Valvestate 40v - Mostly no sound

Gen-Xer

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First post so bear with me. I was cruising FB Marketplace this morning and I saw someone selling a Valvestate 40v for parts only with a $50 price. I messaged and it was a local pawnshop. The owner said he suspected the Jack solder points. I can solder and I figured the 12” Celestion was worth the price so I took the plunge.

I took it home and everything lit up but no sound. If the reverb was turned up, you could hear a tremolo effect. I did some research, watched some vids and took it a part. I resoldered the Jack points but it didn’t work. I put everything back together, and did some more research. I found a video from a UK tech with a 40v on the bench. As he was going through the test, he put a cord in the power amp with the other end in the preamp. When he did it, the amp worked perfectly without the boost and it worked on boost but the volume was low. I tried it on mine and sure enough, I had the exact same result. His assessment was corrosion of the jacks. I’ve sprayed contact cleaner in the Jack inputs and worked the jack in and out of the inputs but I get nada. It seems like something is cutting the sound kind of like how a headphone jack will cut the volume to the speaker. Any thoughts? Also thank you in advance.
 

SkyMonkey

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Hello @Gen-Xer and welcome to the MF :welcome:

As there is no Effects mix knob on the 40v, I assume the loop is series.
Often, the FX Send/Return (Preamp out/Power Amp in) on these type of amps are like so:

With nothing plugged in, the signal is passed internally from the preamp to the power amp.

The Send is a splitter, which allows you to send a preamp signal to another amp's power amp whilst the internal signal still goes to the power amp.
This means you can set up a dual mono rig with a 2nd amp.

The Return jack may be a switching jack that cuts off the internal signal when a lead is plugged in.
This stops the Preamp from sending a parallel signal to the power amp, which can cause phase cancellation issues with Time Based Effects (TBEs) in the loop.

So, the Return jack could be at fault, and the jack is cutting the internal signal (without a lead plugged in).

Can you test the Pre Out and Power In jacks to isolate a possible problem?

If you have access to another amp with an FX Loop, connect the Preamp Out from the 40v to the FX Return of the other amp.
If the other amp gets a signal then the 40v preamp is fine.

Can you competently open up the 40v to pull the chassis safely?
If so, have a look at the Power Amp in jack for signs of bent or highly corroded contacts causing an unwanted permanent connection.
If the video you watched shows the open chassis of the amp, freeze frame it and compare it to yours.

You may have to replace the Power Amp in jack.

Have a look at this thread: https://www.marshallforum.com/threads/jacks-for-8040-valvestate.119863/
 
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Gen-Xer

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Hello @Gen-Xer and welcome to the MF :welcome:

As there is no Effects mix knob on the 40v, I assume the loop is series.
Often, the FX Send/Return (Preamp out/Power Amp in) on these type of amps are like so:

With nothing plugged in, the signal is passed internally from the preamp to the power amp.

The Send is a splitter, which allows you to send a preamp signal to another amp's power amp whilst the internal signal still goes to the power amp.
This means you can set up a dual mono rig with a 2nd amp.

The Return jack may be a switching jack that cuts off the internal signal when a lead is plugged in.
This stops the Preamp from sending a parallel signal to the power amp, which can cause phase cancellation issues with Time Based Effects (TBEs) in the loop.

So, the Return jack could be at fault, and the jack is cutting the internal signal (without a lead plugged in).

Can you test the Pre Out and Power In jacks to isolate a possible problem?

If you have access to another amp with an FX Loop, connect the Preamp Out from the 40v to the FX Return of the other amp.
If the other amp gets a signal then the 40v preamp is fine.

Can you competently open up the 40v to pull the chassis safely?
If so, have a look at the Power Amp in jack for signs of bent or highly corroded contacts causing an unwanted permanent connection.
If the video you watched shows the open chassis of the amp, freeze frame it and compare it to yours.

You may have to replace the Power Amp in jack.

Have a look at this thread:
Thank you. I had the chassis open yesterday to
 

Gen-Xer

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Okay, a little closer today. I ran my little Fender amp into the Marshall. That worked perfectly. Next, I pulled the chassis and took a look at the soldering joints. Some seemed to not have any solder on them so I freshened them up. The foot switch jack had a weird glob that looked like someone had tried to resolder along with a weird yellowish mark near the glob. I cleaned it up a bit but since it didn’t appear to be the issue, I moved on for fear of making it an issue. I also shot the pots with contact cleaner and gave the jacks a dose from the backside. I still couldn’t get sound so I decided to put it back together figuring I may just need to replace some jacks.

Once I got it back together, I was able to get sound by playing with the power in jack. At first I used a cord but I found that if I put a screwdriver in and wiggled it a little bit that I would be able to play fine on the normal channel. However, you have to repeat the process every time you turn the amp back on. It leads me to believe that my next step is to replace the power amp and possibly the preamp jacks to see if that doesn’t resolve the issue.

The next issue is that if it is switched to boost, the sound is greatly diminished. I can move gain, amp volume, and guitar volume all the way to ten but it sounds like everything is at 1. Maybe the tube? At first, I thought the boost issue was related but now I’m not so sure. I am going to order a couple of jacks from the website linked in the other thread. Once they arrive, if I haven’t figured out the boost issue, I may swap another tube in to see what It does. Thanks again.
 

Gen-Xer

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Fixed! I had two issues. After messing around with it, I felt pretty sure the issue was with the “power amp in.” I replaced the jack today and finally had sound. The second issue was that the boost was low with everything turned all the way up. Decided to go simple and replace the tube with a cheap Chinese 12ax7 I had in my parts drawer. Suddenly I had the boost channel back. I’m going to swap in a better preamp tube but all in for price and parts, I have a working 8040 for less than $100. Thanks everyone for all the help.
 


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