Noisy mixer channels, please help

[paulrichards7]

hey gswan
thanks for that
no the phantom power isnt switched on on those channels that have noise
but thanks for telling me what it does
more information added to my learnings
thanks
paul

 

[paulrichards7]

So i changed all the electrolytic caps last night from a good channel and the noise is still there
noise sample:
http://www.zshare.net/audio/542785911368e7e2/
any last advice before i ditch it?
thanks for all your help and advice
Paul

 

[1954U1]

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

There is always the case in which you have a noisy psu and 2 different type of channels,
one good in noise rejection/shielding and the other less good.

I'd quickly check with another psu, just to cover all the possibilities.

 

[paulrichards7]

thanks
though unfortunately i dont have another one
paul

 

[gswan]

Thanks for the audio sample. It's not psu noise.
Does the noise change when you hit the tape switch?
What happens when you move the gain control?
What were the settings for the sample?
The 2-transistor LTP input circuit is not very good, there are much better topologies for this, however I think the noise is coming from around here.
What did you replace the transistors with? Exact values or equivalents?
Do all channels have the same transistors?

 

[paulrichards7]

Hey gswan
i dont have the module in front of me right now, so i cant say about tape switch off the top of my mind
the gain switch effect the noise
sample settings? do you mean how loud is the noise?
i replaced the transistors from one of the working channels
all transistors have the same transistors 2SA970
thanks for your help
paul

 

[gswan]

Hey gswan
i dont have the module in front of me right now, so i cant say about tape switch off the top of my mind
the gain switch effect the noise
sample settings? do you mean how loud is the noise?
i replaced the transistors from one of the working channels
all transistors have the same transistors 2SA970
thanks for your help
paul

I meant what were the mixer channel settings when you took the sample. What was the input gain set to, tape, phantom, eq switches etc. That way we can eliminate various things from the cause.

Is the noise able to be sampled at the insert output with gain set to max, eq out, tape switch set to mic input, phantom off and the input socket shorted (ie all pins connected together to remove any external effects).

This puts only a small handful of components in the signal chain that can be traced out. Did you test/replace the 33uF supply decoupling caps around the transistor LTP section? Check that they are inserted the correct way if you did. If not, check their ESR or replace them.

 

[peranders]

So i changed all the electrolytic caps last night from a good channel and the noise is still there
noise sample:
http://www.zshare.net/audio/542785911368e7e2/
any last advice before i ditch it?
thanks for all your help and advice
Paul

This type of noise did I have in a preamp with old LF357 opamps. I replaced them and the noise disappeared. Sound like popcorn noise = coming from semiconductors. It was very helpful to listen to your problems.

 

[paulrichards7]

Hey gswan,
thanks fro that
tomorrow i will record noise the way you suggested and post
im at work right now
thanks for all your help though

Ive added some pics of difference:
1. Components for line input, but only on this channel not the other 2 good channels
2. Capacitor has its own track on quiet modules, but on noisy modules it is soldered on back of pcb for mid eq freq
3 Phantom resistor on good modules, non existent on bad modules
4. Larger cap 100v on good modules, 63v(i think) on bad modules
5. Different resistors(metal, i think) on good modules, normal on bad modules
6. Different bridge(not sure of correct term)wires on the 2 kinds of modules

peranders
thanks for your post
glad to help

I hope this helps
thanks
paul

 

[gswan]

Paul,

If the 'noisy' channels used to work OK, then it's unlikely that the component values or whether they are fitted are the cause. On the other hand, if it was always noisy then there may be a problem.

I forgot to ask, you did replace the TL072 devices near the input transistor pair, didn't you?

Geoff

 

[Harpo]

I forgot to ask, you did replace the TL072 devices near the input transistor pair, didn't you?

or pull them out and back in?. The socket might be oxydated over the last ~20 years and this pull out and back in procedure might cure this.
Next question: Did you already check or have replaced the two 1N4148 Diodes. Increased leakage current might be this noise source.

 

[paulrichards7]

hi guys
i have replaced the ICs, but didnt think about oxidation
i pulled the diodes and checked them for continuity with a DMM
they seemed ok
thanks
paul

 

[AudioJohn]

Lots of good advice contained in the  messages above - but just how bad is the noise on the 'faulty' channels? Is it high enough to show on the output LED bargraph meters?

Check the DC voltages around the input transistors and output pins 1 and 7 of the IC and compare them with a good channel - this might give a clue.

Leaky diodes have been mentioned - this is indeed often a problem - normal meter testing often doesn't indicate a problem, unless it is really bad - so just replace them with new diodes.

 

[paulrichards7]

gswan question:Is the noise able to be sampled at the insert output with gain set to max, eq out, tape switch set to mic input, phantom off and the input socket shorted (ie all pins connected together to remove any external effects).

http://www.zshare.net/audio/543689204485ab28/  0dB
http://www.zshare.net/audio/543689961cb56c3f/    +6dB

Audiojohn question:Check the DC voltages around the input transistors and output pins 1 and 7 of the IC and compare them with a good channel - this might give a clue.

                            Good chan      Bad chan1        Bad chan2

Transistor 1 
Base   .02v       .02v   .02v
Collector -8.95v            -8.84v         -8.75v
Emitter        .63v         .62v             .64v

Transistor 2
Base   .02v .02v   .02v
Collector         -8.92v       -8.82v         -8.50v
Emitter           .63v .64v           .64v

IC1:
Out 1   .01v         .01v     .02v to -0.03v
Out 2 -.82v         .10v     .00v to 2.00v

IC2:
Out 1   .01v                 .00v             .00v
Out 2 -.01 .00v to .01v            .00v

Links i forgot to add earlier for this
Ive added some pics of difference:
1. Components for line input, but only on this channel not the other 2 good channels
2. Capacitor has its own track on quiet modules, but on noisy modules it is soldered on back of pcb for mid eq freq
3 Phantom resistor on good modules, non existent on bad modules
4. Larger cap 100v on good modules, 63v(i think) on bad modules
5. Different resistors(metal, i think) on good modules, normal on bad modules
6. Different bridge(not sure of correct term)wires on the 2 kinds of modules
http://www.zshare.net/audio/543689204485ab28/  0dB
http://www.zshare.net/audio/543689961cb56c3f/    +6dB

Thanks once again
Paul

 

[paulrichards7]

Hi, the latest
Ive replaced all TL072, All transitors and the diodes
Re-soldered every connection
And guess what?
Still noise
Anymore ideas
Thanks so much for all your help
Paul

 

[gswan]

Going back to the beginning. This is happening on more than one channel, right?
It has operated without noise on these channels before?

 

[gswan]

What is the PSU topology? Do you have a schematic?

 

[paulrichards7]

hey gswan, at the moment i have 4 channels that are fine
3 channels which are the newer version, phantom resistor and all
1 channel which is the old version which i have managed to get working
i changed all the ICs, transistors, diodes and re-soldered all joints/connections and it worked
i thought if i did the same for the others they would work too
but i guess not
if the others are fine i would guess its not a psu problem
however i would like to upgrade it while im at it
so i will post it later
thanks
paul

 

[gswan]

if the others are fine i would guess its not a psu problem
however i would like to upgrade it while im at it
so i will post it later
thanks
paul

Not necessarily, there are some PSU problems that could manifest them in some channels and not in others, given that the circuits of some channels are different and could be dependent on loading (and other factors).

Let's have a look at the PSU just in case.

 

[paulrichards7]

Ok gswan,
here she is
http://i390.photobucket.com/albums/oo345/paulrichards7/PSU-1.jpg
when i tested her on the scope, there was a liitle bit of noise off of the +15v
the problem is my scope only goes to 30mv, so i could see a bit of noise
hope this helps
thanks
paul

ps the other thing is that there is a newer version of schematic
http://i390.photobucket.com/albums/oo345/paulrichards7/PSUschem16-8-16.jpg
the resistors under the 2nd and 3rd LM317T are different, now i dont know if this is to do with the increase of channles on that version
you would know though