Boss HM-2 Repair - part 1

So I'm cleaning out my studio a few weeks ago and I come across the first Boss pedal I ever bought, a Boss HM-2 Heavy Metal pedal. A high-school acquaintence (Jon-Paul B.) sold me his Squier Bullet strat, a Peavey practice amp, and the HM-2. Finally I could make some real noise! I was super stoked.

Sometime in the 1990's I accidentally stepped on the pedal wrong and broke the Distortion knob off. Only a little nub of the potentiometer shaft remained, enough so I could adjust it.

Now reading online about the popularity of this pedal for Swedish Metal tones, and considering it's no longer in production, I decided to try to fix this problem.

Starting with the HM-2 Schematic (thanks to Hobby Hour) I was able to determine it's a 250K pot.

Today I called Roland support and spoke to a nice guy named Carlos. The Distortion pot for the HM-2 is no longer manufactured...BUT I was able to find a pot on another production pedal, the DS-2 Turbo Distortion. This is another 4-pot Boss pedal (so the pot dimensions should be about the same), and the value of the Distortion pots are the same on both pedals (250K), so I figure it should be a direct swap.

FYI, the "Distortion" pot for a DS-2 is Boss P/N 1327-9854. It's a 12mm pot. They don't have it in stock, so I have to call back in a couple of weeks when they do.

This Alpha pot might work too, but I'd have to buy one to see if it will fit. It's a good bit bigger (16mm), and without knurled shaft. Seems tricky to find a 250K pot, 12mm body, with a knurled shaft.

I was able to order a knob; that was about $12 shipped...steep, but fine for a vintage pedal.

So commence with the disassembly. Tools needed:

Philips head screwdriver

12mm socket 

11mm socket

Socket wrench

1. Unscrew the back case (4 Philips screws)

2. Lift out the back and corrugated plastic insulator. 

For reassembly, note that the ridges of the insulator go towards the PC board. 

2. Unscrew the input and output jack nuts using the 12mm socket, and reserve the washers and nuts. 

3. Set the knobs to zero (full counterclockwise) then gently pull off the 4 control knobs. This is a good time to clean off the dust and grime with a damp microfiber cloth. 

Then unscrew the 4 nuts on the potentiometers using the 11mm socket. They're not on there very firmly, they don't take a lot of torque to loosen them, remember for reassembly. Reserve the nuts and washers. 

4. Now you have access to the potentiometer "control" board. I didn't disconnect the switch and battery snap. For now. 

5. Then I set about measuring the pots. Screw shaft is 10.5 mm in diameter, base diameter is 12mm, knurled shaft height is 10mm (~3/8") from threads, knurled shaft diameter is approximately 4.6mm (3/16"). 

More to come!