Marshall 8040 DC on pot? anyone help as Im having a really bad day!

[g1]

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]

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]

Hi Allante.
You *need* a scope to check some things or weŽll 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 weŽll 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 donŽt 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 itŽs better than nothing.


EDIT: damn Photobucket.
again:

[allante666]

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]

Nor you need it
Problem seems to be solved