allante666 10/9/2017 11:05 AM
Marshall 8040 DC on pot? anyone help as Im having a really bad day!
Anyone point me towards the possible culprit? Had a Valvestate 8040 which worked but hissed so loud it sounded like a tsunami. Plugging direct into the fx return it was perfect. As it was doing it on th eclean channel as well I figured that either IC3 or IC4 had gone noisy so I replaced IC3. Success, or so I thought until I put it back together and reconnected its proper speaker. Because I was using teh shop speaker which is 8 ohm instead of the amps 4 ohm and was a distance away I didnt hear what I heard after i put it together. It was like a motorboat but not varying in frequency with anything, probably running about 1500rpm! Only thing I found which altered it was if I turned ch1 treble all the way down it got louder. mid range I could hardly hear it and full up I coudl hear it but not as loud as with it right down. As I pulled and pushed on the pot shaft I got a few dodgy noises so to be sure replaced the pot (VR4). With the brand new pot in I got crackling as I turned it like DC on it? I did try to measure it and metered between the wiper and chassis but that overheated R66 in the speaker. Only briefly, no harm done.
I know Im being lazy here but Ive had a really crap day and am hoping someone takes pity on me! Anyone suggest a solution? Ill try post the schematic. Thanks.

g1 10/9/2017 11:12 AM
Do you have all the pots, jacks, and grounds tightened up?
allante666 10/9/2017 11:54 AM
Yep, all the nuts are on the pots, earth wire on chassis etc.
Mick Bailey 10/9/2017 2:02 PM
Turning the treble down makes the noise louder because it's getting filtered by the tonestack and appearing on the bass side of the treble pot and getting blocked by C2 and most likely the problem is happening before the tone stack. If the amp didn't make this particular noise before you changed the opamp, inspect the board for solder bridges or a bad joint. If you installed a socket pull the chip and see if it stops. There's no path for DC to appear on VR4 unless a cap is faulty. Bear in mind that pots can be noisy from new.
allante666 10/9/2017 2:51 PM
No way of knowing if it made the noise before as the noise it did make would have drowned this out. This noise doesnt increase with volume it stays the same volume regardless which is fairly low. Do you have any idea why R66 would overheat when I tried measuring for dc on the pot wiper?

And I suggested dc on the pot as I was going by the crackle it made when I turned it, its kind of a different crackle to a dirty pot.
Mick Bailey 10/9/2017 4:06 PM
R66 forms a Zobel network with C54. Maybe your probe is causing HF oscillation that's getting amplified by the power amp IC. Try a dummy plug in the FX return and see if it still does it.
allante666 10/10/2017 4:05 AM
Ill try that next time I look at it. Im not missing something and doing something wrong by measuring for dc between chassis and wiper am I?Would I be better off measuring for dc between wiper and track?
J M Fahey 10/10/2017 7:19 AM
You have no DC at the pot.
You amp is unstable and randomly oscillating, you must have lost a ground, to be determined.
The crackling you hear when moving the pot does not mean DC on it (that's why you don't find any ) but randomly getting in/out of oscillation; frequency is too high to be heard by itself, but sudden random start-stop sounds similar to crackling.
Further proof of oscillation is smoking the Zobel network.
On a borrowed computer now, mine died, but when I get back normally (meaning I buy a new one and transplant the old hard disk there) Iīll suggest some tests.
That said, be aware that grounding/oscillation/instability problems are hard to catch.
Mick Bailey 10/10/2017 7:28 AM
Did you double-check the solder joints on your replaced opamp?
allante666 10/10/2017 8:08 AM
But it only smoked when I touched the meter probes to the wiper/chassis, prior to that it was fine? the resistor I mean. wouldnt it have been smoking from the start? and Mick, yes i did plus the fact that the first opamp I put in was a secondhand one which cured 95% of the noise so just to be sure I then replaced that one with a new one. both same result. In a gig situation you wouldnt even hear it unless you went right up to it as it doesnt get any louder with the volume but thats not the point of course!
Mick Bailey 10/10/2017 11:39 AM
Touching the meter probe on the wiper is what can introduce the instability if you have a fault situation. Just like some tube amps can go into destructive ultrasonic oscillation by probing the screen, plate or sometimes the PI. HF has to be generated in the preamp stages and amplified by the power amp to cook that resistor. Bear in mind that once the frequency is high enough the reactance of the Zobel cap drops and this leaves the resistor to take more voltage than it was designed for.
allante666 10/10/2017 11:55 AM
I understand that touching the probe to the wiper may have started the whole process causing it to oscillate and thats fine, but the noise was there before I did that. Im not just meaning the crackly pot I mean the motorboat at cruising speed noise. Im thinking the resistor thing was just somethingn that I caused myself and is unrelated?
J M Fahey 10/10/2017 1:27 PM
Quote Originally Posted by allante666 View Post
I understand that touching the probe to the wiper may have started the whole process causing it to oscillate and thats fine, but the noise was there before I did that. Im not just meaning the crackly pot I mean the motorboat at cruising speed noise. Im thinking the resistor thing was just somethingn that I caused myself and is unrelated?
ALL those can be explained by bad grounding.
The "resistor thing" is a symptom and not a cause.
allante666 10/10/2017 3:33 PM
OK, I understand that. So, let mem take you back to the start and tell you what i did and why then you tell me if I took the wrong path. It all worked except that it had a loud hiss/white noise on it. This would come on immediately on switch on, before the tube had even warmed up. I plugged direct into the fx return and it was perfect, good tones and no hiss or noise. I took this to mean the power amp section was ok. Because it came on straight away I took this to mean that the problem lie in the clean channel as the od channel needed the valve to warm up first. Looking at the diagram I figured the clean channel preamp stage only really consisted of IC3 and IC4. I changed IC3. This almost cured it. So much so that I thought it had an dI put it back together. It was only when I connected its proper 4 ohm speaker which was right in front of me that I heard the other noise. I hadnt heard it before as I was plugged into a 8 ohm speaker (so it wasnt quite as loud) and it was a way away from where I was. thats how low this 'motor boat' sound is. I then figured I would change IC4 just to be on the safe side but it made no difference. I then found that by turning the clean treble all the way down it made it a little louder and turning it to mid way made it all but disappear. As there was a few clicks and crackles in this pot when i pulled on the shaft and it was quite loose to turn I fitted a new pot. Thats when I noticed the crackling as I turned it which led me to think it was dc. That brings us to the present. The only other thing ihave found that slightly alters the volume of the sound is the reverb pot, turnong it fully up makes it a tad louder, thats with or without the tank connected. again, the main volume controls have no effect on its volume at all. So, am I going in the right direction do you think or have I gone astray? The fact that IC3 cured the initial noise by a good 98% makes me wonder if there were two faults and this motor boat is the other one?
J M Fahey 10/10/2017 6:07 PM
Hard to tell.
You did nothing "wrong" and replacing that Op Amp was probably a good thing, replacing a bad/flaky pot too, but as I told you grounding/instability problems are pesky, thereīs nothing obviously at fault.
Some of us *might* solve it having it on the bench, or might eventually crash it against a wall in frustration , nothing is guaranteed.
Nobody can tell you by "remote control" do this or do that and it will be solved, besides generic: tighten pot and jack nuts, check connectors, move wiring a little one way or the other, you may find a "clean spot".

Just curious: are you still using the original insulated plastic jack?

More important: can you get a scope?
Thatīs the most useful tool to find oscillations and instability.
g1 10/10/2017 6:12 PM
Also, are there any screws holding down the circuit board that may be serving as board grounds?
Did you use exact replacements for IC3 and 4? IC4 is a 5201 which are no longer available, if you found one it could be fake.
Motorboating is low frequency oscillation, the crackling and fried resistor are from high frequency oscillation. It's possible the LF and HF oscillation are caused by the same fault, but just changing due to settings or something.
Mick Bailey 10/11/2017 1:44 AM
Here's how I would try to narrow things down; Firstly, check the +/- supply rails are correct and there's no modulating AC on them. Then insert a dummy plug into the FX return to cut off any issues that could affect the power amp. For good measure remove the tube. Now you have isolated the clean preamp as far as you can without removing components.

Is there any voltage (AC or DC) on the FX send? Rotate the preamp controls to see if something happens. Now, do you read any AC or DC on the opamp outputs?

For me if I got this far without seeing any abnormal voltages I'd patch the FX send into my bench amp and see if I got a clean signal and I could play a guitar through it. I would have scoped it to make sure there was no HF on the output, though.
J M Fahey 10/11/2017 3:40 AM
Hi Allante.
You *need* a scope to check some things or weīll be running in circles forever.
Do you have or can get one?
If possible, then please say so.
If not, I can suggest you a cheap/simple adapter and software so you can use a notebook as a virtual scope.
It has 2 problems though: although a useful better than nothing tool, it saved my skin when being on tour with some band or simply far away from home, even in a hotel room in another Country, problems which might diminish usefulness here are:
1) it does not see or work on DC, since it uses the computer Soundcard it shares its limitations: AC only, and that above 20 Hz so a tremolo speed motorboating will still be seen, somewhat, but weaker than actual value
2) it will see nothing above 20 kHz either, and I am quite sure your oscillation is way above that , but I guess weīll see the test waveform with spikes or fuzzy areas where the actual oscillation burst is : you move the pot wiper and see the image jump.
Now if the amp is oscillating continuously at, say, 40kHz, you will not see the oscillation waveform itself , not even blurry, soundcard antialiasing filters **kill** anything above 20 kHz.
Yet, if you can *hear* something annoying through the speaker, you will see "something" abnormal on screen.

You probably have something like this:
notice the small narrow oscillation waveform, above hearing range so you donīt hear *it* , , BUT it is not continuous, it starts and stops in "bursts".
You hear the start/stop as "thump thump thump thump ..... "
A real scope will show what you see here; I guess a narrow band software simulation will show just rhythmic peaks or jumps, but itīs better than nothing.

EDIT: damn Photobucket.
allante666 10/11/2017 9:45 AM
Well, many thanks to all who have given advice it really is appreciated. I followed the advice from Mick above and found no voltages etc either from the fx out or the opamp outputs. I did find while doing this that the crackly pot actually only crackled if i turned it with my fingers without its knob. knob on and it was silent. I began to wonder abouth the mains earthing as I was running through a current limiter bulb which had quite a long lead from limiter to wall and another long lead from limiter to amp. I tried plugging in the lead directly and this reduced the noise by about 60%. Tried a different mains lead and voila! noise all but gone! Its still there barely but only if you turn the treble fully down and literally put your ear to the speaker. treble above 1 and no noise whatsoever. That'll do for me!

And no Juan, I dont have a scope unfortunately.
J M Fahey 10/11/2017 1:38 PM
Nor you need it
Problem seems to be solved