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

[waltschwarzkopf]

Hello, World!

I few years back I saw a used Marshall Valvestate 8040 nearby for a good price, but when I contacted the owner he decided to give it to his brother... Fast forward to this month and I saw the same model elsewhere for a good price, since I always wanted to try it and drove to pick it up.

The ad said "the pots scratch a bit, but otherwise works perfectly", he wanted 50 bucks for the amp, so I thought it was ok. I tried it and ALL the pots were very scratchy, some with dead spots and the volume was coming and going. He said that it had been stored for two years with no use, but his brother (an electrical engineer) had said that the pots needed cleaning and it would be as good as new. He also reduced the price to 20 bucks on the spot. I reluctantly accepted the offer, as I had driven for almost two hours and didn't want to come home empty handed.

I tried again at home and no change, of course, I had only changed it's location. Next day, I borrowed some electrical contact cleaner and sprayed all the pots from behind, this meant removing the metal casing holding the electronics from the box. I turned them a few times, let them dry and then cleaned again. I also sprayed and cleaned the switches and the jacks. I also cleaned the reverb tank and contacts.

When everything was dry I assembled it and tried again. Surprise, the cleanchanel works! Although the treble pot scratches a bit, I now could adjust gain, treble, mid and bass. The reverb works too! The amp is pretty loud, so I never went over 5 on the volume, imagine if it would go to 11! I tried the boost channel and it did not work. I mean, there is a faint distorted sound coming out of the speaker as I play, so something is working inside. Contour and bass knobs seem to do something, but the gain, volume and treble don't, the channel seems to be stuck on a high-gain low-volume setting.

I am very disappointed that my new-to-me amp is not working, but I am keen on looking for the problem and solving it!

This is what I've tried so far:

• Check speaker contacts (ws fine)
• Clean all pots, switches and jacks (better, but still not ok)
• Bridge the pre- and power-amp jacks with a patch cable (this is a known issue, but no change for me)

This is what I will try next

• Blow some air into the pots to remove any remaining crap from the inside
• Check the solder joints
• Re-flow pot solder joints carefully
• Any ideas?

A few additional thoughts:

• After a quick look at the board, nothing seems broken or burnt to my (un)experienced eye, but I'm attaching a few pics in case some of you see something that I have missed.
• I don't think that the tiny 12AX7 tube is problem, as it works only for the clean channel, and that seems to be ok now.
• I've seen new pot sets online, but they sell for a lot compared to what I've paid and to a compareble new amp, so I won't go that route.
• I don't feel that I'm at a total loss, just bummed out. In the worst case, I could sell the parts and recover my "investment". Or just rewire the speaker and use it as a cabinet for other amps.

It's just unfortunate that this is my only amp at the moment, but I'll try to find something that can be combined. I had a tiny terror with the 108 cab that sucked, so it may sound better with a 12". The same goes for my older G15R CD that was a 1x8 combo, nice sounds, but small speaker. I sold it and then bought the same model again later, and then sold it too. I may find it again to try it with the 12" celestion of the 8040.

What do you think?
Any advise will be greatly appreciated!

I'll keep you posted!

Cheers,
Walt

 

[AdrianSD]

Very frustrating, high hopes and crappy end result Try marshallforum. Best of luck.

 

[waltschwarzkopf]

Very frustrating, high hopes and crappy end result Try marshallforum. Best of luck.

Yeah, a real bummer. Hopefully I'll be able to fix it with the help of the guys/gals from here.

 

[blaise.brown.7967]

Did you try the channel switch?

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[waltschwarzkopf]

Did you try the channel switch?

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What do you mean by this?

I cleaned the channel button and also try switching with a pedal. It seems that the "normal channel" is more or less ok, but the "boost channel" is the problem.

 

[idsnowdog]

Pull the main board and look at the solder joints with a magnifying glass. Specifically the inputs and pots. Since they are mounted to the board and not to the front panel they absorb pressure which can weaken the solder joints. It's not uncommon to see fractured solder joints that can be repaired by reflowing the solder.

 

[waltschwarzkopf]

Pull the main board and look at the solder joints with a magnifying glass. Specifically the inputs and pots. Since they are mounted to the board and not to the front panel they absorb pressure which can weaken the solder joints. It's not uncommon to see fractured solder joints that can be repaired by reflowing the solder.

Great idea. I did that yesterday and some pots seem more responsive now, but more are still not. I will try opening the pots to inspect the track and possibly clean them further.

Would it be a good idea to bridge each pot with a wire (like setting them to 10) justo to see if the rest of the circuit is fine?


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[waltschwarzkopf]

Quick update. I contacted the seller, and he was sorry that he sold me a defective amp and reimbursed me the 20 bucks I spent, so only my time and effort were lost. I guess that if I manage to bring it back to life, I'd have a free working amp.

 

[idsnowdog]

Great idea. I did that yesterday and some pots seem more responsive now, but more are still not. I will try opening the pots to inspect the track and possibly clean them further.

Would it be a good idea to bridge each pot with a wire (like setting them to 10) justo to see if the rest of the circuit is fine?


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Like in the video see if any of those large resistors are loose. The problem with those is that as they heat/cool they become loose over time. If an individual pot is bad you can remove the pot and replace it with a standard pot instead of a PCB mounted pot and just run the wire to the board while mounting the pot to the front plate.

https://www.youtube.com/watch?v=bHKuASure74

https://www.youtube.com/watch?v=ogWEhw7stuk

 

[waltschwarzkopf]

Like in the video see if any of those large resistors are loose. The problem with those is that as they heat/cool they become loose over time. If an individual pot is bad you can remove the pot and replace it with a standard pot instead of a PCB mounted pot and just run the wire to the board while mounting the pot to the front plate.

https://www.youtube.com/watch?v=bHKuASure74

https://www.youtube.com/watch?v=ogWEhw7stuk

Will try that too. But first will solve the obvious problem with the potis. Then I will move onto the resistors, although on a first look nothing seemed loose or broken, but I will re-check carefully. Last, I will replace the tube with a new one. I saw a Sovtek 12 AX7 AW that I might order along with the parts for my guitar project.


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[waltschwarzkopf]

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.

 

[Aceman]

OK - that amp - for $20 - now "free". is easily worth $100 fixing it.

And it seems like you have the skills to get it close to right...so go for it.

My opinion, between hot/cold storage and perhaps being dropped (even if it doesn't show) a lot of stuff got loose/cracked etc. But I'm not an amp repair guy.

Also - I'm pretty sure that little tube should be on when the dirt channel runs too!

Worst case - take it to an amp guy for repair. Tell him you have $100 if he can make it go - all or nothing.

 

[JB_From_Hell]

I was gonna ask what type of cleaner you used on the pots and switches. If it wasn’t Deoxit, use that, and spin the hell out of the pots after applying it. They make one that lubes pots and switches after cleaning, so I would use that too.

12AX7s can be had pretty cheap, so a new one is definitely in order.

 

[waltschwarzkopf]

I was gonna ask what type of cleaner you used on the pots and switches. If it wasn’t Deoxit, use that, and spin the hell out of the pots after applying it. They make one that lubes pots and switches after cleaning, so I would use that too.

12AX7s can be had pretty cheap, so a new one is definitely in order.

I used the spray cleaner that we have at the lab, it is specially made for electronics and contacts. I also reworked the tracks with a graphite pencil and spun the post several times. Then I measured resistance to make sure that they were contiinuous and within the specified values.

I will install a new tube too, but now the problem seems to be the heating of the tube. The dark mark on the board is in the place of R62 and R63, which are the resistors that heat up the tube. Also, someone pointed out that these resistors should be identical, but in my amp are different, meaning that someone may have changed them before...

 

[ErikH]

Those spots definitely look like burn marks. If my memory serves me correctly, the clean channel uses one half of the 12AX7 and the dirty channel adds in the second half or uses the second half by itself. If there is an issue there with the traces, that will cause the problem you are seeing. Take a DMM and see if there is continuity between all start/end points around those traces. If a trace is burned, use a jumper wire from the start to the end to restore connectivity between the two points.

 

[Securb]

Worst case - take it to an amp guy for repair. Tell him you have $100 if he can make it go - all or nothing.

Agreed. Very worse case you have a free 1x12 extension cab.

 

[waltschwarzkopf]

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?

 

[blaise.brown.7967]

What do you mean by this?

I cleaned the channel button and also try switching with a pedal. It seems that the "normal channel" is more or less ok, but the "boost channel" is the problem.

I have had several amps with the channel selector switch being the fault of only one channel being operable. Replaced them and voila, made music on both channels.

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[devastone]

Should it be brighter?

That looks right, some preamp tubes you can't even see the heaters. If it lights up like a light bulb there is probably something wrong.

 

[ErikH]

That looks right, some preamp tubes you can't even see the heaters. If it lights up like a light bulb there is probably something wrong.

Agreed.

At this point, it might be necessary to take it to a tech.


Wait, are those LED clippers in there? (seeing them to the left in the pic of the tube)