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Greetings, folks. Haven't posted on music-electronics-forum in a few years, life has changed and I don't play much with amp repair lately. However, I messed with a Boss Hyper Metal HM-3 last night and I thought I would pass on my results. This pedal came to me several years ago but I never used it; my metal days are far behind me, and even at its lowest distortion setting this was a 1980's "angry box of bees" sound--at higher settings, think Swedish Death Metal. I wanted something more Tube Screamer-ish. This isn't a highly-demanded pedal so selling it would not have gotten me much cash, might as well try to play with it.
Schematic is posted below. First step: lift one leg of D5 and D6 (shunt clipping diodes). Still way over-the-top, too much gain. Hmm, from my fuzzy recollection of a Tube Screamer, isn't this pedal kinda the same topology as a TS? I found R. G. Keen's excellent writeup on the anatomy of a TS, and whadya know, this HM3 is very similar, only with an extra gain stage and a slightly more complex tone stack. First gain stage half of the HM3's IC can give gain from unity to about 240, second stage gives a gain of about 10. Let's make a simple change: reduce gain to about 1.5 in the second half by changing R12 to 33K.
Wham...huge difference. At lowest Dist level, a slight boost but still quite clean, increasing the level gives progressively more dirt until maxed out, when there's still some serious distortion. I call this mod a winner!
Oh, this is not SMT and is easy to work on, by the way; merely undo the four screws for the bottom plate and the PC board gently pries out.
Summary...lift one leg from D5 and D6 and change R12 to 33K and this becomes a nice boost/overdrive pedal.
Schematic is posted below. First step: lift one leg of D5 and D6 (shunt clipping diodes). Still way over-the-top, too much gain. Hmm, from my fuzzy recollection of a Tube Screamer, isn't this pedal kinda the same topology as a TS? I found R. G. Keen's excellent writeup on the anatomy of a TS, and whadya know, this HM3 is very similar, only with an extra gain stage and a slightly more complex tone stack. First gain stage half of the HM3's IC can give gain from unity to about 240, second stage gives a gain of about 10. Let's make a simple change: reduce gain to about 1.5 in the second half by changing R12 to 33K.
Wham...huge difference. At lowest Dist level, a slight boost but still quite clean, increasing the level gives progressively more dirt until maxed out, when there's still some serious distortion. I call this mod a winner!
Oh, this is not SMT and is easy to work on, by the way; merely undo the four screws for the bottom plate and the PC board gently pries out.
Summary...lift one leg from D5 and D6 and change R12 to 33K and this becomes a nice boost/overdrive pedal.
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