My studio is based around a 24 channel Yamaha PM2000 console from the late 70's. It's a great console in many ways, but some of the features and signal flow make it tricky to integrate into a recording setting. Fortunately it's pretty easy to modify - it's fully modular, and each channel uses lots of flying wires to make connections, so it's simple to just move those around rather than having to do more invasive surgery on a circuit board.
One of the biggest issues is with the placement of the EQ circuit block in the channel. The mic preamp feeds the EQ circuit, which in turn is sent to the channel direct out (labeled as "insert send" but it serves the same purpose). Then the tape return/insert return gets fed directly to the channel fader, meaning the EQ can only be used on the signal being sent to tape, NOT on the tape return signal. Personally I like having EQ available when I'm mixing, so this has gotta change. I found the general switching arrangement in Douglas Self's Small Signal Audio Design book, which I highly recommend.
In order to make room for this new switch, I decided to sacrifice the "Pan On/Off" switch. It's a sort of interesting feature but not one that I see myself ever using. Instead of totally removing that switch, I just unscrewed it from the front panel and tucked it back into the module, permanently engaged to enable panning. I tried to do all this without being too destructive to the module, so that it wouldn't be terribly hard to return it to stock if someone ever chooses to.
To implement this EQ switching scheme I needed a 3PDT switch. You can in fact get pushbuttons that will more or less match the ones used throughout the console (C&K F series pushbutton switches, and the corresponding mounting brackets), but at the time I was having trouble finding the correct hardware so I settled on toggle switches instead.
So, there are several things that need to be switched around simultaneously - the source feeding the direct out, the destination of the tape return signal, and the source feeding the channel fader. In addition to all that, I used this opportunity to move the channel on/off switch as well. In stock form it's between the mic preamp and the EQ, again meaning it's only going to affect the signal being sent to tape. I moved it directly before the channel fader, meaning it affects the tape return signal instead, so you can mute channels while still recording them. I also installed JLM Audio's solo-in-place modification along with everything else, but that's not a necessary part of all this.
The toggle switch has a total of six leads connected at various points in the channel as follows (I'll also list the colors of wire I chose to use so maybe the pictures will make more sense):
- Mic Preamp Output/IC2 Output (white wire to main PCB)
- Direct Out Jack (yellow wire to edge connector)
- Tape Return Jack (red wire to edge connector)
- EQ input (black wire to main PCB)
- EQ Output/IC6 Output (brown wire to main PCB)
- Input to On/Off switch and channel fader (green wire going to On/Off switch)
- Another wire going from the output of the on/off switch to the top of the channel fader
There are also four existing wires that need to be disconnected - two at the edge connector and two at the on/off switch. For these I just disconnected one end and put shrink tubing over it to prevent any accidental shorts. Also worth noting that much of the original wiring is shielded. I chose to use regular unshielded hookup wire and did not measure any increased noise afterwards.