Marshall Valvestate 8040 Quiet Boost Channel

[chilidawg]

Hi, I'm new here. Just registered to get some help in repairing this amplifier.

Schematic https://www.audioservicemanuals.com/...tate-schematic

It came in with a power switch problem which I already fixed. And while I was doing that I saw the burnt out tracks under 68 ohm 3W resistors that are connected to the tube heaters.

Repaired the tracks also and replaced the resistors with ceramic 5W ones.

Turned on the amp, noticed the tube glowing dimly, and only the Clean channel is working fine.



In Boost channel with the pots turned to max, I could hear a very faint distortion sound when I played my guitar, so faint it's almost like it's not there. Tried recording the line output but no guitar sound, only the noise floor.

I have read another thread here by trevor when he was repairing his VS 8080, and I think I may have the same problem, that the tube is not getting the high DC voltage supply.

At the moment, I only have the board on my bench. I left the main transformer and the tube at the owner's place. It was cumbersome to hand carry it with the metal chassis while taking a ride in a busy subway. Didn't want accident to happen to the tube.

Anyway, I have checked D3 and the 220 ohm resistor in series with it. They're ok.

I have a 2A toroid power transformer with a secondary center tap and dual 18VAC taps. Can I use it to power up the board for a brief moment just to check if I get a somewhat high DC voltage at D3?

Thanks in advance!

[g1]

Did you check R54?
They don't show voltages for the main +/- supplies. What are the voltage ratings on the C50 and C53 caps?

[chilidawg]

The clean channel is working. The power amp is working.

Only the boost channel is not.

I have already measured the values of every passive components and they are correct. No leaky or high ESR electrolytics.

Originally I thought the tube was glowing dimly because the tube plates were not getting high DC voltage supply and supected the auto transformer (T5899) maybe dead, but I just tested it by supplying 10V AC at input, and I get 80V AC at the output and 40V DC at D3 cathode all the way to the tube socket HT pins, so this rules out the auto transformer being faulty.

I'm putting the repair on hold until I get can bring home the main power transformer and the tube which is sometime around next week.

[g1]

The reason I asked for the voltage rating of those caps was to see whether it was safe for you to test with the toroid. It seems you have found a way around that and the HV transformer is good.
Make sure you have healthy signal coming out of the 5201 IC (pin5) that feeds the tube.

[chilidawg]

Hey thanks! Now that you mentioned it, when I was at the amp owner's place, I seemed to recall that the two clipping red LEDs weren't lighting up even tho the boost channel indicator LED was, all the control pots were maxed out and I was strumming his guitar like crazy.

I don't have an electric guitar that I can use to test, but I can inject a tone signal at the 5201 opamp input and check if it is also present at the output.

[g1]

If it does turn out to be a bad 5201 IC, watch out for fakes.

[chilidawg]

If I add a jumper wire like this, it should momentarily bypass the tube and the tone controls, right?

If it does turn out to be a bad 5201 IC, watch out for fakes.

I have NJM2120D which is a direct replacement.

[DrGonz78]

What connection are you cutting? If you jump something you have to show what you are bypassing. Just adding that one wire as illustrated would just parallel the two paths and might cause issues. Imagine lifting the grid side of R33 and it floats up in the air to a jumper wire. That interrupts the signal path from entering the tube grid and it then can enter the next opamp stage. This would work better having R33 between stages of the signal path.

Edit: I would think that a quick trace would show where the signal is cutting out. I was assuming you wanted to eliminate the tube from the circuit. I am sure the way you drew it out will work. I think I just read your post and sort of reacted to just that one post. Sorry to stray.

[chilidawg]

Amp is repaired. Thanks for the help!!

[g1]

Bypass is not really a repair, but I guess it works and you are happy with it.
If you have done something else to actually repair the circuit, please let us know what you did.

[chilidawg]

The bypass was only temporary. It was to help me check if signal passes thru the rest of the boost channel circuit all the way to output without having a tube in the socket because I left it at the amp owner's place.

I wouldn't consider this a repair at all because there is nothing wrong with the board. I guess after I reflowed every solder points, whatever previous bad/dodgy connection is now gone.

On a side note, I do have a question. Is it ok to replace the 22K pots with 20K and 220K pots with 250K, or do I need to modify the circuit in which they are situated to accomodate the change?

[g1]

On a side note, I do have a question. Is it ok to replace the 22K pots with 20K and 220K pots with 250K, or do I need to modify the circuit in which they are situated to accomodate the change?

Those values are close enough. +/-20% is fine for pots.

[chilidawg]

Ok, here is something weird.

I had the whole thing assembled, tube was in the socket, switched to boost channel and... nothing. Same dimly lit tube.

In DC voltage mode, I probed the 1N4007 cathode, there's only 0.2V all the way to the HT pin but it kept climbing at a very very very slow rate, like 10mV increase every minute.

Switched to AC voltage mode, 126V at the anode, and while the probe was still on it, suddenly the boost channel came to life, and when I removed the probe, it went back to silent.

I repeat the voltage measuring process, and found that if I touch the probe at the anode for a few minutes, the DC voltage at cathode will increase exponentially, it gets really high and the tube turns on.

I spent a whole hour trying to figure out what is wrong. There are only 5 components and none of them is bad. Traces have continuity.

I'm at lost. In the end, owner decided to bypass the tube entirely and went full solid state. But I really want to know, if you guys have any ideas/opinions why it is behaving like this?

[Helmholtz]

I didn't read all, but did you check heater voltage and supply?
Plate voltage has no effect on heater brightness.

What do you mean with AC mode and DC mode?

[g1]

There are only 5 components and none of them is bad. Traces have continuity.

You can't really 'prove' the diode D3 is not bad. It can measure fine with your diode check function, yet still be intermittent in circuit. Or the traces measure continuity but some solder joint will not pass more than the tiny current your meter tests with. So either the diode or it's connections seem to be responsible.