Mackie ProFX8 mixer - Low gain fault

Thread Starter

cjeung

Joined Mar 31, 2011
11
I have an 8 channel mixer where the pre-mix gain doesn't seem to work properly. I tested all the adjustable channels and the error is the same. It doesn't matter if its the mic or line input. Adjusting the gain has nominal effect until they are nearly maxed out. At that point the channel level meter and output abruptly jumps into oversaturation. Since it occurs in all channels, I can't be sure it's a channel related issue, but more global. Attached is the manual and schematic of what I'm dealing with. Page 18 of the manual references the gain section in question. Could use a knowledgeable evaluation into troubleshooting ideas. Thx
 

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Thread Starter

cjeung

Joined Mar 31, 2011
11
Did it work properly before? How are you setting the gain controls, did you read page 3 "Set the Gain"
Well the mixer works. I get audio output, but the gain error seems to make the output either too low or distorted. The gain level LED barely responds when tapping directly on the mic (not something normally done). Then on the last 5 degrees of gain adjustment the LED's will become very active just by speaking through the mic (not tapping). This doesn't seem right.

I think the LED level circuit seems to work fine, but the gain error occurs before that.
 

Thread Starter

cjeung

Joined Mar 31, 2011
11
The first things to check are all the supply voltages, +15V, -15V, +15V-M, -15V-M, +7V, +5V, +48V.
I did a check on the power board and the voltage measurements were good. I will double check. Is there a possibility that there might be a load drop in the voltage? (thinking a bit beyond my expertise here). I just measured the power board while disconnected from the mixer circuits. Should I try to measure with board connected? and perhaps a line/mic input?
 

abrsvc

Joined Jun 16, 2018
138
Using a scope, check the signals at the insert point. Since you are having the same problem on all channels, the problem is not likely at that point but at a common point later on in the signal path. The insert jack will be the easiest point to check signal integrity and level for channels 1 and 2 to confirm that the problem is not at the channel level. I suspect that the signal will appear and act correctly here.

You can check with a set of headphones too to isolate the final op-amp stage as the potential problem too.

The bottom line here is you likely have a bad 4580 IC somewhere in the chain. Since these are dual channel OP amps, one failure will affect both channels. Check the levels at the effects insert jack. Here too you will get the signal broken in the chain that will help to isolate where the problem is located. The same for the FX insert. Once isolated, check the 4580s in that area for signal integrity and levels.

Keep us posted.

Dan
 
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