Need help debugging noisy BOSS MT-2

[Andreas]

Hi all,

I've been reading along on this forum and trying to learn for some time, but this is my first post. I recently bought a Boss MT-2 Metal Zone in dead condition in the hope that I could fix it up and use it to try some different mods on. So I quickly discovered that the protection diode was fried and had actually taken part of a pcb trace with it, the one carrying the negative voltage from the dc jack. So after fixing those two things the pedal worked and after playing with it just a little I did this set of mods to it:

http://members.shaw.ca/lauriepedals/Stormchaser.pdf

I basically did steps 1 - 7 but used BS170 mosfets for the clipping as that's what I had - they have a different pinout from the 2N7000 specified but I accounted for that.

So the pedal still works and actually sounds good, but is excessively noisy with a loud hiss that is just about tolerable with all controls at or below 12 o'clock but makes the pedal useless if level or gain is turned up any higher. With level, gain and all the eq knobs turned all the way up the hissing is loud enough through my little practice amp to drown any conversation in the room. Unfortunately I didn't test the pedal very thoroughly before doing the mods so I don't know if it was noisy before the modding.

So some helpful folks at the bossarea.com forum suggested that one or more of the opamps might have been damaged by whatever initially fried the pedal. So i set to audio probing the opamps and found that the noise originates at the very first opamp stage (3b) shown here:



The hiss cannot be heard on pins 5 or 6 but is quite loud on pin 7, the output of the opamp stage. So I pulled the IC (an NJM4558LD in SIP )out and put in an RC4558D in a makeshift SIP to DIP converter but the noise was the same. I then tried an OPA 2134 but there was still hiss. I then lifted C033 so there is no input to the opamp stage but the hiss remains.

So right now I'm trying to first of all understand the gain setup of the opamp. I am still struggling with these things but here is what I think is going on:

According to

http://sound.westhost.com/dwopa.htm#noninv

the gain of a non-inverting opamp is

Av = (R1 + R2) / R2

where

R1 = resistance between output and inverting input

R2 = resistance between inverting input and signal ground

So in this instance

R1 = 220K

R2 = 220K + 10K +10K = 240K

Av = (220K + 240K) / 240K

Av = 1.92

So if this is correct and the gain of this stage is just under two, surely it is not normal for there to be audible hiss on the output? Can anyone suggest what might be causing the hiss or other things I might investigate?

The complete schematic is here btw

http://www.hobby-hour.com/electronics/s/boss-mt2-metal-zone.php

Thanks,

Andreas

[PRR]

> R2 = 220K + 10K +10K = 240K

The R044 220K was already accounted.

Ignoring a major complication, and asserting the caps act "short", and taking R053 || R054 but too large to matter much,  I see R046 as 2.2K and R054 as 10K..... where do you see another 10K?

Still using blinders, we have R1=220K and R2~~12K, gain of almost 20. Yes, this may hiss.

The actual operation is more complicated. If R053 clamped the transistor Base to audio ground, then the effective impedance at transistor Emitter is very low, a few hundred ohms. Therefore R2 is more like 2K-3K and stage gain is more like 70. This sure will hiss.

The caps are not dead short. Skipping a semester on filter theory, this is an "active tuned circuit". For more fun, drawn sideways. It has a very low impedance near 800Hz, higher at other frequencies. The bottom impedance is nearly the resistor R046, 2.2K. So yeah, the gain in the middle of the audio band is 101. It should be very strong midrange (signal OR hiss), weaker in bass and treble.

> used BS170

I don't know that this is equivalent to 2N7000, especially when used as diode.

> might have been damaged by whatever initially fried the pedal.

Good thought.

> I didn't test the pedal very thoroughly before doing the mods so I don't know if it was noisy before

Bad plan.

Between the blow-up, the repair, the mods, and the MOSFET change, debugging will be A Problem.

Is there any chance of un-doing this (admittedly complex) mod to check the "stock" 9fixed) performance?

Have you taken it in bright light to look for dubious joints or solder bridges?

[Andreas]

First of all many thanks, your reply was very helpful, if nothing else it showed me how much I still need to learn. I think where I went wrong was that from reading opamp tutorials I had come to see caps as a sort of "boundary" for the gain stage so i only looked inside C033, C034 and C029 and the only path to ground from the inverting input I could find was through R044+R045+R042. But I see now what you are saying about C034, R046 and R054. What I guess I don't understand is why you would have so much gain and hence hiss so early in the circuit that will get amplified by all the later stages. I can think of three scenarios right now

1. This is just the normal noise level of the pedal and I'm chasing ghosts here. Once you get the heavy metal thing going the noise doesn't matter 

2. The hiss is "normal" for this stage but is normally filtered out later in the circuit. Either the blow up or something I did while modding has caused this to no longer happen.

3. The hiss is not normal for this stage and is caused either by something being damaged by the blow up or by something I did.

Ad 1. I know someone who has a stock MT-2 which I can borrow next week to A/B with.

Ad 2. When I get the other pedal I should also be able to test this by audio probing pin 7 and comparing the hiss there with my pedal

Ad 3. this seems the most likely to me. The only part of the mod that happens in this section is to remove C035 and change C034 to 47nf. For starters I tried putting C035 back in and this does actually lower the noise on pin 7 or at least makes it appear more "muffled", but with all the later gain in the circuit the pedal is still excessively noisy in use.

Here's a thought - Q010 has its collector straight to V+, could it have been damaged by the reverse voltage or whatever else caused the blowup and be causing the hiss. If it somehow now makes the apparent R2 even lower it would drive the gain and hence the hiss through the roof?

I've inspected the board very closely for suspect joints, reflowed all solders that I've done and measured all resistances between the opamp pins and to ground for this gain stage against the schematic. I've also replaced all the electrolytics that remained on the board after the mod in case they were damaged.

I'll also get some 2N7000 for the clipping stage and replace the BS170. Thanks again for any help.

Andreas