Boss HM-2 Mods

[jwbink1500]

Hi all,

I have a Boss HM-2 that I modded withe distortion mods via Brian Wampler's books and the sound improved. Didn't do the tone mods yet though.

My problem is the Distortion pot on the pedal has a small increase in distortion initially to a plateau from about 10:00 to 3:00 then increases again. Does anyone have a way of altering this? I would like more range from the distortion and to remove the dead spot. I'd like to hear any other mods anyone has tried. I loved the Heavy Metal Setting on a Boss GT-6 so I though I'd try this pedal but always sounds fuzzy and not very clear or powerful.

Thanks
Joel

[Mark Hammer]

The gain control should not do that.  It would seem the pot is dirty or otherwise malfunctioning.

[jwbink1500]

Thanks for the reply Mark.

I don't think it should either but I have read other posts with the same behavior. See this post: http://www.diystompboxes.com/smfforum/index.php?topic=58370.0 There is no scratchiness and the transition is smooth  up to a point then it maintains the dist then continues again. No big sudden jumps when turning the pot. The other post never really got the problem solved so I thought I'd try and get some ideas.

What does anyone think?

Thanks
Joel

[Mark Hammer]

Just for the heck of it, try bridging the pair of germanium diodes with a piece of wire on the copper side of the board, and see if that makes a difference in the behaviour of the gain pot.  The thing with the HM-2 is that there are three different sets of diodes to introduce clipping, as you can see here: http://www.freeinfosociety.com/electronics/schemview.php?id=134 .  Where two of those sets do not impair the circuit's ability to pass signal, the GE diodes present a kind of "wall" where, unless the signal is above their forward voltage, no signal will pass.  My suggestion is to take that "wall" down temporarily and see if it makes a difference in the behaviour of the pedal.  It will certainly change the tone, so don't confuse that with the behaviour of the Dist pot.

[jwbink1500]

Thanks Mark,

I'll give it a try when I get the chance. Might be a bit though. Just had a baby boy on Dec 21st so there goes my time! Not a bad thing though. 

I'll post the results when I get them.

Thanks
Joel

[Jurgen]

This is a known issue with the HM-2. The distortion increases from minimum to about 9 o' clock on the gain pot, then stays pretty much constant until about 3 o' clock, after which it ramps up again. I'm quessing it has to do with the way the gain control is implemented, simultaneously functioning as a variable ground leg resistance in a voltage divider after the gain stage built around Q6 and setting the resistance to ground in the feedback loop of IC1B, as you can see on the schematic here: http://www.freeinfosociety.com/electronics/schemview.php?id=134

[Mark Hammer]

Good point.

[Renegadrian]

resurrecting an ol' thread, just got a HM-2 with a broken pot for cheap. (I have to find a 9mm pot, regular one don't fit in the box)

Spent the whole afternoon modding it, trying new things and implementing known mods (wampler)

ACA to PSA mod is mandatory.
D6-D7 diodes are out.
Removed C28
subbed all 1µ electros in the signal path with non electros (C3, C4, C12, C15, C32)
C30 is now a 47n from the original 68n

tried to implement the mids pot, mmm I don't think it's that useful...
tried to remove D8-D9 or put leds instead of Si - mmm nope, sound is bigger but I don't llike how the notes fade out.

output buffer removed, wire #6 is wired to q10/r48

For now, the main mod is the removal of C28, it give a whole different sound at the end of the high pot. and yeah those germ diodes out.

I'll report other mods if I find something new...

[gorgieboy]

Resurrected again!
Mine is broken, began to sound very sloppy and dirty all of a sudden, do you recommend doing this mod to trie to fix it or do you have any other ideas?

[gorgieboy]

Or anyone for that matter....?

[bluebunny]

If it's broken, then fix it first.  Otherwise you're modding from a completely unknown starting point.

[gorgieboy]

Ok, asking as a layman, what should i start with? Where do i start with the failsearch?
I'm not a soldering veteran so i'm a little lost. Got som basic soldering skills though....

[Kipper4]

If it where me I'd start a new thread.
eg "My HM 2 failed, help me debug or fix it."

Thouroughly look at the pcb and componants with a magnifier to see if you can see any possible areas of concern.

Go here
http://www.diystompboxes.com/smfforum/index.php?topic=29816.0

follow up with the debug info posted in your thread.

Be a little more precise about what it does and doesnt do.

"began to sound very sloppy and dirty all of a sudden,"
could mean so many things it's hard to tell where to start.

I hope we can get you sorted.

Welcome to the forum
Rich

[czech-one-2]

Resurrected again!
Mine is broken, began to sound very sloppy and dirty all of a sudden, do you recommend doing this mod to trie to fix it or do you have any other ideas?resurrecting an ol' thread, just got a HM-2 with a broken pot for cheap. (I have to find a 9mm pot, regular one don't fit in the box)

Spent the whole afternoon modding it, trying new things and implementing known mods (wampler)

ACA to PSA mod is mandatory.
D6-D7 diodes are out.
Removed C28
subbed all 1µ electros in the signal path with non electros (C3, C4, C12, C15, C32)
C30 is now a 47n from the original 68n

tried to implement the mids pot, mmm I don't think it's that useful...
tried to remove D8-D9 or put leds instead of Si - mmm nope, sound is bigger but I don't llike how the notes fade out.

output buffer removed, wire #6 is wired to q10/r48

For now, the main mod is the removal of C28, it give a whole different sound at the end of the high pot. and yeah those germ diodes out.

I'll report other mods if I find something new...

Just want to chime in to save any other potential modders a headache. The 'remove the output buffer,wire #6 to Q10/R48 is nonsense. I did it and while there was a slight improvement in the tone, all my true bypass pedals had footswitch 'pop' downstream. Not recommended....