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

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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Aceman said:
This is very exciting! I love my VS100R. If/When it goes south good to know it can be revived!
Click to expand...

I had to replace the power transistors on both my 8100 and 8004. As long as you can do the work yourself, they're easy to fix. The problem comes when you have to pay labor, that makes it less viable pretty quickly.
 
richard parker said:
I'm pretty certain the clean channel on this amp doesn't use the tube.
Click to expand...
It doesn't, the original Marshall amp manual says that the ECC83 is used only on the boost channel.

richard parker said:
I remember reading someone claim that the dirt channel still worked when they took the tube out.
Click to expand...
I read exactly the same comment! Was it the gear page or harmony central? Anyway, this got me confused too, but after reading the manual, everything was clear.
 
richard parker said:
I'm pretty certain the clean channel on this amp doesn't use the tube. I remember reading someone claim that the dirt channel still worked when they took the tube out.
Click to expand...

BTW, I tried this last night, and the Boost channel does not work without a tube; the normal channel did work fine.


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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...
 
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!
 
waltschwarzkopf said:
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.
Click to expand...

Looks to me like that resistor behind the 2 central Electrolytic Caps took one for the team.
 
NegativeEase said:
Just be mindful that a blown resistor is often a symptom of another component having issues -like a dying cap or that 12AX7
Click to expand...

Good to know! I think that the resistor may have damaged the tube. Although when I replaced them, the resitance values were ok, and the PCB tracks seemed to be continuous too. I will try again tonight with the new tube, and if it still does not work, I will recheck all the tracks in the board.
 
waltschwarzkopf said:
BTW, I tried this last night, and the Boost channel does not work without a tube; the normal channel did work fine.


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Click to expand...

Good to know. My Vox AV30 works differently. I had thought some of the valvestates worked the same way.
 
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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As a further step, I measures the voltage on all altice components, that is OP amps and the tube. Can anyone make sense of the values?

voltages.png
 
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