Boss - HM-2 Heavy Metal  [schematic]

All about modern commercial stompbox circuits from Electro Harmonix over MXR, Boss and Ibanez into the nineties.
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slammer88
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Post by slammer88 »

Hi again.

I think I'll recover this project which I've made years ago and threw to the depths of my cabinet and never used ever since, and just wanted to thank GodSaveMetal for his great job. May he rest in peace..

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slammer88
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Post by slammer88 »

Hi,

I re-made the pedal with a better etched board and an enclosure printed on 3d, but there's a problem; it doesn't work :)

Thing is, I lost R54 (3.3k resistor) to the carpet monster and while mounting, one of the germanium diodes split into half and I replaced a silicone one instead, however, I could not put anything instead of R54, only resistors I have are around 100-150ohm for LED's and there's a holiday here and I can't get one.

Traced everything one by one, the board seems to have no problem, no short circuits. I put the parts into the very same places that I removed from the old one.

Any ideas?

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FiveseveN
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Post by FiveseveN »

You're gonna have to be a bit more specific than "it doesn't work".
Voltages? Audio probe? Does it smell like magic smoke?
Ignorance more frequently begets confidence than does knowledge. (Charles Darwin)

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Intripped
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Post by Intripped »

R54 is on the signal path, so without it the pedal would be mute, if activated.

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slammer88
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Post by slammer88 »

Intripped wrote: 02 May 2022, 16:46 R54 is on the signal path, so without it the pedal would be mute, if activated.
Hi,
Sorry for not giving much info, you're right, I didn't want to go deep further as this resistor is missing it would be kind of useless to look for a problem, once I soldered it I'll give it a go and see how it goes..

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slammer88
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Post by slammer88 »

All right, all's good now. Noticed that I soldered a polarized capacitor in reverse, fixed that, mounted R54's 3.3k resistor and fixed a cold solder on volume pot's molex side, all's good and working now.

Time for some improvements I believe. Wish I'd put female pins for every hole in the board, so that I'd toy around in every stage of the circuit :)

Another question is, I've seen that some people changed GE diodes with regular LED's. Is it a thing really? :)

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slammer88
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Post by slammer88 »

Hi again.

Seems i need some boost around 1k to catch that real chainsaw sound, like around 7-8 dbs. When I do that from Amplitube, it sounds perfect, however, in default, mid-scoop seems weak. Any ideas?

Ok some more addition; seems like lowering the 31-62hz around 2-3db, 500hz -7db and boosting 1k around 7db gives the sound what I want (what it should be)

Right now, it sounds bassy, a bit muddy and not mid-scooped that much. I also noticed that the project GodSaveMetal (r.i.p) made has some different parts from the original schematic and I don't know why. Here are the differences;

C35 - In original, its 1.5uf tantal or electrolytic - In layout its 1uf
C28 - In original, its 0,15uf tantal, in layout its 0.33uf
C29 - In original, its 0.1uf tantal, in layout its 0.33uf
C27 - In original, its 6,8nf, in layout its 5,6nf

Also, some capacitors have 50V limit, some has 16V, I don't know if it makes a difference.
Besides, I don't know how above mentioned changes would affect the final sound.

Mod's I've done so far :
Changed D8 and D9 to LED diode,
Changed C6 from 47n to 1uf (2nd message I guess, for bass response)
Changed C15 to 1uf non-polarized capacitor,
All IC's are 5218, tried 5532 and 4558 but 5218 sounds best dynamically.
Lifted leg of C28, gave a bit more treble it seems, but I don't know if I should connect a 3rd pot to it for mid range.

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Post by Rievezahl »

slammer88 wrote: 09 May 2022, 10:55 I've seen that some people changed GE diodes with regular LED's. Is it a thing really? :)
This is hard for me to believe. You can do this with the hard clipping diodes, but not the germaniums. Germaniums have forward voltages around 0.2-0.5V, while LEDs are more like >1V. Actually my "true" 1n60 germaniums range up to way higher Vf's, but those are not usable. I did some testing regarding GE substitutes. Anything with Vf >0.5V does not sound pleasing at all. Too large crossover gap gives you crackling and cutoffs like a noise gate as far as I remember. You can however go with some shottkys, you get them in or close to the germanium range of Vf's. I don't really hear much difference with my "fake 1n34", 0.25V shottkys. I have a Video ready for posting on Youtube comparing shottky vs germanium in my clone of this pedal. I can post it there and hope I'm able to put a link here then - given your request!

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Post by slammer88 »

Rievezahl wrote: 25 May 2022, 15:56
slammer88 wrote: 09 May 2022, 10:55 I've seen that some people changed GE diodes with regular LED's. Is it a thing really? :)
This is hard for me to believe. You can do this with the hard clipping diodes, but not the germaniums. Germaniums have forward voltages around 0.2-0.5V, while LEDs are more like >1V. Actually my "true" 1n60 germaniums range up to way higher Vf's, but those are not usable. I did some testing regarding GE substitutes. Anything with Vf >0.5V does not sound pleasing at all. Too large crossover gap gives you crackling and cutoffs like a noise gate as far as I remember. You can however go with some shottkys, you get them in or close to the germanium range of Vf's. I don't really hear much difference with my "fake 1n34", 0.25V shottkys. I have a Video ready for posting on Youtube comparing shottky vs germanium in my clone of this pedal. I can post it there and hope I'm able to put a link here then - given your request!
Im all ears for any ideas.

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Post by Rievezahl »

Hi, here the promised video. Sry, took me some time as it wasn't as prepared as I remembered it. What you are hearing is raw device output, no amp, no ir, no speaker, no simulation nothing, just out of the "pedal" into the audio controller. Seems like the audible differences come more from my inconsistent playing than anything else.

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Post by slammer88 »

I made some tests yesterday.

Seems like, when I connect the lugs of hi pots' 1st lug to low's 1-2 and lo pots' 1st or 2nd lug to vol2, I get the real chainsaw sound, no humming, pure raw distortion, but this renders the hi or low pot useless. I don't know if there's something wrong with the layout Godsavemetal did, but i'm pretty sure there are no cold solders or broken/misconnections around. Without this, it sounds so bassy and muddy. I'll try to create a short video for that.

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Post by slammer88 »

Ok, I seem to find the real sound out of this circuit. Removing the C28 helped that. Tried some other capacitors there (soldered 2 empty female connectors to quickly interchange but leaving that connection empty seem to give the authentic sound that I want.

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Post by Umulamahri »

slammer88 wrote: 25 May 2022, 17:38
Rievezahl wrote: 25 May 2022, 15:56
slammer88 wrote: 09 May 2022, 10:55 I've seen that some people changed GE diodes with regular LED's. Is it a thing really? :)
This is hard for me to believe. You can do this with the hard clipping diodes, but not the germaniums. Germaniums have forward voltages around 0.2-0.5V, while LEDs are more like >1V. Actually my "true" 1n60 germaniums range up to way higher Vf's, but those are not usable. I did some testing regarding GE substitutes. Anything with Vf >0.5V does not sound pleasing at all. Too large crossover gap gives you crackling and cutoffs like a noise gate as far as I remember. You can however go with some shottkys, you get them in or close to the germanium range of Vf's. I don't really hear much difference with my "fake 1n34", 0.25V shottkys. I have a Video ready for posting on Youtube comparing shottky vs germanium in my clone of this pedal. I can post it there and hope I'm able to put a link here then - given your request!
Im all ears for any ideas.
Those GEs are pretty much just crude implementation of a noise gate. Changing them for higher Vf type would be only beneficial if you wanna use it for "start-stop" riffs with lot of pauses. If you want something cool to try with this circuit - tune that low bandpass to match the actual tuning that you use, stock is around low E. Cheers.

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