Personal Project: Hacking an Obscure Boss Distortion (help requested)

[somnif]

Story Time!

A local used book store here in town has an extensive used (and new) instrument section, and being the nerd that I am, I tend to hang out at the pedal case and look to see if anything interesting has come in. I've made friends with the old guru who works the desk, and we got into a conversation a while back about odd, obscure Boss pedals.

He told me one of his favorite distortions was the MZ-2 "Digital Metalizer", an odd effect that spat a distortion into a digital delay/chorus circuit with stereo output. He didn't care much about the delay side of things, but the distortion was "just awesome". I decided to dig up the schematic to look at it. Turns out the distortion is a discrete analog circuit in front of the delay chip, and could in theory be chopped out and used on its own. I took up the task.

Finding the schematic was easy, but its one of Boss's more messy service manuals. Tons of JFET switching going on, odd voltages, part values not matching gut shots, all that fun stuff. I simplified matters by going Mono with it.

Basically, I would love a few new sets of eyes to look at my attempt, and see if anything really glaring stands out as wrong.

Things I am most concerned about:
1) On either side of the JFET switches, there is a 100k resistor to 4.1V ( represented by R4.5 and R34.5 on my schem R4 and R5, R34 and R35 on Boss's). If cutting out JFET switching, would this stay at 100k, drop to 50k still at 4.1v, or 50k at 8.2V (I know parallel resistance, but don't know what to do in regards to the voltage).

2) Why on earth were 8.2V and 4.1V used. The power supply is strange (to me anyway), using a couple transistors and a zenner to drop the voltages. 8.2V after the first tranny, 5V after the second tranny with its zenner.

3) Voltage on the non-inverting input on the output buffer op-amp. Given how this pin is set up, I'm not sure if I should drive the 4.1V via the 100k resistor and 47nF capacitor (from the "Direct" line) or via the 220k resistor coming off the stereo output. Another case of me not being sure how to deal with the voltage having two inputs.

4) ...anything else I have likely missed really. My insomnia has been kicking my ass the past couple weeks and this mostly has been something to keep me occupied, but it does mean my eyes are a bit blurry and my mind not at its sharpest.

I have tried to match part numbering and values to the Boss documents. Unfortunately given its an effect built around 3+ PCBs, some parts have the same part number, so I have amended those indicate (adding a "p" if its in the power supply, "vr" if its on the pot board, etc).  Most of the distortion circuit is in SMD form, and as such Boss's schematic doesn't actually match. I have dug through Toshiba documents and found what I think are the most likely parts-on-board based on a gut shot and the package markings. Unfortunately the resistors are too small to read values in this picture, so no idea if those values were altered.



(Also included as a PDF attached below)

(As a note, judging by clips I've found on youtube, the distortion is a bit harsh to my ears. But I'm bored and can't sleep, so what the hell I'll give it a shot.)

[Scruffie]

Q6 in the power supply is a capacitance multiplier, it makes that 10uF look like a massive capacitor (it's simply xhFE IIRC) this will have been to reduce noise especially when there was digital stuff going on in a high gain pedal, the transistor happens to drop 0.6-1V by nature so the supply is probably really 9V instead of 8.2V from a 9.6V supply. The 5V, probably just for limiting the headroom, distortion not being about headroom after all.

The output opamps, I'm just looking quickly but it looks like as it was stereo one was used as a diff amp and the 47n/100k is audio signal and related to J-FET switching, long story short, just bias it with a 220k resistor to 4.5V.

Nothing is glaringly wrong but I'll look it over further later.

[somnif]

Oh one other thing. In the boss schematic the JFETs are drawn symmetrically. I am guessing therefore it doesn't matter where how the drain and source are oriented, but the datasheets don't have any indication about it. (What little I know about the construction of JFET meshes with this idea, but I'd rather have conformation before I go much further. Very little self confidence!)

[somnif]

I'm wondering if it would just be easier to redesign the power supply section just for space/part numbers sake. Cut out the transistors, use the filtered 9V on its own and divided to 4.5. Not sure if I want to use the Zener or a 7805 for the 5v/2.5V though. I've never worked with a zener regulated circuit, and reading up makes it seem somewhat touchy.

[Scruffie]

The power supply has been designed for you by BOSS engineers, why change it?

[JC103]

BUMP! The distortion is awesome, I've been waiting for someone to agree with me. Yes, forget all the digital stuff... it's not great, and clone the distortion. Through a 100w EL34 head you can sound like EVH with this pedal. It is very plexi sounding in response and tone, almost has an opamp BMP flavour to it with a lil' HM-2 thrown in.

[JC103]

I'll do a youtube demo eventually this summer. I use the MZ-2 as a mono distortion before the CE-3 stereo chorus split then run it in stereo to 2 MIJ DD-3's and then into a pair of 1969 Traynor YBA-3 Custom Specials and a stereo 4x12.

[somnif]

Next question is what do I do about these capacitors. All the 1uF polarized caps on the analog board are SMD tantalum. Do I move to aluminum electros, big fat film caps, tiny little MLCCs, or keep them as through hole tant's.... choices choices.

[JC103]

You may want to cross reference this project with the Metal Zone MT-2 . It's possible there are some similarities (sans mid control). Electric Druid has a decent break down: https://electricdruid.net/boss-mt-2-metal-zone-pedal-analysis/

I can help when it comes time to compare your results with an original Boss MZ-2.

Just to confirm, this is the smd distortion pcb?

[somnif]

Yep, that's the analog bit of the pedal. The power supply, pots, and digital stuff is all on separate boards. The chips labeled "XG" are the Jfets, the JY's are the switch fets, and lots and lots of jumpers.

[somnif]

Funny enough, the circuit isn't too close to any of the Boss distortions I've seen schems for. Closest I've found is actually the OD-2 overdrive, weird as that sounds. Not identical, but similar.

[Scruffie]

Yeah it's nothing like the MT-2 or HM-2, it will look similar to the BD-2 though as that and the OD-2 are both using discrete op amps.

There's no gyrators or even clipping diodes so all the clipping is 'op amp' based, it's basically high pass op amp gain, low pass filter & high pass filtered dry blend, op amp gain, filter, tone and output.

Capacitors, I wouldn't worry about the construction at all, just use whatever you prefer, judging by when this was made I would think the use of tantalum was probably a cost or size concern for SMD rather than tone.

[somnif]

Well, already caught one mistake on the schematic. I'd accidentally ended up with all 4 power rails having the same name. THAT would've made for an interesting circuit. (Might've simplified the routing on this bloody board though....)