Outmoded Electronics: Shure M67 Preamp Mod

I purchased a Shure M67 a few years ago with the idea of performing the individual direct out mod. I likely gave up after being overwhelmed by the messy stock wire layout.

I want to give the RodC M67 mods a try, to relocate the gain knob to use the preamp for louder sources. His website was now gone, with a few other sites preserving the information.

Channel 1: gain control to be relocated; switchable unbalanced direct out to headphone jack chassis hole.
Channel 2: gain control to be relocated; switchable unbalanced direct out to “2” banana jack chassis hole.
Channel 3: stock gain; switchable unbalanced direct out to ”3” banana jack chassis hole.
Channel 4: stock gain; stock mix bus connection intact.
Line Output will be rewired to the Microphone Out XLR jack, with 600 ohms resistance across pins 2 and 3 if possible

I hastily yanked out the headphone jack before I started taking pictures to document the stock wiring.

Stock wiring on switches.

Headphone circuit removed. Balanced output remaining to be removed.

Edit 6/2/20. I started to make the changes to the output section of the preamp. I spliced the line level tap of the output transformer to the Microphone Out jack.

I have so far rewired the gain pot of channel 1 to come before the transistor stage. Next will be the wiring of the direct outs.

Edit 6/18/20. I completed performing the modifications to the M67.

The repurposed mix bus output has an 820 ohm and 2.2kohm resistor pair in parallel on pins 2 and 3 for an equivalent load of 597.35 ohms. I can directly connect the mix bus output to a balanced line input and load the preamp properly.

The direct outputs are lower in gain as expected, as the taps are before the final amplification stage. I can understand how relocating the gain pot to attenuate the signal into the transistors is useful for louder sources like drums.

I currently have the new unbalanced direct outputs of the M67 hooked up to my the RCA inputs of the Roland VSR880 recorder as a few extra channels of mic preamps.