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

  1. #36
    Old Timer J M Fahey's Avatar
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    Quote 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]

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    Juan Manuel Fahey

  2. #37
    Senior Member vintagekiki's Avatar
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    About burnt output tubes

    Click image for larger version. 

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  3. #38
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    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

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  4. #39
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    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.

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  5. #40
    Senior Member vintagekiki's Avatar
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    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

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  6. #41
    Old Timer J M Fahey's Avatar
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    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

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    Juan Manuel Fahey

  7. #42
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    Quote 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.

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  8. #43
    Senior Member vintagekiki's Avatar
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    Where we left off with Marshall?

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    Who does not know and knows that he does not know - teach him Confucius)
    Who knows and does not know that he knows - wake him Confucius)

  9. #44
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    Quote 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 =(

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    Last edited by spunko; 10-18-2019 at 10:21 PM.

  10. #45
    Senior Member vintagekiki's Avatar
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    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)

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  11. #46
    Stray Cap DrGonz78's Avatar
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    Wow!! Read many threads here where the OP posted about getting the flu and hadn't gotten to work on the amp lately. But civil unrest against the government and a chaotic situation like that takes the cake. Hope all is going better where you are and that you can get back to amp work soon.

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  12. #47
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    Quote Originally Posted by vintagekiki View Post
    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)
    Hello, and sorry for the late reply, you already know

    Thank you very much for all the help.
    I checked again all the pots as suggested, and the volume pot for the ch B wasn’t reading well on my meter, I know it’s audio taper, but it went up and down in the reading when it is supposed to just rise when turning up.

    I changed the pot and “glory”, both channels sound really loud.

    I’m happy with its sound so far (1 hour playing, neighbors are complaining)

    Also all the correct fuses has been installed.

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  13. #48
    Senior Member vintagekiki's Avatar
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    I am glad for your fixing Marshall and what you are happy how the amp works.
    Welcome to the club.

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