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Abaddon
Copper Member
Denmark
3 Posts |
 Posted - 03/17/2010 : 13:36:14
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Hi all,
I recently bought an MT-2 for real cheap in dead condition in the hope that I could bring it back to life and possibly do some mods to it. Well, it turned out that the pedal must have been subjected to some serious reverse voltage, the protection diode was complete burned to cinders, and a track on the pcb underneath carrying the negative rail from the dc jack was actually burned completely away. So I replaced the diode and fixed the pcb track and the pedal actually worked after that so I decided to do the Stormchaser mods to it which also went fine - I did steps 1-7 in the pdf from Laurie and used BS170 mosfet's in place of 2N7000 as that's what I had.
The pedal still works and I really like the sound of the mods, but it is now also extremely noisy. With all knobs at 12 o'clock or below into a clean amp the noise is just barely tolerable, but if you turn either the level or the distortion any higher it becomes very loud. If you dime all the controls including the eq the noise is enough to drown out any conversation in the room. It's basically a hissing noise with a bit of hum and swoosh in it. I must admit that I did not test the pedal very thoroughly after first fixing it, so I don't know if the noise was already there before the mods. I replaced all of the remaining electrolytics on the board thinking that they might have been damaged by the reverse power, and that _may_ have helped some but I'm not sure. Do you have any suggestions for what I could try / test / replace? or is this just the regular noise floor of the MT-2 
Thanks a lot for any help,
Andreas |
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Laurie
Double Platinum Member
Canada
4854 Posts |
Posted - 03/17/2010 : 14:18:31
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G'day and welcome!
The stormcahser mod doesn't really make the pedal any noisier than standard.
I'd say maybe you have a "stressed" opamp?
I'm also not sure about the BS170 MOSFETS... are they pin compatible with the 2N7000's? I guess I'm politiely asking if you have the right legs in the right holes? |
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Dr. Bob
Moderator
Australia
6593 Posts |
Posted - 03/17/2010 : 14:45:18
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Hi Abaddon
Welcome to the forum from Australia.
I was about to ask you about the pinouts of the BS170's verses the 2N7000's as well.
Do you know if it was excessive voltage or reverse voltage, that nukes the Diode & track?
Laurie is correct - Opamps absolutely hate reverse voltage applied to them.
Laurie & I see many nuked opamps that have been fried or really stressed from reverse voltage.
It's one of the most common things to look for, in pedals that hit our Lab bench for repair/servicing.
Regards Dr. Bob  |
Edited by - Dr. Bob on 03/17/2010 14:46:36 |
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zerksies
Double Platinum Member
USA
3406 Posts |
Posted - 03/17/2010 : 14:46:44
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Abaddon
Copper Member
Denmark
3 Posts |
Posted - 03/17/2010 : 16:52:20
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Hi and thanks for the welcomes and the suggestions. The bs170 are very similar to the 2n7000 which is why I used them but they do have a different pinout which I accounted for when I installed them
Also, I don't know for sure what happened to the pedal but as I understand it the only thing that will fry the diode is reverse voltage. Overvoltage or a shortcircuit between the + and - rails won't cause the diode to conduct any current and hence burn itself, unless it was in excess of its breakdown voltage which I believe is 50V for the 1n4001 and higher for the other 1n400x.
I know about opamps and reverse voltage, I've fixed a number of pedals with burnt diodes and dead chips before, but I've never come across one that survived but in a damaged / stressed condition. I guess I'll give the opamps a go with the audio probe and see if I can isolate the noise to one of the chips - do you have any likely candidates or could it basically be any one of them?
Again, thanks! |
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Abaddon
Copper Member
Denmark
3 Posts |
Posted - 03/18/2010 : 12:02:19
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Ok, possibly some progress. I've been poking the opamps with my audio probe and have determined that the hiss comes from the very first opamp stage, section B of IC3. There is no hiss that I can detect on either the non-inverting or inverting inputs (pins 5 and 6) but on the output (pin 7) there is lots of hiss. With a big bunch of hiss so early in the circuit which gets amplified in all the later stages it's not so strange that the pedal is very noisy. Here's what i've tried so far:
Took out the IC (an NJM4558LD) and put in an rc4558p via a makeshift SIP to DIP converter. The noise was the same. Tried a Burr Brown opa2134, still the same noise.
Measured resistances between pins 5,6 and 7 and they all check out against the schematic.
Put C35 back in (not the original cap but a WIMA 10nf) - this actually reduced the noise substantially, I'd say to about half or 1/3 of its previous level, but even with this the pedal is excessively noisy in use.
So do you have any idea why the output of the first opamp stage is so noisy and why putting C35 back in reduces the noise? I can't see any components around this opamp stage that would normally be damaged by reverse / overvoltage except maybe Q10 but that should be removed when C35 is taken out right? |
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cctsim
Silver Member
United Kingdom
418 Posts |
Posted - 03/19/2010 : 00:19:07
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quote:
Originally posted by Abaddon
So do you have any idea why the output of the first opamp stage is so noisy and why putting C35 back in reduces the noise? I can't see any components around this opamp stage that would normally be damaged by reverse / overvoltage except maybe Q10 but that should be removed when C35 is taken out right?
I would remove Q10, C35 and possibly R53 and see if the problem persists. |
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Help with noisy MT-2 after Stormchaser mod
