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Marshall Valvestate 8080 fizz

Started by deadear, September 18, 2014, 01:51:57 PM

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deadear

Help!
I bought this amp cheap and have been playing it for three years. It would hardly run when I got it and I replaced all the pots. Later have replaced ic line chips #3,#5,#7.(a you tuber suggested this but did not help)

It has a lot of fizz and as you turn it up on volume and gain it gets worse. I use it at home to practice but would be no good on a gig.OD 2 is real bad and not even usable. It seems that pedals in front help to quiet it down a bit. Pull the instrument cable out and it sounds like a blizzard is going on.

Also if you run the guitar into the effects loop instead of the input jack and run the blend at full effect setting, it is as quiet as a mouse. So the problem is pre effect loop which is 95 % of the amp. I am not a tech just a little handy. Thanks for viewing.

http://www.drtube.com/schematics/marshall/pc0689.pdf

Enzo

If it is goo from the Fx return, then more like half the amp.  After that jack is IC6,7,8 before the power amp.

Let's go further.  Does switching channels ever kill the noise?  or just better and worse?

The channels have separate controls so:  The clean channel has a gain and tone controls.  Does THAT gain control affect the level of noise?  And do THOSE tone controls alter the tone of the noise?  The lead channel has a pre gain and post gain and four EQ controls.  In the lead channel do ALL of those affect the sound of the noise?    ie. do BOTH pre and post turn the noise up and down or only the post?

The two channels join in IC5,so if those controls have no effect, then IC5 is suspect, but if they do, then it is likely OK.

The lead channel also has the vacuum tube.  Pull that 12AX7 and see if the noise is gone.  Yes, there will no longer be guitar through the lead channel that way, but we are looking for noise now, not guitar.  (In my personal case, that is not much of a distinction)

If the tube pull stops the noise, then the source is before the tube or in the tube.  And any control that affects the sound of the noise is after the noise source.  Controls that do not affect the noise are either before the source or not in its path.

J M Fahey

Quick test.
Get a short piece of wire, say 8"/20cm .
Solder 1 end to ground, near the input jack.
With the other end touch momentarily the following points:

1) the node R34/R68/Led6/Led7.
Node means any point where all those parts are connected, including any component leg or the copper track or wire which joins all.
Since they are all connected, grounding one grounds all.

2) C43/R11/Sw3

3) IC1A-pin3/IC1B-pin5/R2/C1

Does any of them kill the noise?

deadear

#3
JM Fahey
I tried out your suggestions with a ground jumper. No difference in noise when doing this. Allthough on r2/c1 i got a big speaker crack but the fizz was still there.

Question; when looking at the schematic they have ic chip  6A and 6B. Are they refering to one side or the other of the ic 6 chip? There is only one ic 6 chip.


Enzo
Pulled tube and hiss was real quiet but the guitar volume was real low also. Switching channels does not kill hiss. More hiss the higher the gain or volume is set no matter what channel you use. Tone controls do not alter hiss but treble makes it higher pitch.

Enzo

Well think about it, if the treble control makes it sound higher pitch that IS altering the sound, isn;t it?  That means the noise is coming from before that control.

IC1A or B.  Look at the pin numbers on the schematic.  Pins 5,6,7 are section B and pins 1,2,3 are section A.

If pulling the tube substantially cuts the noise, it is either after the source or IS the source.  have you tried a different tube?




deadear

#5
Enzo. I have tried a different tube ,same deal. I will have to look at these chips again, 8 pin as i recall but I think I am missing something . Computer and amp are in two different places. Good point about the treble changing the pitch.  Thanks for help.

phatt

Some observations,

IC3b and IC5b are controlled by switching circuit TR1,2,3.

What about the switching control?  If faulty could be leaving OD mode engaged all the time.

I'll leave better minds to suss that out. :tu:

Also If the hiss is worse when you remove the cord then likely the ground switch is cactus inside the input socket, the second switch there also grounds the grid to shut up the hiss.

Keep in mind that quite a few hi gain Amps do tend to hiss even when working correctly and although this would not be noticed much in a gig situation, at home it would be pronounced and quite annoying.
Phil.

deadear

#7
Jack is good . Continuaty is good to ground points ,and the hot lead to C1 is good. I have a ohmn meter.

You tube post of a similar problem  " Marshall 8080 broken " will get you there if you are interested anyone. Mine is identical up to the 1 minute mark but not as loud.