Noisy mixer channels, please help

[paulrichards7]

So, I have noise on multiple channnels of my Studiomaster
Ive checked them individually connected to power supply, there are 2 that are fine which means that this is not a power supply problem
It seems to me to be white noise/static with some intermittent spikes
I have been using a crystal earpiece and checking from the tape/line input, which on my mixer is the unbalanced tape input marked on schematic(see pic) and the noise seems to be starting at the collector of the 1st transistor(see pic)
I have been advised to check caps, IC and diodes
All of these have been checked and/or replaced(see pic and yellow circles for those)
Unfortunately the noise still seems to be present
As I am kind of new to electronics I am relying on your guys experience
I know, I know, take a class. Well if there was one around my area I would. Unfortunately there isn't
Could you please help
Also, even though the schematic I have is for my mixer, things dont seem to be exactly correct leading up to 1st IC!
See pics for evidence
The middle pic of the multi 3 way view has been flipped to help me in locating components on the trace side
Thanks for your help
Paul

http://i390.photobucket.com/albums/oo345/paulrichards7/3-way2.jpg
http://i390.photobucket.com/albums/oo345/paulrichards7/Channel-2.jpg
http://i390.photobucket.com/albums/oo345/paulrichards7/Noisestartshere.jpg
http://i390.photobucket.com/albums/oo345/paulrichards7/Partsreplaced.jpg
http://i390.photobucket.com/albums/oo345/paulrichards7/Studiomasterchannelschem.jpg

 

[gswan]

Check the switches. Sometimes the contacts inside the switch can have corrosion and dirt on them which can lead to noise and crackles.

 

[rodabod]

Can you name which electrolytic caps you swapped? It looks like you left some.

 

[paulrichards7]

Thanks guys
Whats the best way of checking those switches to see if they are the problem?
Could you put a cicle around one of my pics to show me which other ones i should check
Thanks
Paul

 

[gswan]

Any switches that are in the signal path should be checked.
You can try exercising them several times to see if it helps reduce any noise or crackle.

 

[paulrichards7]

Hi Gswan, ive lubed them up multiple times
Is there a way to test with scope or should i just pull em out of circuit an dtest?
thanks
paul

 

[paulrichards7]

There seems to be 2 kinds of channels on ,my mixer
3 x which are screen printed and have all or partial components for the mixdown gold
and 13 which are not screen printed and are minus the balanced line input and the 1k2 resistor off of the phantom switch
Here is a revised schematic
http://i390.photobucket.com/albums/oo345/paulrichards7/Studiomasterchannelschemminuslinein.jpg
Paul

 

[gswan]

Hi Gswan, ive lubed them up multiple times
Is there a way to test with scope or should i just pull em out of circuit an dtest?
thanks
paul

If you've exercised and cleaned them and it makes no difference then they are unlikely to be the culprit.
Is the noise present with no input connected?
Have you checked the board for cracked or dry joints?

 

[radiance]

If you've exercised and cleaned them and it makes no difference then they are unlikely to be the culprit.
Is the noise present with no input connected?
Have you checked the board for cracked or dry joints?

Was thinking the exact same thing.
I had the same with my Sony mixer. Cleaned all contacts and the noise was gone.

 

[paulrichards7]

Hey gswan, yes there is noise with no input(meaning nothing plugged in)
Ive checked the board for breaks and dry joints
Nothing?
Thanks again
Paul

 

[gswan]

Did you check/replace the pair of transistors?

 

[paulrichards7]

Hey gswan
yes i did
paul

 

[paulrichards7]

This slipped by everyone
How many ICs do you see in this schematic?
http://i390.photobucket.com/albums/oo345/paulrichards7/Channelfulminuslineinputl.jpg
How many ICs do you see in this photo?
http://i390.photobucket.com/albums/oo345/paulrichards7/Channel-2.jpg
hummmmm
Paul

 

[radiance]

Two in schematic & two on photo right? The Tl072 is a dual opamp...

 

[Harpo]

2 dual op-amps.
Did you already pull these op-amps out of their socket and push them back in ?

 

[paulrichards7]

Dont worry, ive just found out that there is only 2 ICs in a dual mode
Thanks
Paul

 

[paulrichards7]

Hi
so the latest is that there is noise present when the channel is plugged into psu but channel on not engaged?
There is noise when the EQ section is bypassed, but when it is engaged it does effect the tone of the noise!
thanks
paul

 

[radiance]

Paul, can you mark in the schematic which parts have been replaced?

 

[paulrichards7]

hi radiance, thanks
i will, but i have to look at board first which will be in a few days

I dont know if this has any relevance to the noise issue
But if you look at th pic where the resistor is circled
http://i390.photobucket.com/albums/oo345/paulrichards7/Phantom-resistor.jpg
this resistor is not on the channels that are noisy!
If this is not an upgrade from earlier versions
Then what is the purpose of it?
Thanks
Paul

 

[gswan]

It's a soft turn-on resistor for phantom power, to prevent surges on the audio lines causing a pop.
I doubt that it has anything to do with the noise unless the noise is only appearing on channels with phantom power and when phantom power is switched on.