Goals of this modification:
•Convert mixer to 4 channel mic preamp
•Eliminate low cut filter
•Tame high gain so preamps are useful for loud sources (Guitar amps, Drums) do this by relocating the gain control between the Mic transformer and the transistor stage.
•Make as few changes as possible, use minimal new parts. The only new parts required are ¼” jacks for the outputs.
•Optional, remove unused componets/switches.
 

Using channel 1 as an example:

1.Remove the Orange wire going from the center of R9 (Channel 1 Gain control) leading to S2 (Low Cut).

2.Remove the Blue wire going from S2 to R10 on the PCB. Note where the 4 33k resistors are on the PCB (R10, R17, R10, R24) This is where the individual preamps are connected to the mix bus. You will remove these 4 blue wires, one for each channel.

3.Leave all the black ground wires that connect to the top of the gain controls in place.

4.Repeat 1 and 2 for channel 2

5.Mount the output jacks:
Remove the headphone jack; use this hole for ouput 1.
Remove the 2 lugs for the line out.
Use the 3 spot for output of channel 2.
Use the 2 spot for the output of channel 3.
Remove the ground lug and ream out the hole or use a step drill bit, use this hole for the output of channel 4.

6.The wires on the bottoms of each Gain pot will be the outputs for each channel. Cut the wire from each pot, not the PCB. Run each wire to the proper output jack.
Brown = Channel 1
Red = Channel 2
Orange = Channel 3 (Extend this wire)
Yellow = Channel 4 (Extend this wire)
The Master is yellow also, but we won’t need this pot
Ground all the jacks at one point. I used the lug that all the input transformers connect to.

Still using channel 1 as an example:
7.Unsolder the white wire from the PCB that connects to the output of the Mic transformer for channel 1. Connect this wire to the top lug (Side connection not connected to ground) of R9 (Channel 1 Gain)

8.Solder a wire from the center of R9 to the 1 spot on the PCB. (Where the white wire from the mic transformer was removed in step 7)

9.Repeat 7 and 8 for channels 2-4 if you wish to relocate the gain for each channel.

NOTES:
You can also leave the 1st channel in the mix bus. I still relocate the pot, but this gives you one high gain channel with 2 places to adjust the level (Gain and master vol), and 3 that are suitable for Loud stuff, guitar cabs drums ect.

I Sometimes leave the 1st channel stock (Don’t relocate the gain pot, and leave it in the mix bus), this is great for very quiet stuff, acustic guitars and such.

If you choose to relocate the gain pots, you can replace them with 500K audio taper for a bit better top end. In the new circuit the pot becomes the dampening resistor for the transformer.