Boss HM-2 Repair - Part 2

So I spoke to Leanna at Boss support today and was able to get  the "Distortion" pot for a DS-2 is Boss P/N 1327-9854, it's $4, $11 shipped. They had it in stock...hooray! Hopefully will get it next week. 

Got the Boss knob a few weeks ago, so I am back in business.

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! 

Old Ironsides - Transformers

So here are the transformers that came with the chassis...old iron for this Old Ironsides. 

First the output transformer, made by Stancor, Part A-3311. Conveniently, I found an old Stancor catalog from 1961-1962 here in PDF format (thank you internet!) that lists the following specs for it:

Application: 10KΩ CT (center tapped) to 500Ω /15Ω /8Ω /4Ω 

Max primary DC: 70mA

Audio watts: 25W

There are 5 wires coming out of this output transformer:

• Black: output jack negative (sleeve)
• Dark brown: output jack, positive (tip)
• 3 other leads were tied off (i.e, not connected to anything):
• Yellow
• Red/Orange
• Light brown

What the hell do they mean? Well, this web site has a convenient guide to standard output transformer wiring; in this case, it's a"push-pull" transformer, so it should follow:

 Plate lead (pri.) -------------- blue (start)
 B+ (power supply) lead --------- red (center tap)
 Plate lead (pri.) -------------- brown (finish)
 speaker (typ. +) (sec.) -------- green (or yellow)
 speaker return (sec.) ---------- black

Next, the power transformer, another Stancor model, Part number P-6012. Again, thanks to that old Stancor catalog from 1961-1962 here in PDF format, here's what I learned:

Stancor P-6012: 

Plate supply: 350-0-350 AC Volts, DC 90 mA

Rectifier filament: 5.0V CT @ 3A

Other windings: 6.3VCT @ 3.5A

Here's a picture of the wires coming out of it. 

What the hell do they mean? Well, this web site has a convenient guide to standard power transformer wiring:

Primary leads -------------- black

   (if tapped)

     Common ----------------- black

     Tap -------------------- black/yellow

     End -------------------- black/red

 High voltage secondary ----- red

     Center tap ------------- red/yellow

 Rectifier filament winding - yellow

     Center tap ------------- yellow/blue

 Filament winding No. 1 ----- green

     Center tap ------------- green/yellow

 Filament winding No. 2 ----- brown

     Center tap ------------- brown/yellow

 Filament winding No. 3 ----- slate

     Center tap ------------- slate/yellow

Here's what I identified, and how it's currently wired:

• 2 black: connected to 120VAC (one side to power switch, one side directly to AC power wire) - this appears correct.
• 3 dark green (black?), currently wired as follows:
• 1 to ground
• 1 to 6V6 pin 2
• 1 to 6V6 pin 7
• 2 brown, as follows (on closer inspection, and scraping down insulation anti-mold coating, it's clear these are RED):
• 1 to rectifier pin 4
• 1 to rectifier pin 6
• 3 yellow, as follows:
• 1 to rectifier pin 2
• 1 to rectifier pin 8
• 1 to ground
• 1 red: wired to ground (on closer inspection, I can see this has tiny yellow dashes on it, indicating red-yellow)

The starting point - Old Ironsides...

So I'm starting this journey (again) with an old amp I started building from spare parts, sometime in the late 1990's, until about the early 2000's. I'm pretty sure it was based on a vintage Fender Deluxe, although I wish I could remember which schematic version. I located a schematic in my copy of Aspen Pittman's excellent reference The Tube Amp Book, and started wiring it point to point from there.

Here's a picture of where I'm starting from:




The thing needs a name...I think Old Ironsides is nice. Here's a shot from the front panel, input jack on left, power switch on right. I don't remember what the 3 controls are for, I'll dig around and figure it out.



Another shot from the top. I used a little copper plate to enable me to mount the jacks for the 12AX7s.




From what I remember last working on it, I added the tubes, fired it up with the guitar plugged in, but got no sound. I think the rectifier did light up though, that's a good thing I think.

So now back to it.

Some the parts sources for this franken-amp:

Chassis and transformers: from Angela Instruments, from the mid to late 1990's, when they used to have a cool old catalog printed in newsprint, sort of retro-looking, lots of irreverant commentary. I bought this around the same time I bought a Sovtek Mig50 from them, not long after they came out. They were so cheap, I think I paid $200 or so for it new. It actually had a pretty good clean fender sound, good iron but parts were kind of crappy: loose knobs, etc.

Small parts: Antique Electronic Supply, also Mouser, and Radio Shack. Some new, some used...including some crazy surplus shit I found in the trash near the facilities department at LMU in the early to mid 1990's...they were throwing out all of these cool panels and things with banana jacks, and huge bags of electrolytic caps.