Allen & Heath GS3000 schematic

[greenmeanh1]

I have an old GS3000 recording console that went on the fritz today. Powers on but all the mutes and overload lights are stuck on and no LCD window. Mic pre's still work and i see levels on the meter bridge. I have not checked any further yet other than to pull the PS out of the rack. I am going to check the voltages on the supply to see if they are correct. Fortunately the back panel is labeled with pin voltages so i can at least check that out. Seems to be pin 1 is 16v+ pin 3 is 16v- pin 4 is chassis ground. Pin 5 is audio 0v ??? Not sure what this is and finally pin 10 is 48v phantom power.
I will report back on those findings shortly, as i am going to check the pins after i finish this post.
Likely i will be needing a schematic. ???
I strongly suspect its probably a secondary power supply in the mixer itself, probably in whatever controls the mute system.

[g1]

http://www.allen-heath.com/media/GS3000-SCHEMATICS.pdf

[greenmeanh1]

Thanks g1. That schem will prove very useful im sure. It already has!! Pretty sure i gotta go inside the board as the external PS checks out as per the pin out tests.
Looking through the schematic it appears there are 3 supplies that branch off from the main outboard PS. VA VB VC. They all appear to be fed from the 16v+ and 16v- plus ground from the external power supply I think my problem may lay in the VA part of the supply. Looks like the filtering and IC regulation is inside the mixer so the next step is to de wire the mixer and get it into an upside down position to work on. Ugg!!
Probably a 50 cent cap causing all this grief.

[greenmeanh1]

Had some more time to look at the schematic. Also de wired the console and got it flipped over and opened up.
Here is what i do know
Mixer powers up
External PSU has all the correct voltages
Phantom powered mics make the meters work
The tube preamp lights up
LCD does not light up
All mutes are lit up and non operational
All peak lights are lit up but zero sound.
The last three items seem to be all tied to the same 5v supply. Best i can tell is they have taken one of the 16v legs from the external PSU and run it through a 5v regulator circuit (7805) to run the MPU board, the LCD screen and possibly the mute logic chips. So this is the central computer in the GS console which also does the midi.
What do you think the chances are that i am on the right track?
In the meantime i have to track down some special screwdrivers (torx i think??) to start pulling cards.
Anyway probably in a bit over my head but im trying.

[TimmyP1955]

I expect that 0V means signal ground but not chassis ground. Who knows where the chassis ground is - maybe in the PSU, maybe in the surface, maybe in both. You might have a look as to whether that console has "the Pin 1 Problem" and if you want to tackle it.

Grounding and Shielding Audio Devices

Pin 1 Revisited

[greenmeanh1]

All power supply voltages are present inside the console meaning that the psu is moving the voltages to the console.
I have narrowed it down to a couple spots, the most likely being the P80C32 MPU. The second most likely is the 5v power supply itself.
To me it looks like the P80C32 may be trying to protect itself during the power on sequence. The chip has a Break feature on P1 Pin12 and i think this is what is happening. P2 is reset which should happen everytime the console is powered on.
So would this be caused by a power supply issue. I notice there is a couple of BC549's in the 5 volt power supply. Are these part of the Break circuit??

[greenmeanh1]

I thought i would update. The problem ended up being the MPU Eprom and it needed to be re seated. Once doing that a couple times, the mixer now fires up properly. So memory was corrupting due to oxidized connection. I love the zero cost fixes but they do take longer to figure out.
Mixer is back in its place and wired back up after a complete cleaning