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

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

  1. waltschwarzkopf

    waltschwarzkopf Member

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    Good idea! I will try that.

    You read my mind, I wasn't able to find a 3W resistor, but I would a 5W one that I thought would work better
     
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  2. waltschwarzkopf

    waltschwarzkopf Member

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    Alright, so I removed the old resistors (R62 & R63) and checked their values, they were both very close to 68Ohm. I cleaned the connections and checked for continuity along the tracks up to the resistor hole, all four tracks were ok and continuous. I also checked continuity between the holes and (as expected) there was none, as this would have meant a short circuit.

    I also checked continuity among the ground and different pins of the tube PCB solder joints and it was only continuous between the ground and the resitor tracks. See below, red dots with blue arrows mark the pins.

    Once everything was clear, I checked the new resistors (both close to 68Ohm) and installed them. Then, I assembled everything together and mounted the tube to test the glowing. The tube does glow, although very faintly. Below are two close ups of the tube, I let it warm up for a couple of minutes with no instrument connected and all the knobs set to 0. Is this normal? Should it be brighter?


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  3. SkyMonkey

    SkyMonkey Well-Known Member

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    That looks OK, but to really see what the tube is doing you need to view it in the dark.
    BTW I don't think you said if the tube glowed before the resistor change.

    And I am not sure of the significance of the last photo (with the arrows).

    Be sure you have the speaker connected when the power is on too.
    Some will say that a solid state power amp needs no load when powered, but my VS65R smoked when I accidentally powered it without the speaker connected.

    Let us all know if the amp now works.
    If not then the tube is next, as the old resistors may have cause damage to the original.

    Nice neat work BTW.
     
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  4. waltschwarzkopf

    waltschwarzkopf Member

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    Thanks!

    Then I'll try tonight and see if there is any improvement. Either way, I will install a new tube next week.
     
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  5. waltschwarzkopf

    waltschwarzkopf Member

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    I tried it again tonight. The normal channel now seems to be ok, just some slightly scratchy pots, but no big deal.

    The boost channel, on the other hand, is improving but still not good. Very low volume, contour knob now seems to affect the sound. Volume know seems to change then saturation, but not the overall volume. The gain knob affects a bit the gain at the beginning 1-3 but then is distorted. The treble and bass don’t seem to do anything at the moment.

    Bridging the pre amp and the power amp does not change anything. Then footswitch works with no issues. The line out does not mute the speaker (don’t know if it should), and headphones don’t work on the line out (don’t know if they should either).

    Next steps: Install a new tube next week and if it doesn’t work, I’m gonna check continuity across all PCB paths.

    Any other ideas?
     
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  6. fitz288

    fitz288 Well-Known Yinzer Gold Supporting Member

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    Do this before trying to diagnose problems that may not exist.
     
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  7. waltschwarzkopf

    waltschwarzkopf Member

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    Will do! I already ordered it, they didn’t have it on stock, so it’s taking a couple of days.
     
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  8. SkyMonkey

    SkyMonkey Well-Known Member

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    I think the Line Out may be for live recording, so you'd still need to hear the amp.
     
  9. waltschwarzkopf

    waltschwarzkopf Member

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    Quick update, I got a pair of 16Ohm V-Type Celestions, like new and with original packaging.

    I tried the amp with the stock speaker to have a baseline and then I installed one of the new speakers. Turned it on with all the same setting, and it sounded clearer, a bit louder and with a better bass response. I really liked it! I was surprised that the volume did not drop, I was expecting quite a big volume difference given the 4/16 impedance mismatch. I did the same with the other identical speaker, and (as expected) the results were the same.

    The amp has a really nice sounding clean channel, even with all the knobs at 12 o'clock. I'll try to turn it up to see if it starts to break up, but I might need to hide the ampsomewhere as it will be too loud...
     
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  10. fitz288

    fitz288 Well-Known Yinzer Gold Supporting Member

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    Louder in spite of the impedance mismatch may be due to the stock speaker just being junk.
    I upgraded the stock speakers in my G15MS (VT jr's - same impedance) and it was definitely louder, and tone change was amazingly better.
     
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  11. waltschwarzkopf

    waltschwarzkopf Member

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    Amazing, now I literally can't wait to try it on the little G15...
     
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  12. waltschwarzkopf

    waltschwarzkopf Member

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    BTW, the new tube just arrived. I ordered a ECC83S Tesla/JJ and from the build quality it looks already better than the (possibly) stock tube. I will install it tonight and let you know about the outcome. Stay tuned!
     
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  13. waltschwarzkopf

    waltschwarzkopf Member

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    Now that you mention it, the build quality was also a lot better on the V-Type, the stock speaker felt quite cheap...
     
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  14. waltschwarzkopf

    waltschwarzkopf Member

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    Bad news…

    I installed the new tube tonight and the was little-to-none improvement. The gain knob seems to have some effect until 3/10 and then it’s just fully saturated. Contour and treble knobs do something, but not much; the bass knob just does nothing at all. The volume knob does adjust the volume a little, but with both the gain and volume cranked all the way, the overall volume is less than the clean channel at 3/10.

    I am running out of ideas of where to look for a problem. Otherwise I’m gonna have to call time of death on the amp…
     
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  15. Gunner64

    Gunner64 Emotional Support Animal Gold Supporting Member

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    You have to figure out where you lose the signal on the gain channel.

    If I didn't have the tools to do that I'd take it to someone who does before I'd gut it.I mean you got it for free right? Myself I'd spend $100-$150 tech time to get it working. You could still sell it and make a decent profit at that price.

    It's not an MG..these amps have a serious following, destroying it seems a shame.
     
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  16. waltschwarzkopf

    waltschwarzkopf Member

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    Yeah, I got it for free in the end. That’s why I’m trying to fix it myself. The thing in Switzerland is that man/hoyrs are so expensive, that sometimes is not feasible to tale something to a tech. A friend had a Palme tube amp that stopped working, and he eas told that it would cost 170 bucks just to look at them and see what’s wrong. Sadly, he threw the amp away. I contacted that shop and they don’t even service SS amps.

    So, I have to solve it myself or gut it. I will look at the schematics and see where the issue could be. But since this is my first time repairing/modding an amp, I don’t have many ideas…
     
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  17. tschrama

    tschrama Well-Known Member

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    Really dont wanna see a 8040 in to the garbage bin, so I really want to help.

    The diagnose the preamp, it helps a lot if we know the DC voltaga around the active parts: opamps and tubes.

    A really handy way is to print out the schematic and note the voltages in that schematic and post a photo of that here. If any of the active parts are faulty, we will see that due to these voltages.
     
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  18. waltschwarzkopf

    waltschwarzkopf Member

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    Thanks for your help, I really appreciate it! I wouldn't throw it away, I'd most likely turn it into a cab for my G15RCD, but I see what you mean.

    I like the clean channel, so even if it doesn't work 100%, I still got a 40W clean amp, which is not a bad deal at all! Plus with the new speaker, it sounds even nicer.

    Back to the issue, I already had the circuit printed out, now I've identified 1x tube and 6x amps. I guess we can discard the main 50W OP amp (IC6 TDA1514), the OP amps on the normal channel (IC3 TL072, IC4 5201), and the reverb (IC5 1458) as these seem to work and be loud enough. The issue seems to be in the boost channel, where the tube (V1 ECC83) and the OP amps (IC1 5201, IC2 TL072) are located. The tube is brand new, but there might be an issue with the board/socket or the other amps. Is this what you mean?

    I have a multimeter and can hook one end the chassis' ground and use the other probe to carefully measure the voltage on each of the pins. I can do this tonight and post the values as soon as I have them. Hopefully they will make more sense to you than to me...
     
  19. Spanngitter

    Spanngitter Active Member

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    Before diving deep into the Amp and doing measurement and signal tracing I would first check all Poti Solder joints, mainly VR5, VR6, VR7, VR8, VR9.
    If there is no finding like a cold solder joint or a defective Poti then the following ICs are involved for the Lead Channel:
    IC1(A/B), IC2A, IC4B. Signal itself branches of at IC3A Pin1 and rejoins the (common with the clean channel) circuitry at IC4A Pin5 and routes thru FX Loop to the Power Amp.
     
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  20. waltschwarzkopf

    waltschwarzkopf Member

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    I already did that... I removed all ten potis, opened them to clean and rework the tracks. The measure them on both ends and with the wiper, then soldered them back in. This presented an improvement, but the core of the issue seems to be elsewhere.

    Great, so I guess I'll check those amps first.
     
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