifier out and measure it.
    Prove it first.
     
    Last edited: Jul 4, 2018
  6. Publius

    Publius Member

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    Hi again,

    The following is a reply to the response and method of ampmadscientist.

    I followed the steps you described and as I understand from my measurements there is no bias shifting. Here is what I measured in the past two hours or so in PIN 5 (Control Grid). The numbering is from left to right as we see the tubes from the back of the amp.

    DSL_100_Pin5.jpg

    Well, there is a slight difference but it is in the floating part with a fraction of mV, (Gaussian distribution) so it is steady.

    A friend of mine gave me a matched pair (NOT a Quad matched) unfortunately, so I am going to test them as well. I am going to test the pre-amp once again before removing the chassis and dig deeper. Still, I think there is a cap screwing my fun.


    Any other suggestions, I am all ears.




    P.S. ampmadscientist, as said I am an engineer and do not pay attention to fart mouths who post only criticism with no substance, I only believe hard numbers and willing to listen to people who actually contribute like yourself. I am not going to buy or destroy anything because someone tells me to. Unfortunately, common sense is not that common nowadays. Thanks a lot, mate.
     
  7. charveldan

    charveldan Well-Known Member

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    Substance would Dictate your Capacitors are only a few years old [at best] and can easily last 20 years or more.

    Why don't you measure the Capacitance of your Caps Mr. Engineer ?

    While you're at it why don't you order a MATCHED QUAD OF NEW TUBES AS WELL dingle-berry.

    Then you can bore us to Death with more Charts & Graphs.

    https://www.ebay.com/itm/New-4x-Tun...548975?hash=item1eb2aaadef:g:HNwAAOSwwbdWKSal

    100 Watt amps work best with 100 Watts of tubes.
     
  8. Publius

    Publius Member

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    To charveldan.

    You know mate that at all times you can say "No" and not bother with this or any other post, sorry for boring you to death, watch the other way and spare both this post and anyone else that may have come across such a problem. Just because you are a member, sorry a "Well-Known Member" does not mean that you have to post on every thread, you are not obliged to do so.

    In order to measure the capacitance, you have to remove them from the circuit and this is NOT how one should address such a problem. There other ways to snuff out possible reasons and other priorities before doing so. Otherwise, you end spending to much time and money and risk doing even more damage. I am sure you know that. And stop calling other people names it is not that polite. The comment for fart-mounths was not destined for you in person I am sorry that you categorised yourself with them, I didn't name you. Probably you found some resemblance between you and the description of a fart-mouth I gave in a previous post. You did it yourself and probably this tells something about you as well.

    Is there a way you can contribute to a solution, then I am all ears. If not, then do not bother, you are not obliged to respond, man up and just say "No" mate.

    P.S. Thanks for the link on ebay and the trouble you went through in order to indulge me, but I do prefer the SED-C any time. Check them out...
    https://www.ebay.com/itm/NOS-WINGED...074688&hash=item2aaaa72f97:g:BzAAAOSwGllaaQ2b
     
  9. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    Seems like you eliminated a bias problem.
    You also eliminated possible effects loop bad jacks, which can also cause the audio to cut out. (you went straight into the power amp and it still didn't work)

    Then you need to swap preamp tubes one at a time with a new one, and if there is a bad preamp tube you can find it that way.
    This would be the last preamp tube before the power amp (the phase inverter tube).

    If you do that and it still doesn't work, you have covered all the usual (user fixable) failure points.

    So I will hope that it is a bad phase inverter tube.

    There is a whole bunch of solid state parts (FETs, opto-isolator, etc...) and you need to trace through with a scope and see where the signal is being lost.

    There is also reports of bad foot switch jacks.
    The jack contains switches...and if the contacts corrode, the switch in the jack will fail conceivably causing the channel switching to malfunction.

    If you have the foot switch: plug it in and see if the audio comes back.
    The foot switch will bypass the bad switch contacts inside the jack.

    But we don't have a schematic which is what we need to go any deeper into this.
    It's complicated and we don't have the map to go through it.
     
    Last edited: Jul 8, 2018
  10. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    Actually
    real technicians make measurements before they order parts....they narrow down the causes first. (hence the charts and voltage readings)

    Just throwing money at the problem is not usually very successful (but you may get lucky).
     
  11. Publius

    Publius Member

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    My thoughts exactly and have already swap two of them. The problem is that the time period when that amp goes off is extremely random, e.g. I have the amp off chassis in order to measure voltages/oscillations and has not gone off for the past four hours or so. I was able to swap only two of them, I will do the rest in the following hours.

    I checked the bridge rectifier, and is actually working as supposed to. Anyone interested on the method just check the following video.


    The unseeable/unheard problem that remains is that even it Pin5 of all power tubes is -42,7mV | -42,6mV | -43,6mV | -43,7mV (left to right), still the bias of the left pair is down to 74,6mV and the right 88,4mV even though I rebiased the amp a few days ago.

    I will also swap each pair with a new matched pair and see where this leads.

    My point exactly, I first read, measure, think of what the problem and its cause may be, think again, then plan, do, check, act, thanks for the support. Unfortunately, charveldan has other opinion, let's agree to disagree on this.
     
  12. Publius

    Publius Member

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    My thoughts exactly and have already swap two of them. The problem is that the time period when that amp goes off is extremely random, e.g. I have the amp off chassis in order to measure voltages/oscillations and has not gone off for the past four hours or so. I was able to swap only two of them, I will do the rest in the following hours.

    I checked the bridge rectifier, and is actually working as supposed to. Anyone interested on the method just check the following video.


    The unseeable/unheard problem that remains is that even it Pin5 of all power tubes is -42,7mV | -42,6mV | -43,6mV | -43,7mV (left to right), still the bias of the left pair is down to 74,6mV and the right 88,4mV even though I rebiased the amp a few days ago.

    I will also swap each pair with a new matched pair and see where this leads.

    My point exactly, I first read, measure, think of what the problem and its cause may be, think again, then plan, do, check, act, thanks for the support. Unfortunately, charveldan has other opinion, let's agree to disagree on this.
     
  13. charveldan

    charveldan Well-Known Member

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    Just some internet humor man. I don't take whats said on internet forums very seriously and neither should you.

    Good luck with your amplifier, you have to be smarter than the amp.

    If you suspect a component is bad but wont measure its value then your Theory can't be proven or eliminated, therefore the rest is just lip service.

    Vietnam DSL's aren't that big of a challenge in the world of Amplifiers.
     
    Last edited: Jul 8, 2018
  14. Publius

    Publius Member

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    Sure thing mate and exactly my initial point regarding fart mouths overwhelming the internet.

    To tell you the truth mine is WIA Vietnam DSL (Wounded In Action, wounded directly to the Head), take a look at that hole I was forced to make due to a spark during a live many years ago.

    WIA-DSL100.jpg

    During a gig, almost in the middle of the gig, the fuse was blown and I knew something wrong happened to the power tubes but unfortunately due to logistic I was able to carry only one amp and that DSL is my main amp. It took just a minute to locate the spare fuses and change them. I changed four of them during that gig and fortunately, all were blown after finishing a song or two. The last fuse was blown a minute after finishing the encore song, talking about luck. Unfortunately, that V7 power tube was destroyed to its core, causing a spark which destroyed the socket but fortunately enough it did not destroy the PCB. That next week I changed the socket and all the power tubes and has been playing since then without any problem.

    That was the first thing I checked when I visited the circuit, thankfully everything was OK with that. It may not show but I was very careful the first time placing silver paste and solder both sides of that socket and to this day it is still working... well with that random problem I mentioned, entitling this thread.

    I also swapped all pre-amp tubes and that "damn" thing is still working, its a true veteran. The caps are OK, the bridge rectifier is OK. Tomorrow I will focus on the power tubes alone, I am still narrowing it down to the cause.

    So anyone else has any ideas for the cause to contribute? Feel free!
     
  15. charveldan

    charveldan Well-Known Member

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    Oh shit thats not good. Those Nam DSL's had warranties, many people who had issues could just swap for a new amp.

    Don't know if thats your case.
     
  16. Publius

    Publius Member

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    No, unfortunately, it was five-six years due to the warranty. No matter what, I pulled myself together and was able to revive it. Even to this day I don't remember even breathing when I made that decision, that hole and the rest of the works, but perhaps its because breathing is underrated.
     
  17. charveldan

    charveldan Well-Known Member

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    [​IMG]

    As a US VETERAN it is my Duty to Award you this PURPLE HEART for being SHOT IN THE HEAD IN VIETNAM & actions above & beyond the Call of Duty ... SEMPER FI, CARRY ON ...
     
  18. J5684

    J5684 Member

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    These DSL's are notorious for leaking high voltage into the grid circuit through parasitic PCBs. I had several, one I took a dremel tool and completely isolated pin5 and rewired the bias circuit out of the PCB.. There are a few older threads here about the bias drifting on these amps. For me DSLs are a love/hate relationship LOL ... Good luck
     
  19. Publius

    Publius Member

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    Well. now I'm totally puzzled.

    As I have gone through many measurements and everything seemed to be alright and have not been able to pinpoint the cause of the symptom, I changed two of the tubes that a friend gave me. I only did this in order to make sure that the bias remains the same, just in case my original tubes were starting to wear out. Well, the bias with the new matched pair remains intact, namely, my original tubes were actually wearing out. I played more than two hours without any problem whatsoever.

    Yesterday we rehearsed with the band and within 15minutes from starting, the amp went mute once again and remained mute for 2minutes and then came back with a fade in effect. After 5-10 minutes of playing and while I was, I noticed that the amp went mute with a fade-out effect and then after 5 minutes it came back with a fade-in effect.

    Differences between the testing period and rehearsal.
    1. More people in the room, more equipment requiring power supply, more moisture, more mobile phones in the room etc.
    2. Another side/row of lights switched on
    3. Amp placed in its position which is very close to the testing site but not exactly. The amp was at all times powered by the same outlet in the same manner.

    I can only assume that since the amp does not experience a power outage, but only goes mute with power on, then the symptoms can only be caused by EMC when it comes to external factors.

    Moreover, the fade-out/fade-in effect resembles the discharge/charge of a capacitor which was my initial hypothesis. The problem is that during test I had no instance of the amp going mute which means that at all times all the capacitors were acting as supposed to. Therefore I either have to artificially push it to go mute which is rather difficult or take the amp out of the chassis and have a rehearsal with it out of the box which is rather dangerous.

    Any suggestions???
     
  20. charveldan

    charveldan Well-Known Member

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    Take it to a competent Amp Tech and get it fixed.
     

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