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Marshall JCM 900 fried! Need help :)

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  • #46
    Again, thanks!
    I'll look into the diode tomorrow but I did a quick recheck on R30 and it shows 38.5k again. What's a voltage dropper? Might this be part of the problem?
    “Women are meant to be loved, not to be understood.”

    ― Oscar Wilde

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    • #47
      Originally posted by Audiotexan View Post
      Another quick item of interest is R27.
      ...
      Previously we said:
      R27: 150k (good, 150k accord. to schem g1 posted / should be 1.5k according to mine/2nd schem)
      R27 is *clearly* 'smoked', and was either mis-measured, or read 'as expected' strictly by chance. Replace.
      Good eye! Yes it should be replaced with 150K, even if it still reads 150K it has been overheated. Makes me worry about C11 a little.

      Recheck R30 with one end disconnected. It should measure 22K, if not, replace it. The voltage dropper reduces the supply voltage that is going to the preamp tubes. I don't think it would cause your problem but it is possible.
      Originally posted by Enzo
      I have a sign in my shop that says, "Never think up reasons not to check something."


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      • #48
        Originally posted by Audiotexan View Post
        Diode tests should be done in 'diode mode' on the meter (the symbol for a diode >| ), and you'll have to flip the leads (pos. and neg.) as you do it. If it's a good diode, it should read approx .7v when forward biased (pos. red-lead on anode, neg. black-lead on cathode [striped end]), and it should read 'infinite' when reverse bias
        Ok so I did that and the digits shown on my display are 616 with right polarity and infinite the other way around. What does that mean and what exactly am I measuring?
        “Women are meant to be loved, not to be understood.”

        ― Oscar Wilde

        Comment


        • #49
          Originally posted by sibo View Post
          Ok so I did that and the digits shown on my display are 616 with right polarity and infinite the other way around. What does that mean and what exactly am I measuring?
          What that shows is that the diode is testing correctly.

          By swapping the leads around, it shows that it conducts in one direction only, which is what makes it a diode.

          The 616 display represents 0.616 volts dc. This is the voltage that it takes for the diode to start conducting or to turn on.

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          • #50
            Here we go again, part two of my component-changing list. I will "over-do" it this time to minimize the risk of something else being bad. I found nothing on the +- tolerances, what should I aim for?
            I would love some help on the missing data.

            Power section:
            R27: 150k, Effect?, Wirewound resistor with silicon coating, Axial connection
            R30: 22k, 1W, Ceramic resistor, Axial connection
            R35: 100Ohms, Effect?, Ceramic resistor, Axial connection
            C11: 0,047uF, 250V (is this ac?), Connection type?
            C12: 0,047uF, 250V (is this ac?), Connection type?
            C15: Unknown

            Pre section:
            IC3: Does this need changing? I scraped some goo off it.
            C3X: That big capacitor next to TR2, 33uF, 450V, Type?, Axial connection
            C11: 0,1uF, 250V AC, Connection type?
            C13: Capacitance?, 63V, Electrolytic, Axial connection
            C18: 100uF, voltage?, Electrolytic, Axial connection
            C31: Capacitance?, Voltage?, Type?, Radial connection
            C32: Capacitance?, 35V, Radial connection
            C45: 2,2uF, 63V, Electrolytic, Axial connection
            R47: Unknown

            If this doesn't help I guess I'll have to hack my axe through it...
            “Women are meant to be loved, not to be understood.”

            ― Oscar Wilde

            Comment


            • #51
              Originally posted by sibo View Post
              Here we go again, part two of my component-changing list. I will "over-do" it this time to minimize the risk of something else being bad. I found nothing on the +- tolerances, what should I aim for?
              Shotgunning parts at it will only cost you money and time. So hold off just yet. But I feel your frustration. We've all been there. Just breath a moment, and it'll be alright! lol

              Originally posted by sibo View Post
              I would love some help on the missing data.
              Will try to help where we can!

              As for the list above that you drew up...The only things that are needed thus far are:

              Originally posted by sibo View Post
              Power section:
              R27: 150k, Wirewound resistor
              and if you did as g-one said here:
              Originally posted by g-one
              Recheck R30 with one end disconnected. It should measure 22K, if not, replace it.
              As, while in circuit, it could measure differently simply due to other components affecting the reading.

              So possibly adding R30 to the needed parts list:
              R30: 22k, 1W, Ceramic resistor, Axial connection

              Originally posted by sibo View Post
              Pre section:
              IC3: Does this need changing? I scraped some goo off it.
              No. Not unless it's been verified that it's bad.

              Originally posted by sibo View Post
              If this doesn't help I guess I'll have to hack my axe through it...
              Again, I feel your pain. Patience is a #$^$@ sometimes.

              Once you get R27 changed out (which was already confirmed as needing replacement), and confirm verification that you used g-one's procedure, and possibly replacing R30 as well, then we'll have you get some voltage readings.

              But we need to know where things stand once R27 has been replaced and R30 has been addressed/confirmed/replaced (if warranted) before we move to voltage readings. =)
              Start simple...then go deep!

              "EL84's are the bitches of guitar amp design." Chuck H

              "How could they know back in 1980-whatever that there'd come a time when it was easier to find the wreck of the Titanic than find another SAD1024?" -Mark Hammer

              Comment


              • #52
                Originally posted by sibo View Post
                I would love some help on the missing data.
                Again, this is just to cover your questions...but not needed unless mentioned in the previous post.

                Originally posted by sibo View Post
                Power section:
                R27: 150k, Effect?, Wirewound resistor (part of voltage divider network for bias)
                R35: 100Ohms, Effect? (think of it as an extra fuse)
                C11: 0,047uF, 250V (is this ac?), Connection type? (non-polarized coupling cap to pass signal, rating is DC)
                C12: 0,047uF, 250V (is this ac?), Connection type? (non-polarized coupling cap to pass signal, rating is DC)
                C15: Unknown (47nf -not sure what "Class X" is)

                Pre section:
                C3X: That big capacitor next to TR2, 33uF, 450V, Type?, Axial connection (standard electrolytic polarized PSU cap)
                C11: 0,1uF, 250V AC, Connection type?
                C13: Capacitance?, 63V, Electrolytic, Axial connection
                C18: 100uF, voltage?, Electrolytic, Axial connection
                C31: Capacitance?, Voltage?, Type?, Radial connection
                C32: Capacitance?, 35V, Radial connection
                C45: 2,2uF, 63V, Electrolytic, Axial connection
                R47: Unknown
                R47 pertains to the preamp circuit if I'm not mistaken, so I'll not make any further assumptions due to we've already made established there are revision differences and a preamp schem hasn't been posted and (more importantly confirmed) as matching your amp.

                Basically though, most of your questions about things like radial vs axial, polarized vs non-polarized, are basics that need to be learned by reading some basic sites. Not meant to be rude/snarky in any way whatsoever, moreso that, some folks have taken a lot of time to do those types of wonderfully informative sites, and it'd be far easier than explaining it all here.

                But the extreme basics are:
                Radial: both leads on one end
                Axial: one lead on each end

                Polarized: has a side for +v, and a side for "ground" (can also be "less-positive" voltage: meaning not 'true' ground, but 'ground' with respect to the circuit, or what we refer to as a 'floating ground')
                Non-polarized: typically used for passing AC signals (at least in the majority of our applications)

                Ratings: WVDC (Working Voltage, DC)

                Capacitor - Wikipedia, the free encyclopedia

                Introduction to Capacitors, Capacitance and Charge

                https://www.youtube.com/watch?v=CmII_BmOf0I

                Hopefully that'll help get you off on the right foot
                Start simple...then go deep!

                "EL84's are the bitches of guitar amp design." Chuck H

                "How could they know back in 1980-whatever that there'd come a time when it was easier to find the wreck of the Titanic than find another SAD1024?" -Mark Hammer

                Comment


                • #53
                  Agree with Audiotexan about resisting the urge to shotgun replacing components not proven bad. For starters, you won't learn anything that way . But the bigger problem is you run the risk of introducing new problems which will interfere with the troubleshooting process.

                  "C15: Unknown (47nf -not sure what "Class X" is)" Class X & class Y caps are special safety rated caps that are approved to run "line to neutral" (class X) or "line to ground"-AKA "death cap" (class Y).
                  C15 is the bias feed cap, between the HV winding and the bias diode. I'm not sure if they are using class X here because of legal requirements, or because of how much damage can occur if that cap fails (loss of bias). In any case, it is quite a critical component.
                  Originally posted by Enzo
                  I have a sign in my shop that says, "Never think up reasons not to check something."


                  Comment


                  • #54
                    I guess they use it for the guaranteed reliability.
                    And even so they are using it beyond its rating: it´s meant for "up to" 250VAC lines, they are using it with some 350VAC .
                    Juan Manuel Fahey

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                    • #55
                      Big thanks again. I took your advice and calmed down. And you're right, I do this because it's interesting. Otherwise I could've just turned it in at a tech shop and got it fixed! Also you're most definitely not being rude, Audiotexan. I have no prestige in this whatsoever, I'm just thankful for all the help you give me and I will start reading those links right now. Also, I will order R27 & R30 and after they're changed I will get back (I'll measure R30 with one end loosened first).

                      I'll keep you updated in a couple of days when the components have arrived and gotten changed <3
                      “Women are meant to be loved, not to be understood.”

                      ― Oscar Wilde

                      Comment


                      • #56
                        On a sidenote!
                        I actually have to admit smelling solder getting heated up turns me on I will be needing my own multimeter (the one I'm using is borrowed); would this be a good first invest? https://www.elfa.se/elfa3~se_en/elfa...%2C+AM-510-EUR
                        “Women are meant to be loved, not to be understood.”

                        ― Oscar Wilde

                        Comment


                        • #57
                          Originally posted by sibo View Post
                          ...I will be needing my own multimeter (the one I'm using is borrowed); would this be a good first invest? https://www.elfa.se/elfa3~se_en/elfa...%2C+AM-510-EUR
                          At first I panicked, thinking holy crap if you're going to spend 379.00 might as well get a Fluke 87...then common sense kicked me in the teeth again, and I noticed the Zurich IP. (Thank you again Flagfox!)

                          At current conversion rates, I'm showing 1.00 SEK = 0.141336 USD, so that puts that meter around $54 USD. Modestly priced, and some nice features! True RMS, .1ohm accuracy, does small caps (up to 100uf) does frequency, and a count up to 3999. Not bad at all!

                          However, it all depends on how 'serious' (read as: how often) you'll be needing a DMM, as well as what you're working on. If you plan on doing a lot more in tube amps, I'd then suggest going '1 step up' and considering the 1000v model. Bear in mind, it's triple the price, but, considering that many tube amps operate with B+ in the range of 500+ volts, you're not leaving much room for 'disaster' to strike with the 600v model. [A guy I worked with cooked my 1000v Fluke 87 once on a TV flyback by not paying complete attn when he should have (hey, we all have off days!)] Regardless, if you're only going to be working on 'small' (25-50w) tube amps, and stuff around the house, then the meter you linked should be more than adequate.
                          Given you're already into a 100w amp that runs at 500v...

                          I look at it like this. IF you cook that meter even once, and replace it with the same, then you're already 2/3 of the way to the price of the 1000v model. The question lies with your outlook. Is it worth spending 3x as much for 50% more "insurance" that you won't even come close to cooking it (at least by raw B+)?

                          On a slightly different note, the single most important thing in a meter to me (aside from accuracy), is that it's True RMS.

                          A perk of the 1000v model I linked is that it's 10x more accurate in VDC readings, which is the majority of what you'll use. (Accuracy to within 100uv on the 1kv model, vs 1000uv accuracy on the 600v model) But again, that's a *perk*, the 1000uv reading is still quite accurate! Let your needs be your guide.
                          Start simple...then go deep!

                          "EL84's are the bitches of guitar amp design." Chuck H

                          "How could they know back in 1980-whatever that there'd come a time when it was easier to find the wreck of the Titanic than find another SAD1024?" -Mark Hammer

                          Comment


                          • #58
                            Originally posted by g-one View Post
                            "C15: Unknown (47nf -not sure what "Class X" is)" Class X & class Y caps are special safety rated caps that are approved to run "line to neutral" (class X) or "line to ground"-AKA "death cap" (class Y).
                            Ahhhhh! TY g-one!
                            Start simple...then go deep!

                            "EL84's are the bitches of guitar amp design." Chuck H

                            "How could they know back in 1980-whatever that there'd come a time when it was easier to find the wreck of the Titanic than find another SAD1024?" -Mark Hammer

                            Comment


                            • #59
                              Originally posted by J M Fahey View Post
                              I guess they use it for the guaranteed reliability.
                              And even so they are using it beyond its rating: it´s meant for "up to" 250VAC lines, they are using it with some 350VAC .
                              Yeah, I was wondering about that as well...

                              I wish all manufacturers would put the damned voltages down.
                              /minor annoyance rant off
                              Start simple...then go deep!

                              "EL84's are the bitches of guitar amp design." Chuck H

                              "How could they know back in 1980-whatever that there'd come a time when it was easier to find the wreck of the Titanic than find another SAD1024?" -Mark Hammer

                              Comment


                              • #60
                                Yeah, I ate too many $15.00 sense resistors on my Fluke 600 volters.

                                I have sinse gone to the 1000 volt model.

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