Mackie Mixer fault

[diskosteppas]

Hi all can anyone help point towards where the fault would be for my mackie mixer please?

It will pass audio but the pan control seems not to work - When the pot is turned, the volume changes but does not move signal from left to right.

Also, the main volume pot only passes audio when it is at zero. It is as if it shorts when turned up and the audio just cuts out. I have changed the pot out for a new one but no change.

I have attached a schematic if anyone can please help me? I have tools; scope, desoldering, lots of parts and experience building but I know little about audio mixer tech!

[Audiorepair]

 What does it do on headphones monitoring the output?

I think you may have something out of phase.

[diskosteppas]

Hello thank you for your reply! I have checked in headphones the behaviour is the same as the main outputs.

[Audiorepair]

Well which pan controls don't work?  Every channel?

Maybe there's a missing ground - the pan pot wipers (middle pin) should be connected to the same ground all the jack sockets are.


Edit:  My mistake, only the stereo channels' balance controls have the centre pin grounded, not the channel pan pots.

[diskosteppas]

Hello and thank you so much for getting back. It is every pan pot is not working! 1 - 4 (mic channel) and the 5 - 12 (stereo lines) no response although there is some kind of volume change but no shift across stereo field.

[wasedadoc]

Hello and thank you so much for getting back. It is every pan pot is not working! 1 - 4 (mic channel) and the 5 - 12 (stereo lines) no response although there is some kind of volume change but no shift across stereo field.

Feed a signal into LINE 1 and switch the meter LEDs to show Left and Right (ie not mono mix and Mic bus).  Do the meter LEDs show the levels change as you would expect when the Channel 1 pan control is operated?

Edit.  Scratch that.  It isn't going to give any new information.

[Audiorepair]

Dunno mate.

I'd be looking for a missing ground somewhere, possibly due to a cracked PCB.


Edit: you won't see a cracked PCB on a schematic.

[Brianf]

As the fault affects all channels, and you've said that the master volume is also faulty, then it's going to be in the output section. I'd start with a long hard look at that master volume control.

[Brianf]

Ah, but you say that the phones behave the same?

The feed for phones is before the master volume so scrub my suggestion. Which pushes the fault back towards U3. Or, as others have said, a missing audio ground connection. Time to get the meter out and prove that everything that should be connected to AGND really is.

[Audiorepair]

The feeds for the Master Volume, Phones, and Meter driver are the same.

They then have this odd thing of series resistors to the pots shorting the signals to ground rather than appearing at the top of the pot track and the wiper going to the op-amps.
Something odd is happening around here.
If the phone pot has lost its ground, for example, you can have left and right signals connected together by 2 x 4.7k resistors.

[diskosteppas]

Thank you for your support!! I went and swapped U3 like for like opamp no change so ill try find any missing grounds and check the master volume area.

[Vicus]

In this case the scope is your best friend.

[Audiorepair]

In this case the scope is your best friend.

Well it probably isn't in this case, as it will just show the same symptoms.