NAD - Marshall Valvestate 8040 40W 1x12 - I got a lemon... help please!

Discussion in 'The Workbench' started by waltschwarzkopf, Nov 30, 2021.

  1. Kutt

    Kutt Well-Known Member

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    What @Spanngitter wrote.

    The single preamp tube in the first generation Valvestates are not used in the clean channel circuit, only the distortion channel. This changed in the second generation Valvestates.

    The fact that your clean channel is healthy but the dirt channel is problematic calls for a new12AX7. Also hit the preamp tube pins with contact cleaner and work it in/out of the socket a few times to remove any oxidation.
     
  2. fitz288

    fitz288 Well-Known Yinzer Gold Supporting Member

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    Cool! Add that to the repair budget.
    ^^ Do This! ^^ $30 for some Deoxit and a tube, you may be rockin' a Marshall for the cost of some typical maintenance items.
     
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  3. waltschwarzkopf

    waltschwarzkopf Member

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    LOL! He actually had a VS100 that he was asking 250 bucks for it, it was reserved and now sold. I wonder if it was in the same condition.
    I am now ok with my free purchase, in case the amp does not work, I have some plans to turn in into a cab an get a small head. Although I am very tempted to build my own tube head. But one thing at a time…
     
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  4. waltschwarzkopf

    waltschwarzkopf Member

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    Will do that too! A 20+ year old amp that as sitting for the last two years unused in a cellar sure calls for maintenance. I am also thinking about getting a new speaker. I read about some cool mods that one can do, but first get it back to normal.
     
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  5. waltschwarzkopf

    waltschwarzkopf Member

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    Yeah, that’d be great. I never had tube amps for long for the fear of the tubes breaking and calling for an expensive repair. But now I feel more confident as Im learning to change the tubes and do proper maintenance on the amp.

    Wish me luck!
     
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  6. SkyMonkey

    SkyMonkey Well-Known Member

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  7. Gunner64

    Gunner64 Emotional Support Animal Gold Supporting Member

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    For $20 I would have kept it working or not.
     
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  8. waltschwarzkopf

    waltschwarzkopf Member

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    He said he needed the space…
    Now I’ll try to bring it back to life, but if ir doesn’t work, I’ll turn it into a cab and scrap the electronics.
     
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  9. Gunner64

    Gunner64 Emotional Support Animal Gold Supporting Member

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    You'll get it. Clean channel works, can't be too f-ed up, could be simple .

    And use a lubricating cleaner on the pots and jacks. I only mention this because I read a post recently where someone was using the stuff from the auto parts store..cnc or something like that that, no lubricant in most of that stuff.
     
  10. waltschwarzkopf

    waltschwarzkopf Member

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    I opened the amp again, unsoldered all pots and removed them. Then I opened them up to clean and rework the tracks, then I measured the resistance between the outler legs and also during a swewp to check for dead spots. Most values seemed to be within what was specified, and the sweep was smooth for all posts.

    Here are the resitance values, in order of the schematic:
    1. 77k (100K)
    2. 0.9M (1M)
    3. 20k (22k)
    4. 197k (220k)
    5. 0.87M (1M)
    6. 20k (22k)
    7. 0.74M (1M)
    8. 190k (220k)
    9. 75k (100k)
    10. 20k (22k)
    Afterwards, I remounted all posts and soldered them back carefully. I also removed the tube and cleaned the pins and the socket and mounted it back. Assembled everything and took it back home.

    The normal channel is still working, pots are smoother, although the treble scratched a bit. The boost channel is still bad; however, the volume and gain control seem to have a better response than before. This means that the gain changes a bit and the volume too, although it is considerably quieter (at 10) compared to clean (at 4). The contour, bass and treble don't seem to do anything noticeable. Although, it just occured to me to try it with headphones, as I may notice some small chanches.

    I guess the next step would be to order and install a new tube and see if that changes anything. But if it doesn't, then I'll have to leave it at that, as I'm running out of things to try. I won't throw it away, I just may scrap the electronics and turn the casing into a 1x12" cab.

    Any thoughts or ideas?

    One last thing! What is this dark spot in the middle of the board? I thought something was burnt, but it doesn't look like that, plus the connections seemed also ok.

    [​IMG]
    [​IMG]
    [​IMG]
     
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  11. SkyMonkey

    SkyMonkey Well-Known Member

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    The 8040 is a decent amp (when working). The trick might be to keep the $20 amp for spares, for when you spot another one.

    Get a new JJ ECC83S to install.
    I put a new one on my working 1997 VS65R and it still improved the tone of the amp.
     
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  12. SkyMonkey

    SkyMonkey Well-Known Member

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    That dark solder spot seems to be under R63 (@ V1 Heater) which the schematic says has a value of 68/3W.
    You could test its value and the soldered connections.
     
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  13. Spanngitter

    Spanngitter Active Member

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    This is where R62 connects to the ECC83 Heater/Filament. I would recommend to
    1. Desolder and Re-Solder this point, clean everything with IPA or Alcohol before resoldering
    2. Ensure R62/63 did not drift ( 68 Ohm/3W)
    3. Measure between the outer legs of R62/63 with tube in place if you have something around 140 Ohm resistance to ensure there is proper contact
     
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  14. SkyMonkey

    SkyMonkey Well-Known Member

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    The adjacent resistor to R63 is R62. They should both be the same, but on your OP photo (2) they look different.
    Has one (R62 by the look of it) been replaced?
     
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  15. tschrama

    tschrama Well-Known Member

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    Please measure the DC voltages of the tube , all pins, when the amp is on, all volume or gain pot at zero.

    Then atleast you get closer to a diagnosis instead of randomly soldering , desoldering, exchanging parts.

    Check that filament PCB track carefully. Does the tube filament glow?
     
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  16. waltschwarzkopf

    waltschwarzkopf Member

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    Not by me, but who knows if the previous owner (or his brother) did something!
    Here's a close up.

    Ok, so I will remove both resitors and measure them. Repalce if needed, and resolder.
     

    Attached Files:

    Last edited: Dec 2, 2021
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  17. waltschwarzkopf

    waltschwarzkopf Member

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    I will try this too. Do you know which DC voltages I should expect?
     
  18. johan.b

    johan.b Well-Known Member

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    Screenshot_20211202-155607_Samsung Internet.jpg check that the legs on either of these didn't break from vibration. It'll kill the heaters. The fakt that they are different suggests it has happened before
    J
     
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  19. waltschwarzkopf

    waltschwarzkopf Member

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    Will do! I will put in new (identical) ones to be sure. Maybe even reduce the leg length a bit...
     
  20. Spanngitter

    Spanngitter Active Member

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    Don't reduce leg length as it will cause issue with heat dissipation, better is to support these legs with a short piece of glass fiber tubings.
    Also when replacing the R you can upgrade it to a 5W Wirewound (No Cement Block!) which handles the load a tad better...
     
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