Boss Pedal Mods

[Ripper]

Hi, I'm new to the forum.  I have a Boss MT-2, HM-2 and OS-2 that I would like to mod.  I was wondering if anyone here had any mods they would recommend and if so, do you have the notes for performing them.

I like the MT-2 and HM-2 very much but I feel they could be rounded out a bit better, less "harsh" if that is the right term.  The OS-2 is a great little pedal but I find it really trebley for most of what I do.

Take care
Ripper

[Peter Snowberg]

Welcome to the forum.

I have an HM-2 and I don't really like it that much because it's just not what I go for. What it does do, it does very well though.

One Mod I would suggest right away is to jumper across the pair of germanium diodes and one of the 10K resistors they connect to. That eliminates one of the three sources of distortion and I think It makes the pedal sound quite a bit better. You can always remove the jumper to return it to stock or put it on a switch to keep it an option.

Find R23, D6, and D7. These are what you'll jumper across. Now for reference, find R30 (10K) and C12 (a little electrolytic). Attach a wire to the junction of R23 and C12. Attach the other end to the junction of D6, D7, and R30.

Another HM-2 mod is to unsolder one side of D8 or D9 and add a second diode in series to get asymmetrical distortion from the third set of diodes. Either a germanium or a silicon diode can be used... try both, or try two germaniums in series. Just keep all the bands pointing the same way with series strings.

You can also try shorting across D5 which will go from asymmetrical to symmetrical distortion in the first clipper stage. I like the former, but to each their own.

To change the amount of highs present, adjust C16 (.0022µF). Make it .001 to be a little harsher or .0047 for a little mellower.

Disconnecting C29 or C28 will affect how the hi tone control operates. Normally the HI control operates two gyrators resulting in two high peaks.
disconnecting one of those two caps reduce it to one peak.

You can adjust the tone control gyrators by adjusting C27, C26, and C30 as well as R43, R44, and R51. The amount of influence they have depends on C28, C29, and C35 being large enough. If you fiddle with the parts above you may want to make C28 or C29 larger.

Good luck! I think it's a great pedal for a "base". With some mods you can make it do a huge number of different tricks.

[Ripper]

Thank you for these mods Peter.  I'm going to try some and see what works for me.  I prefer asymmetrical as well, and I have thought that the HM-2 is too trebly so will adjust that too.  I'll let you know how it goes.  I redid an old Ibanez SM-9 that I have had for years and I am now really happy with it.  It nows can be used for anything from a good blues growl to all out snarl for hard rock.

[Peter Snowberg]

Cool. I would love to know what you try and what you thought.

Shorting the D6/D7 pair transforms the distortion in a way I really like. I would try that mod first because it will affect how the other mods act. Those two diodes create crossover distortion which generates lots of high frequency mess.

If you don't have it already, I would suggest getting the schematic to this pedal so you can see where the mods take place in the circuit.

I decided the Boss box was too small for the number of knobs I need so my HM-2 sits half alive in a 1590BB.

[Coriolis]

Hi Ripper
I've got an OS2 that I'm planning to do a lot of mods to as well.
First up is true bypass, next thing on my list is to try and get a more linear frequency response (always found Boss distortion pedal to be weak in the lows and harsh in the highs). I believe what the os-2 does, is blend two different distortion circuits through the "color" pot. If I remember correctly, one half of a dual opamp has diodes in the feedback loop, and the other half has them across the output.

I think I'm going for something a little more natural sounding, and might put a cleaner opamp in there and try different diode combinations.

It would be fun to compare our results!  

C

[Coriolis]

Ripper,

I just PM'ed you a link to an os-2 schem...

Peter, would it be alright to post it to the forum? I don't think the os-2 is in production...

C

[triskadecaepyon]

Welcome to the forum.

I have an HM-2 and I don't really like it that much because it's just not what I go for. What it does do, it does very well though.

One Mod I would suggest right away is to jumper across the pair of germanium diodes and one of the 10K resistors they connect to. That eliminates one of the three sources of distortion and I think It makes the pedal sound quite a bit better. You can always remove the jumper to return it to stock or put it on a switch to keep it an option.

Find R23, D6, and D7. These are what you'll jumper across. Now for reference, find R30 (10K) and C12 (a little electrolytic). Attach a wire to the junction of R23 and C12. Attach the other end to the junction of D6, D7, and R30.

Another HM-2 mod is to unsolder one side of D8 or D9 and add a second diode in series to get asymmetrical distortion from the third set of diodes. Either a germanium or a silicon diode can be used... try both, or try two germaniums in series. Just keep all the bands pointing the same way with series strings.

You can also try shorting across D5 which will go from asymmetrical to symmetrical distortion in the first clipper stage. I like the former, but to each their own.

To change the amount of highs present, adjust C16 (.0022µF). Make it .001 to be a little harsher or .0047 for a little mellower.

Disconnecting C29 or C28 will affect how the hi tone control operates. Normally the HI control operates two gyrators resulting in two high peaks.
disconnecting one of those two caps reduce it to one peak.

You can adjust the tone control gyrators by adjusting C27, C26, and C30 as well as R43, R44, and R51. The amount of influence they have depends on C28, C29, and C35 being large enough. If you fiddle with the parts above you may want to make C28 or C29 larger.

Good luck! I think it's a great pedal for a "base". With some mods you can make it do a huge number of different tricks.

My chips are fried on my HM-2.  What IC's are on the HM-2?  I can't read them anymore because they are all burned up now.   I think it was like a M5216L or something, which they don't make anymore.   I remember the chip on NTE was 778S or something.

[WGTP]

From the Schematic I've seen, and my use of the OS-2, I think it is a mix of the SD-1 on the left side of the Color knob and the DS-1 on the right side.  I think they have done some EQ to both to accentuate the differences between them.  The SD-1 seems to be real midrangey and the DS-1 seems to have a notch filter on it.  That way you can sort of control the midrange at the same time your varying the distortion type.  Cool idea, but to me both sound too compressed.  I would suggest the standard diode mod's to both (I think the balance between the 2 could be messed up without carefully balancing the diode mods).  Personally, I would like some more highs out of the SD-1 side and a little less out of the DS-1.

I've messed with op amps enough, that I would expect the diodes mod's to be much more noticeable than switching op amps IMHO.  

[Ripper]

Hi Coriolis,
Thanks for the schem I hadn't tracked one of those down yet for the OS-2.  My big complaint with the OS-2 is the lack of lows and tin can sounding highs.  I'll check over the schematic.   I always figured this pedal was a type of hybred SD-1/DS-1.  I would be happy to let you know how things work out on this one.  


The IC's in my HM-2 are the M5216L.  I don't have a cross refernce for that chip.  Sorry


WGTP, I would agree on what you said about op amps.  Changing the op amps in some things doesn't produce as much of a result as the diode or even capacitor changes can do.

[Peter Snowberg]

The chips in the HM-2 are just generic opamps, but in SIP form.

You can get a couple different SIPs from Mouser including regular opamps and a SIP version of the NE5532. (current hog..... but wonderful opamp!)

You can also use any DIP dual opamp and a little jumper wire.

Coriolis, if you have a link then sure. Boss releases factory schematics to the general public so that's no problem at all. Thanks for making sure.

[Ripper]

Okay I did the diode jumper and lifted c28.  Now this really changed the pedal for the better.  Thanks Peter.  The schematic I found is really crappy.  It's handdrawn.  I was wondering if you would be able to tell me which resistor is the current limiter for the led.  I want to swap it out for a different color and need to know which resistor to change.
Thanks for the help with the HM-2

[Peter Snowberg]

Cool. Somebody else independently posted that diode bypass mod a couple weeks ago too.

I know there's a good schematic out there. Try a search or maybe somebody will post a link to one.

R46 is the LED current limiter, but there is also a zener diode in series which is D13. If you follow the trace from the LED, it runs right through those two before hitting the flip-flop made fom Q8/Q9.

The zener causes the LED to go out if the forward voltage drops too far as a battery check function. You may want to bypass this diode if you're changing the color of the LED because the Vf will be higher by a volt and a half when you switch from red to blue. I think the diode might be 3.9V?

[Ripper]

Thanks Peter,
I was pretty sure those were the ones (I had traced it back but wanted to make sure).  I'm going to bypass the zener.  I run only power (no batteries to my pedals) so it isn't a huge deal.  I am going to use a super bright yellow in this pedal.  I change out the LEDs in my distortion pedals so I can tell at a glance onstage which one is on.  My MT-2 has a blue, the SM-9 has a green and this one will be yellow.  

I have a tip for those interested in power supplies.  If you can find them, older style laptop computer power supplies work great.  I have picked up three so far, two from Compaqs and one from an IBM.  They take a regular computer power supply cord.  They are rated for 15 volts, but they have an adjustment inside that you can take right down to around 8 volts if you want and they are filtered power supplies too.  No noise.  They work great.  I run mine to a distribution block and then my cables to the effects from there.

Thanks again Peter.  I can now see about beating this OS-2 into something useable.
Ripper