# Boss HM-2 mods?



## devnulljp (Mar 18, 2008)

So when I was 14 I had a Boss HM-2, which I later traded for a flanger with a guy who sounded great with it (how'd he do that??!). I don't really like the whole high gain thing, but nostalgia got the better of me and I snagged one cheap on evilbay. It's not very subtle to say the least. Real synthetic and big hair pointy guitar & spandex metally. 

Any ideas for mods that might make it a bit nicer or more useful?
There's a schematic here but that doesn't mean much to me I'm afraid.

Turning the gain all the way down gets a slightly more interesting sound out of it, but gain up beyond a fraction and it has no subtlety at all.
In my head I remembered it being a bit Rat-like, but that's wishful thinking...


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## moonlington (Jul 3, 2008)

hey, i _just_ got one of these today. mint, mij and $40. it sounds sick! very wall-of-fuzz like. i too would be interested in these mods.


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## Ripper (Jul 1, 2006)

Okay here's a good mod for the HM-2, makes it a better pedal and sounds great

C30 change to .022uf adds bottom end
C6 change to 1uf adds bottom end
change d5, d8, d9 to leds, 
remove d6 and d7 and put in jumper wires
Change d3 to a 1n4001 
The diode changes will take out some fuzziness and really open this pedal up to a full well rounded distortion.
put a 1uf film cap into c15 (this one helps to take out background noise)
change over c12 to a .033uf, it helps tame the fuzzies as well.
You can also increase the value of r52, it will add mids and increase the volume I think the stock value for that resistor is 3 or 3.3k.

Hope that helps


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## devnulljp (Mar 18, 2008)

Thanks - got a source for parts? Other than the film caps you mentioned, anything special about the others or just regular old orangedrop caps (or something else)? New to this...


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## mhammer (Nov 30, 2007)

I have one of these too, except it was given to me by a nephew....who was around 18 and in a speed metal band. Not that enamored of it myself, but it has its place, like anything else.

The schematic you link to shows several sets of clipping diodes in different positions and performing different functions. The first "horizontal" set provide asymmetrical clipping in that first op-amp. The second horizontal set provide "crossover distortion". The third set (vertically oriented) provide the usual diode-to-ground clipping, like you might find in the Rat, the DOD250, and scads of other similar pedals.

"Clipping" is something most of us understand. The signal is boosted and diodes or some other limitation prevents the peaks from being their full amplitude, "clipping" the peaks off and replacing them with something that is flat (i.e., a fixed amplitude) for everything above that amplitude. Crossover distortion is a different beast. Where most squaring up of signal is achieved by lopping the tops off of negative and positive peaks, crossover distortion attempts to square things up by lopping off the *sides* of the waveform.

With that diode pair in series with the signal path, positive and negative half-cycles don't pass unless they reach the "forward voltage" of those diodes. In this case, because those diodes are germanium, no signal passes through them unless it is at least around 220mv or so. That means that the slope up and down from the peak of those negative and positive half-cycles is turned into a straight drop. So, essentially, the HM-2 is trying to square up the signal from both the top and sides.

Is there something sonically desirable in doing this? Well, at one level, it actually acts like a noise gate, since any noise also has to be greater than +/-220mv or so to pass by those diodes. And, given the gain in most “metal” pedals, and sort of noise control is welcome relief. The tone is, like any distortion, a matter of taste. FWIW, the Z-Vex “Machine” pedal is supposedly an attempt to take crossover distortion to the max, so I suppose the use of horizontal and vertical squaring may be something desirable. 

If you wanted to explore the crossover distortion side of things a bit more with your HM-2, then consider doing the following:

1) Change the 220k feedback resistor in that first clipping stage for 330k.
2) Add a 4th diode to the 2+1 complement so that it is a 2+2 complement. You can do this by simply lifting one end of that solo diode and soldering the free end to a 4th diode, whose other end goes to the now-empty pad. This will give a bit ore dynamics and amplitude to what hits the Ge diodes. Alternatively, take the diodes out completely.
3) Add a 2nd pair of GE or Schottky diodes in series with the existing pair that produce the crossover distortion.
4) Replace the 3rd diode pair to ground with a pair of red LEDs.

Those changes will push the GE diodes harder, produce more crossover distortion and “open up” what comes after the crossover stage. Could be interestng. Or maybe not. You never know until you try.

On a final note, the linked-to schem shows that the HM-2 shows two boost-cut controls. In reality, the "high" control is altering two resonant bands at the same time (IC2a/IC2b). I suppose if a person wanted a quick-n-dirty mod, they could simply install a mini-toggle to lift one end of the 0.15uf cap shown in the lower resonant section. That would effectively disable the upper mid EQ-ing, and relegate the "high" pot to controlling only the treble. A simple way of changing the resulting tone without too much fuss.


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## Ripper (Jul 1, 2006)

devnulljp said:


> Thanks - got a source for parts? Other than the film caps you mentioned, anything special about the others or just regular old orangedrop caps (or something else)? New to this...


I get most of my stuff either from Digi-key or Electrosonic and I buy in bulk, but I am sure that there must be a local electronics supplier somewhere in your area. Even if you just took the list to the local tv/stereo repair place (if you still have one they are going the way of the dinos too) and tellthem what you need. Other than that use a metalized film capacitor for the film replacement and use regular polyestery/mylar caps for the rest.


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## devnulljp (Mar 18, 2008)

Thank you guys -- a mine of information as usual. 
I'll keep you posted how it goes.


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## devnulljp (Mar 18, 2008)

mhammer, is that clipping why it sounds sooooooo compressed and kinda synthetic? I read somewhere that Eric Clapton of all people used one for a bit in the 80s until he realised they were crap 
I'd love to be able to shift it closer in the direction of a Rat, but I suspect it's just going to be a paperweight.


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## mhammer (Nov 30, 2007)

The compressed sound (and I just plugged mine in and noodled around to get a feel for it...been a long time) has nothing to do with the crossover distortion. Compression is about putting a clamp on the *amplitude* - the ceiling on the signal - not the sides of the waveform. The compression the two of us hear is a result of the use of double clipping. That is the signal is clipped twice at two different points.

Just to confirm this for myself, I did the experiment and just installed a temporary switch in mine that shunts the two germanium diodes or restores them. So, only vertical clipping, or vertical plus horizontal clipping.

There was a slight change in tone, but no change in compression. What *kind* of change in tone? Well, I'm playing through a 2W amp and 6" speaker turned down such that my kids voice on the telephone is competing with it 25 feet away. So, I'm not going to make claims about what someone else might hear through a 4 x 12 stack. But, to the extent that I could hear anything, it seemed like there was a slight increase in volume and "body" when the diodes were shunted. Slight. It might show up more if one is playing louder but that's what I heard.

The component numbering does not conform to any diagram I've seen posted, however the germanium diodes are easily identifiable because they are clear glass and are MUCH bigger than the other diodes found on the board. All you need to do to bypass them is solder a small insulated wire to the pads on the copper side of the board. Actually, if you bypass the 10k resistor on the one side OR the other, you'll probably get a little more oomph.

I would not describe this as changing the essential character of the pedal, though. It's a double clipper, and produces a compressed sound much the way the other more famous double clipper - the Big Muff Pi - does so.

I also tried lifting the connection of the midrange resonant section to the High pot. This involved lifting one side of the 0.15uf cap (it's a small blue tantalum cap near the edge of the board on mine). Tended to remove all the guts from the sound. Too much midrange loss. Not worth doing.

Tag, you're it.:smile:


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