BOSS HM-2 Modding....This time a Blend control.

I love messing around with HM-2s. This is a super simple mod and very useful. Adding a clean blend to the mighty HM-2

I have done this enough times to have a modified schematic drawing with where to add/cut stuff in.

Modified Schematic

First thing to do is open up the pedal and move the wires around and drill a hole for the pot. I use a 9mm pot as there isn't much room in there.

Guts and hole drilled

HOLE!

Now I take my 9mm pot and put some 80mm (approx.) fly leads on it and then mount it into the hole, put the nut and washer on and tighten.

Wires soldered on

Mounted

Next I build up my little blend PCB. I have designed a PCB for adding to any circuit to add a clean blend. As long as they are in phase it works great for only a handful of components. I don't sell this PCB it's just for me. Alternately if you want to do this mod you can use this. It's a vero layout of the same circuit I have made a PCB of. I'm not taking credit for the circuit it's not mine. I do however use a 100N cap instead of a 10N as I found it sounds better.

My little PCB

All ready to go

So now with my PCB made and the 9mm blend pot installed all I need to do is wire it into the HM-2. To do this I need to cut 1 track on the HM-2 PCB. This track is after the volume but before the FET bypass switching system (as labelled on the schematic above).

Locate the trace at pad #8

Trace cut with Exacto knife.

Now with the trace cut I can start soldering wires onto the main PCB. Two wires from the 9mm Blend pot, Pin 3 and Pin 2 go onto where the cut trace is. Pin 3 goes to Pad #8 and Pin 2 goes to the other side of the cut trace which connects to C32.

Time to connect up the rest of the wires. To do this I have to find the input to the effect on the effect side of the FET for the bypass switching system. Q4 is the FET doing the switching. I need to solder the send wire to the daughter board to somewhere at the junction of Q4, C6 and R16. Blend Pin 1 connects to the daughter board after the 10uF cap. V+ of the daughter board connects to the main board at Pad #2. GND of the daughter board connects to the main board at Pad #3.

Start connecting daughter board

Find Q4

Power wires soldered on and location of Q4 Traces on PCB

Ready to test

With everything now soldered in it;s time to test the mod. Tested it with no issues. Now I need to secure the daughter board with a bit of the old hot snot glue, attach a knob to the pot, put it all back together and send it off the customer.

So that's how you mod an HM-2 to have a blend control. This is a pretty easy mod to do even for a beginner. If you have any questions or comments add them below

Testing

Ready to send.