Amps Marshall DSL40CR

Discussion in 'Gear Talk' started by CapnDenny1, Dec 18, 2018.

  1. CapnDenny1

    CapnDenny1 Student Of The Blues

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    Customer complaint was the amp makes low frequency rumble when off of standby. I can hear it, but having a heck of a time finding the cause.

    It stops when I remove the phase inverter tube. Otherwise nothing affects it.

    I am thinking it is a leaking coupling cap, and it is doing some low level motorboating.

    I will have to remove parts to try and isolate it further. I can't see it on the scope.
     
  2. PapaBear

    PapaBear Guit Fiddlier

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    Do have time to pop my popcorn?
     
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  3. CapnDenny1

    CapnDenny1 Student Of The Blues

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    Sure, no problem. I will tell you what I tell all my customers, it will be a couple weeks before I can pop it.

    I wanted to mention about the DSL40CR, that it has a Hi and Low power setting. On low the reduce the B+ from 450V to around 150V. They also reduce the bias voltage, which in turn increases the bias current through the power tubes from 33mA to about 90mA! I guess they are trying to make sure the EL34 tubes start distorting even at low power settings. I hadn't seen that done before. I got very concerned when I saw 90mA bias current! The previous amp the DSL40C didn't do that. It just reduced the voltage to the screen resistors to 150V, but left the plates at 450V.
     
  4. Cleotis

    Cleotis AKB48 Team-8 Wota

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    That Marshall DSL40Cx certainly is a man's amplifier. I using mine right now (it's under the work desk to multi-task during the work day). Anyway, the first DSL40CST that Sweetwater shipped did have a rumble, maybe it's of the same nature. It kind of sounded like a screw wasn't tight. The annoying sound was worse on some notes more than others. Everything was tightened and all new tubes put in. It still rumbled/vibrated. It was sent back to Sweetwater and they sent out a new one. The new doesn't rumble at low volumes but when it's on the clean channel with gain and volume at about 3 o'clock and the Friedman BE-OD engaged it's hard to tell if it's rumbling or not because everything on this side of the house is rumbling (it hurts my teeth sometimes).
     
  5. CapnDenny1

    CapnDenny1 Student Of The Blues

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    I read some comments on the Marshall Forum. They blamed it on wiring too close to other wiring. I cut the tie wraps and moved wires around. Absolutely no effect.

    There is some questionable solder on the other side of the board, and some weird residue next to that tube near a pin of the tube socket. I am thinking bad solder and heat induced dendritic growth in the flux left on the pcb. Hey it sounds good anyway?

    I will leave some parts out and make it so I can connect or disconnect them from the other side.
     
  6. PapaBear

    PapaBear Guit Fiddlier

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    I
    That's the one I got, but I think I want to upgrade the speaker
     
  7. CapnDenny1

    CapnDenny1 Student Of The Blues

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    I got a response from Marshall, which is great. We shall see if it helps?

    Hello,

    Thanks for contacting us. Schematics are only available to authorized service centers. We would recommend performing this on the amp. Replace R141 and R148 to get rid of the noise. Replace C139 if any residual noise remains.

    If you would be interested in becoming an authorized service center let us know. We do require warranty service work be offered as well.

    KEVIN DRURY

    MARSHALL SALES AND SERVICE SUPPORT

    1649 BARCLAY BLVD. BUFFALO GROVE IL 60089

    [​IMG]

    Here is where I think that is. The parts don't match, but R141 and R148 are 1M resistors and one is connected to C139. They are all in the same Phase Inverter section of the amp, where I traced the issue to.

    [​IMG]Untitled by Dennis Kelley, on Flickr

    [​IMG]zoom pic by Dennis Kelley, on Flickr

    I also asked what it takes to become an authorized service center. Sounds like they want money?


    [​IMG]
     
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  8. OG_Blues

    OG_Blues Guitar Geezer

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    While I am impressed that you got a response, I find their actual response unsatisfying.
    Replace them with WHAT, and WHY?
    Exact new replacement parts? (What would that do, unless they were putting in defective resistors?)
    Different values? (What?)
    Different composition / lower noise parts? (If so, what do they recommend?)
    I could assume several different answers myself, but why don't they spell it out if they really want to help?
    Poor communication IMO.
     
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  9. CapnDenny1

    CapnDenny1 Student Of The Blues

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    Yes, that info would be useful, no doubt.

    I suspect they got a bad batch of resistors. So yes, low noise. I'm sure if you are an authorized service center they give you the full info.

    Hey, I am glad he said something. I put it the pcb back in last night, but I will just pull it out and change all 3 parts.

    When I got my Egnater Rebel 30 MK II, it had a noise issue with the record out line. The fix was to replace a resistor with a slightly lower value. Egnater told me about the fix, but they wouldn't tell me which resistor, or where it was. It was in the authorized repair shop for 3 months. It would have taken longer, but I got Bruce Egnater to send an email to the guy at the US incoming receiver tech bench to send the info the the repair shop. The local shop was called LS Electronics, and were pretty bad. Some of these buggered up amps I get to repair were buggered up by LS Electronics according to Dave the Amp Guy.
     
  10. OG_Blues

    OG_Blues Guitar Geezer

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    A policy of withholding critical information from anyone involved in fixing a product seems like a counterproductive policy to me.
    If I were a manufacturer and had a problem / defect with something I made, I would want it fixed properly and promptly by anyone capable of doing so in order to make my customer happy and preserve my reputation.
    If they feel they are "protecting" their authorized service centers (that paid them $ to be certified) in some way, they are kidding themselves.
     
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  11. CapnDenny1

    CapnDenny1 Student Of The Blues

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    They have to be somewhat careful. Who knows what some Yahoo might do, and then claim Marshall told them to do it.

    A lot of manufacturer's will not supply schematics. Some are out of business, like the Legend amp I have still to fix.

    Peavey will send them if you call or email and ask them for it.
     
  12. CapnDenny1

    CapnDenny1 Student Of The Blues

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    Well, holidays are over, almost, so I got my shop back in action. I replaced the suggested resistors and put it back together. I figured it would have no efec. But to my suprise it did, it buzzes now and the noise is much louder? Happy freaking New Year! So either the resistors I used are also bad, or I messed up the pcb replacing the two resistors, orsomething. I will ring it out and see if the pcb is all connected right or not. I hate negative progress!
     
  13. Rancid Rumpboogie

    Rancid Rumpboogie Blues Mangler

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    How many times did Einstein fail before he finally got it right? :ROFLMAO::ROFLMAO:
     
  14. OG_Blues

    OG_Blues Guitar Geezer

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    I'm curious - did you measure the values of the resistors you removed and the ones you put in?
    "Defective" resistors are pretty rare - tolerances may be significantly off, but most circuits are not all that critical of resistor values (a broad generalization, I know). The fact that they had a noticeable effect of any kind at all surprises me. It will be interesting to hear what you find next.

    For anyone interested in how noise is related to resistors, the following information is quite interesting:
    Thermal and current noise
    There are two types of noise: the thermal noise and the current noise. To understand their principle, they will be discussed in more detail. In all materials, the electrons permanently move. As temperature increases, the movements increase. The vibrations of the electrons cause an electric signal (AC) across the terminals of the component. Because the vibrations are completely random, the electrical signal is noise. This is called thermal noise or Johnson noise. It is the main contributor to noise for resistors. Thermal noise is constant over a wide frequency range. Current noise however, declines when frequency is increased. The thermal noise increases with a larger resistance value, while the current noise decreases.

    Noise standards
    The way to measure resistor current noise is defined in norm IEC 60195. This makes the comparison of different manufacturers possible. The current noise of a resistor is described by the current noise index with a code number.

    Low Noise Resistor
    Thin film, metal foil and wire wound resistors have better noise characteristics than other types. Therefore they are often specified in low-noise amplifying applications. The carbon composition resistor and thick film are of the worst types. They cope with high noise due to the construction and material.

    [​IMG]
    Noise index (dB) for the main resistor types

    Resistor applications
    In every amplifier circuit, the input resistor is critical. Any noise at the input signal will be amplified to the full gain. It is therefore of high importance to choose a low-noise resistor at the first stage, as well as a low resistance value. This is however not valid for a load resistor, since the gain that is obtained from a high resistance value outweighs the higher noise level. Because thermal noise is temperature dependent, it is very effective to cool the input stages to reach a low-noise performance.



    Read more http://www.resistorguide.com/resistor-noise/
     
  15. CapnDenny1

    CapnDenny1 Student Of The Blues

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    Got ‘er Done! Watchin Larry the Cable Guy.

    I looked at the circuit that I changed the resistors in, and it goes to a transistor? So that isn’t where I thought it was. Perhaps it is part of the circuit that adjusts the bias when you switch between full low power.

    I decided to go ahead and replace the 1 meg resistors in the phase invertrcircuit. I also replaced the coupling cap and the cap Marshall recomended I replace. I then buttoned it all back up and I didn’t hear the origanl noise?

    So I plugged in a guitar and it sounded ok. I turned both masters all the way down, and it is silent, so it is fixed.

    I must say this is a nice sounding amp. It’s not a Fender (duh), but it has a nice sound. When you crank up the gain it sounds really nice.

    I don’t know exactly which parts fixed the issue. I have 90% on the couplig cap. This was a tough one.
     
  16. Cleotis

    Cleotis AKB48 Team-8 Wota

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    Nice work! Good to see a Marshall back in service.
     
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  17. CapnDenny1

    CapnDenny1 Student Of The Blues

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    One of the issues with this amp was trying to get the correct schematic. Some stuff can be fixed without one. But when you are tracing through the circuit looking for a noise source or an anomaly, the schematic, the correct one is really needed. I could find the old one, but there were things on this amp, in the area where the problem was, that were way different from the old schematic. I looked an looked but could only find the older version, which from what I read did not have this issue.

    Once I fixed it I was really curious as to why that would work, so I wanted even more to get a schematic. It occurred to me that Marshall also makes this same amp in a 100w version. In fact the extra power tubes have places on this amp's circuit board where they would be added. I assume a bigger power transformer and output transformer are also needed, but perhaps they use the same ones on both amps? So I did a serch for a schematic for the DSL100HR, which is the 100W Head version of this amp. And to my delight it was out there. Someone on Music Electronics was looking for the same on I was, and someone first sent the old one. But then later in the post someone provided the latest schematic. It actually had the parts that Marshall told me to change. I wish I had figured that out a bit sooner, it would have saved me some troubleshooting times and grief.

    Here is the section of the schematic that has the parts I replaced that seemed to fix the issue. R141, R148, and C139. They are in the section with a pot called "Resonance". It is interesting. They are taking the negative feedback, which is also adjusted by the "precense" control, in the Phase Inverter section, Tube V4 in the upper left. That type of Precense control is pretty standard. But the Rosnance control is something new. It appears to use the negative feedback signal from the speaker out, to actually load down the B+ supply. That's the HT1 signal. So I guess this causes the B+ to droop a little, which causes the output to droop, which causes the Resonance signal to let the B+ go back up. So it is a way to add global reverb sort of? Perhaps this makes it behave more like a tube amp that uses a tube rectifier? But you can adjust it to be more like a solid state rectifier, or a tube rectifier, or somewhere between the two.

    The effect must have had a low frequency waver in the output caused by some weird heating effect in the resistors R141 and R148, and cap C139. I don't like to sell myself short, although I do it all the time, but I doubt I would have been able to figure this one out? Maybe, but it would have taken a lot longer.

    Thank you Marshall for refusing to help, but then helping me anyway! And I repay that by divulging their new secret circuit? Just the kind of guy I am, I can't keep a secret. I just thought it was interesting as heck, and sometimes new tube amps have some new innovative stuff going on. Now that I know where the issue is, I need to make sure it doesn't come back with other settings of that pot.

    [​IMG]zoomed by Dennis Kelley, on Flickr
     
  18. DannyB

    DannyB 2 miles from Jim Beam. Oh! Pleasent Hope!

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    One of these days, I'd like to buy you a beer. No real reason, but just because!

    :Beer:
     
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  19. CapnDenny1

    CapnDenny1 Student Of The Blues

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    Cool. Thanks for the complement.
     
  20. FabianoBernardesdeToledo

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    Hi, there!

    I know this is kind of an old thread, but I've just bought a brand new Marshal DSL40cr, and it has the same problem as the one you described, that is, a frequency rumble when off of standby, which is similar to a "wind blowing on a microphone" sound. I'm going to take mine to a local tech here in Brazil, and after reading everything you wrote here, I would like to ask you a question, if you don't mind.

    The solution you found was to replace the R141 and R148 resistors and the C139 capacitor? Is C139 the coupling capacitor you mentioned or did you replace two different capacitors? Did you replace any other components? Your help will be greatly appreciated!!

    Best regards,
    Frank