Announcement

Collapse
No announcement yet.

Cheap Marshall JCM900 4102 "AS IS" - blow fuses, bad power tube and pots - low output

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • #31
    Given the different native anguages and the nuances implied in these borderline undocumented bias and setup conditions I suggest you should both reset the counter to 0, not even try to explain earlier misunderstandings and simply start from 0.

    Ignore what was said before, don´t try to explain it, just suggest and refer to new measurements and tests.
    Start with a white sheet of paper.
    Juan Manuel Fahey

    Comment


    • #32
      Originally posted by spunko View Post
      Quick test: No power tubes = no output // with tubes and no fuses = low output

      Should I post voltages without the fuses, but with the power tubes plugged in?

      Also, is this ok?? That pin8 gets different voltages because of the fuse in use?
      Each fuse value has some resistance, so measuring the voltage drop on the fuse does not indicate anything.

      Look tube schematics
      http://www.drtube.com/en/library/schematics/69-marshall-schemas

      In 99% amplifier schematics pin 8 (cathode) is connected directly to the chassis, so nothing bad can happen to the amplifier, if in parallel with FS1 / FS2 solder resistor 1 ohm to adjust the bias (until you get the original fuse)

      Measuring the voltage drop on 1 Ohm will give true information about the status of the output tubes.

      Make sure look at the output tubes and the sockets between pin 2 and pin 3 for a lampblack (soot, char)
      It's All Over Now

      Comment


      • #33
        Originally posted by J M Fahey View Post
        just suggest and refer to new measurements and tests.
        Start with a white sheet of paper.
        Click image for larger version

Name:	Double like.gif
Views:	1
Size:	12.9 KB
ID:	854699

        Well done master
        It's All Over Now

        Comment


        • #34
          Any progress on 4102 spunko?
          It's All Over Now

          Comment


          • #35
            I'm sorry for the late repply.

            This is what I found for the PC scope. I will have to order some parts from ebay.

            Found:
            - 470K
            - 470 Ohm
            - plug for the pc
            - 1N400 x2
            - metal box and a plastic one for the case

            NOT found:
            - 600 volt =/> 0.1uf or 0.22uf in fact only found 400v in every single store.
            - 20 watts or > 4 ohm resistors for the dummy load

            My laptop has only one jack that serves as headphones and mic at the same time. There is no option to change from Mic to Line. It runs Win 10.
            I have an old PC that ran Win Vista, but the HDD died, I will find a cheap replacement, and try with it.

            There are no signs of lampblack or burnt/black spots in any part of the amp.

            Someone will borrow me some new 6L6s to try, because mine are too old.

            Comment


            • #36
              Originally posted by spunko View Post
              This is what I found for the PC scope. I will have to order some parts from ebay.

              Found:
              - 470K
              - 470 Ohm
              - plug for the pc
              - 1N400 x2
              - metal box and a plastic one for the case
              Cool. Did you find that locally or through EBay?
              NOTE: please get into your Member information page and update your Country to Ecuador so it´s displayed in your avatar, I had to go through 20 answers to find it.
              NOT found:
              - 600 volt =/> 0.1uf or 0.22uf in fact only found 400v in every single store.
              Maybe I mistyped but actual values are .01 to .022
              600V is "safe" and a standard value , 400V may be used but it´s somewhat on the edge, but if there is nothing else available, it will do.
              - 20 watts or > 4 ohm resistors for the dummy load
              Can you get a cheap electric heater like I showed in post #20?
              And a couple spare resistor coils?

              If so, I can explain you how to make a high power load resistor in any value needed.

              Won´t last forever, it will rust, but will be fine for a week or so.

              My laptop has only one jack that serves as headphones and mic at the same time. There is no option to change from Mic to Line. It runs Win 10.
              I have an old PC that ran Win Vista, but the HDD died, I will find a cheap replacement, and try with it.

              There are no signs of lampblack or burnt/black spots in any part of the amp.

              Someone will borrow me some new 6L6s to try, because mine are too old.[/QUOTE]
              Juan Manuel Fahey

              Comment


              • #37
                About burnt output tubes

                Click image for larger version

Name:	1.jpg
Views:	1
Size:	60.4 KB
ID:	855486 Click image for larger version

Name:	2.jpg
Views:	1
Size:	75.4 KB
ID:	855487 Click image for larger version

Name:	3.jpg
Views:	1
Size:	1.83 MB
ID:	855488
                It's All Over Now

                Comment


                • #38
                  Hello, the parts were found locally.

                  I could use two caps in series, and also buy the electric heater, like a resistor for an electric shower, but since I'm ordering some parts from ebay, I better get them all and build a load resistor that last.
                  I saw a really cool video by D-Lab : https://youtu.be/8jFKa8AJkFo
                  He says he uses it for champs, so I better get 100 watts resistors?

                  Resistors for the attenuator (PC scope) should be 1 watt? or it does not matter?

                  EDIT: most important, my house does not have ground line, it is a 70 years old house
                  I'm planing on stick a metal bar in the yard and send a wire to my bench

                  Comment


                  • #39
                    I saw a really cool video by D-Lab : https://youtu.be/8jFKa8AJkFo
                    The type of power resistor shown in the video needs to be mounted on a suitable heatsink to be able to dissipate its rated power without getting too hot. See resistor datasheet.
                    - Own Opinions Only -

                    Comment


                    • #40
                      The power of the dummy load resistor depends on the power of the amplifier being tested.
                      For a guitar amplifier 100W the optimum power of the dummy load is of the order of 150W (dummy load has air cooling)
                      If the construction of the dummy load resistor allows, it is advisable resistor mount them to some heat sink.

                      It is not necessary for the dummy load resistor to be in one piece.
                      An effective dummy load can be made of several resistors of higher resistance and less power connected in parallel, with the total value for resistance and power being within the desired limits.

                      For example, 10 resistors connected in parallel value 82 Ohm/ 15W mounted on heat sink, given a dummy load of 8 Ohm/ 150W

                      http://www.ibselectronics.com/ibsstore/resistor-15w-5-82ohm.html
                      It's All Over Now

                      Comment


                      • #41
                        Agree and add: just to repair this one, kludge a simple/cheap resistor load out of pieces of electric cooker resistor coil, so you repair it NOW or we all lose focus.

                        Later feel free to build a nice elegant one for your workbench
                        Juan Manuel Fahey

                        Comment


                        • #42
                          Originally posted by J M Fahey View Post
                          Agree and add: just to repair this one, kludge a simple/cheap resistor load out of pieces of electric cooker resistor coil, so you repair it NOW or we all lose focus.

                          Later feel free to build a nice elegant one for your workbench
                          I'm sorry, I was out of town again.

                          I got a HDD but unfortunately my old PC did not boot up. The beeps tell me it is a faulty RAM stick, and it is the only one I have.
                          So that leave me with no scope software
                          Is there another way to test?

                          I'm going to try with new tubes from a friend and let you know, it wont hurt.

                          I am considering to buy a used oscilloscope and bring it overseas. Maybe a friend who is in the US could bring it to me.

                          What would you recomend? I'm lost about brands, costs, features needed? Maybe this question should be at another thread.

                          Comment


                          • #43
                            Where we left off with Marshall?
                            It's All Over Now

                            Comment


                            • #44
                              Originally posted by vintagekiki View Post
                              Where we left off with Marshall?
                              Hello, I am really sorry for the so late reply.

                              I wasn't able to get an scope. I was going to buy locally a really old one (used but working they said), but unfortunately my country went into chaos. There were massive protests against the goverment, and people moving from town to town closing roads and destroying anything in their path.
                              I was stuck in the fields for 4 days, totally incomunicated. The natives (indígenas) threatened everyone who won't join their cause. Really horrible situation.

                              Finally I didn't get the scope. I did get the new tubes from a friend. I plug them in, rebias with a tube probe I made with an old tube socket and now the amp sounds really really loud. Maybe my old tubes are too worn and weak.

                              So far the amp sounds really really loud, they way it is supposed (they way I wanted), BUT!! just the clean channel. The dirty channel sounds with less output, like half. (this was the opposite when it has a broken Master volume pot in the dirty channel that I replaced with the correct one).
                              I know we already talk about how to not measure output by ear, but with the new tubes, the clean channel now sounds like a 100 watt amp, really really loud and powerfull, but the dirty channel sounds like half power =(
                              Last edited by spunko; 10-18-2019, 08:21 PM.

                              Comment


                              • #45
                                Good news Marshall half-worked.
                                Since clean ch sounds really really loud, we don't need to test the power amp. If the power amp works properly on clean ch fault is in dirty ch.
                                For this repair do not need scope.

                                https://drtube.com/schematics/marshall/cd0189-iss7.pdf

                                When switching channels does dirty ch have less output in both cases with F / S and with panel push boost sw, and does the LED light up.

                                To start
                                - Check VR2, VR8 and VR10 (1 MOhm) does is it correct and not interrupt.
                                - When switching channels, check that changes voltage at point C of switching circuit (TR1/ MPSA13)

                                - If VR2, VR8 and VR10 are correct, check the changes voltage when switching channels at
                                diode D5 (1N914) and IC6 (M5201 pin 1)
                                diode D2 (1N914) and IC1 (M5201 pin 1)
                                diode D1 (1N914) and IC4 (M5201 pin 1)
                                It's All Over Now

                                Comment

                                Working...
                                X