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Desoldering Boy
Copper Member
Pakistan
23 Posts |
 Posted - 03/01/2009 : 22:47:32
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I've already modified this MT-2, and I'm just going to test it, without assembling it... It 100% works! no problems at all, but suddenly, the body of my DC jack touches the chassis of the MT-2 there was a sudden arcing, until the MT-2 died.(note: my DC jack has no protection at all, the + and - terminals were exposed, which it can touch the chassis of the pedal)
The Problem: When the Mt-2 is not engaged, there is a bypass tone, and when it is engaged, the LED responds, but the sound output doesn't change at all! I only hear the bypass tone...
I've listened to the bypass tone carefully, it has no dirt, but there is a sudden squashy tone, not a distorted tone done by the MT-2 but it has something to do with a transistor affecting the clean tone, I've compared it to the clean tone of my amp, and the difference is when i strum harder with the MT-2, the bypass sounds seems to be a bit squashy.
Circuit analysis: At first, I did some resistance measurements at the transistors, almost all of them are ok, there are some doubtful, but i've replaced them with another one coming from my working MT-2(yup, I have a spare of MT-2 here, 100% working) tested the pedal, and there's no improvement, I've also tested my another MT-2, and there's nothing wrong with a transistors from the broken MT-2.
I've decided to measure the voltages across each pin of the OPAMPS, first, I did it to the working mt2.. most of the pins, reads 4.5V,2.2V.6V, almost all of the opamps has there readings..
so it's time for the broken MT-2, I've measured the voltages across each pins, and there's a big difference! most of the pins read 8.2V!
I did not change the OPAMPS yet, but I started to analyze the voltage source for the transistors and OPamps... at first, I've checked the source of 4.5V (there is 9V and 4.5V source for the mt-2 right)
If i'm correct, the 4.5V came from a voltage divider circuit(two 10k resistors), when I've tried to measure the voltage across R57, instead of a voltage reading of 4.5V, the reading was a freaking 9V!!!(note: I always compare it to the working MT-2)
so I decided to open up the J20(jumper) when I did that, and measured the voltage again across R57, the voltage returns to 4.5V! and I think there's a 9V source, tapping on a 4.5V source(if these are correct)
So do you have any idea on how to solve this? what will be the particular culprit here which can affect the 4.5V source, and made it to 9V? |
Edited by - Desoldering Boy on 03/08/2009 04:18:55 |
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Desoldering Boy
Copper Member
Pakistan
23 Posts |
Posted - 03/02/2009 : 15:35:52
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| Question: What will happen if one of the dual opamps of MT-2 got broken? please elaborate it in terms of voltages, does the 9V supply voltage comes across the + or - feedback of the opamp? |
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Laurie
Double Platinum Member
Canada
4854 Posts |
Posted - 03/02/2009 : 17:27:14
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G'day Desoldering Boy!
I received your PM about your MT-2 problems... I can help for sure! I'll read through the symptoms tonight and see what advice I can provide. |
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Desoldering Boy
Copper Member
Pakistan
23 Posts |
Posted - 03/02/2009 : 22:08:47
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Thanks Laurie :)
will wait or your reply.
-------------------------
what if I pull out all of the ICs/opamps? just thinking if the output voltage from the r56/r57 v.divider will supply a normal 4.5V again, and if that happens, maybe, one of the opamps are broken? |
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Laurie
Double Platinum Member
Canada
4854 Posts |
Posted - 03/03/2009 : 15:03:03
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Hi Desoldering Boy! Some thoughts about your questions in PM...
The Problem: When the Mt-2 is not engaged, there is a bypass tone, and when it is engaged, the LED responds, but the sound output doesn't change at all! I only hear the bypass tone... This is almost certainly caused by the 4.5V rail failing. Without that rail, the FETs won't switch, so even though the LED changes, the signal won't.
There is 9V and 4.5V source for the mt-2 right? Kind of. The supply is actually 9V. The 4.5V rail is a reference voltage (rather than a "supply" rail).
I've measured the voltages across each pins, and there's a big difference! most of the pins read 8.2V! When I've tried to measure the voltage across R57, instead of a voltage reading of 4.5V, the reading was 9V! That's consistent with a failure that affects the 4.5V rail. Probably an opamp
I decided to open up the J20(jumper) when I did that, and measured the voltage again across R57, the voltage returns to 4.5V!
Sounds like one (or more) of the opamps has been fried - seen this a couple of time. Quickest way to check is to swap the known good ones from your donor pedal into the dead pedal - one at a time. When it starts working again, you have found the culprit (it may well be that more than one opamp has died).
Replacement opamps if you need them can be purchased here: http://www.smallbearelec.com/Detail.bok?no=578 |
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Desoldering Boy
Copper Member
Pakistan
23 Posts |
Posted - 03/03/2009 : 22:30:45
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Thanks Laurie,
yup, I've pulled out IC3 and IC4, and compared its resistances together with my extra opamps(sadly, those are DIP type, I need to improvise an adapter for it) and found out that the IC3 was broken, do you think IC1 and IC2 might fail also? or they will not, because, IC3 acted as a fuse wherein, when it got broken, the rest of the OPAMPS were not affected? |
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Laurie
Double Platinum Member
Canada
4854 Posts |
Posted - 03/03/2009 : 22:32:30
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quote:
Originally posted by Desoldering Boy
Thanks Laurie,
yup, I've pulled out IC3 and IC4, and compared its resistances together with my extra opamps(sadly, those are DIP type, I need to improvise an adapter for it) and found out that the IC3 was broken, do you think IC1 and IC2 might fail also? or they will not, because, IC3 acted as a fuse wherein, when it got broken, the rest of the OPAMPS were not affected?
Unfortunately... there is no way to tell how many were damaged without replacing them. If it was on my bench, I'd replace them all because if they aren't actually broken, they were for sure "stressed" by the accident.
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Laurie
Double Platinum Member
Canada
4854 Posts |
Posted - 03/03/2009 : 22:36:08
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quote:
Originally posted by Desoldering Boy
sadly, those are DIP type, I need to improvise an adapter for it
You can buy adapters here: http://cimarrontechnology.com/so8dip8tosiladapterpn060301.aspx
Or almost any 8-pin SIL dual opamp will work (eg. 4558L). Just check the pin-outs are the same - many of the SIL opamps are pin compatible. |
Edited by - Laurie on 03/03/2009 22:36:44 |
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Desoldering Boy
Copper Member
Pakistan
23 Posts |
Posted - 03/04/2009 : 14:06:33
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I will update this soon as i've replaced the opamps of mt2.. thanks!
so ESD is very critical to opamps? :( |
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Laurie
Double Platinum Member
Canada
4854 Posts |
Posted - 03/04/2009 : 14:16:09
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quote:
Originally posted by Desoldering Boy
I will update this soon as i've replaced the opamps of mt2.. thanks!
so ESD is very critical to opamps? :(
ESD is not so much of a problem... the "sudden arcing" has put some sort of voltage spike onto the 9V supply rail. Maybe a reverse voltage spike? (to quote Dr. Bob, "opamps hate reverse voltage")
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Desoldering Boy
Copper Member
Pakistan
23 Posts |
Posted - 03/04/2009 : 22:42:08
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now I know... I'll keep posting here soon as I've replaced those Opamps.
Thanks! |
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Desoldering Boy
Copper Member
Pakistan
23 Posts |
Posted - 03/08/2009 : 04:17:10
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Laurie, after changing the opamps (IC4 and IC3 only) the pedal is now responsive, I've checked the tone control, and it works well...
the only problem is, the distortion sounds splatty, when you stuck a palm muted string, it does a sustain, but it cuts off on a certain time, it ends up like farting...
What seems to be the problem here? TIA |
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Laurie
Double Platinum Member
Canada
4854 Posts |
Posted - 03/08/2009 : 05:53:37
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| What part number did you replace the opamps with? Do you have enough opamps to replace the rest of the opamps? |
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Dr. Bob
Moderator
Australia
6593 Posts |
Posted - 03/08/2009 : 05:58:03
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Hi Desoldering Boy
Farting as you call it, is a very accurate description of the sounds produced when an audio signal from a guitar is being clipped as a you would get from "Half wave rectification.
splatty - is the other nicer, but less descriptive tone.
I would try changing all the op-amps, and see if that fixes your problem.
The others may have been just partially damaged, & producing the 1/2 wave rectified sound.
From what I can make out in your posts, your MT-2 had reverse or-and over voltage applied to it.
The Fating sound is common on poorly designed valve preamps-overdrives-distortion, that are running in low voltage/starved Plate mode, as some crappy tube pedals do.
I have seen Plate/Anode voltages of 12VDC and even as low as 6VDC.
The problem in simple terms, in these starved plate topologies,is that the voltage on the Grid approaches, or goes more positive than the Plate,
the Triode effectively, for a short period of time, becomes a diode, and half wave rectifies the signal.
That's when you will hear the farting sounds.
Believe it or not  , some guys/bands like this sound, sheck out the splatty farty mis-biased sounds you get from some of the ZVex pedal range. 
I'm guessing that, the internals of the other op-amps were damaged, remember that you just replaced (2) of the other op-amps to get it to this SORT-OF working stage.
Good luck
keep us informed of your progress.
Regards Dr. Bob
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Desoldering Boy
Copper Member
Pakistan
23 Posts |
Posted - 03/08/2009 : 08:10:09
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I've already solved the problem
I've looked up on the schemtic, analyzing it, the clipping diodes are responsive, which made me think that there's no problem on the input signal, as well as the OPAMPS that controls the EQ, (IC1 and IC2) which also made me to decide, not to touch those Opamps, so I checked the IC4 and IC3, my problem yesterdays, was to determine, which of the two are the defective one, I have a spare opamp here(NE5532) measured its resistances and compared it to the pulled out Opamps(IC4/IC3), i've replaced the IC3, because I thought that was the defective one(IC4 resistance properties were similar to my spare opamp.
But the big pak pak boom was, my spare opamp, was also a defective one!!! I made a wrong choice(therefore, IC4 was the defective one, and IC3 is normal) I've discovered it when I tried to pull out a JRC4458 on my another trashed pedal, checked its resistances(on every pin, referenced on PIN1) and discovered that, its similar so the IC3 I've pulled out(at first I thought that, every opamp i pulled made it fried by the soldering iron. To make the story short, the pedal is now completely working.
thanks Laurie and Dr.Bob.
I hope that I would not encounter another problems on MT-2.(but if theres any, I will post it here again.) |
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Dr. Bob
Moderator
Australia
6593 Posts |
Posted - 03/08/2009 : 08:26:23
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Hi Desoldering Boy
That's great news.
It's always good to hear that another person,
has saved-resurrected another Boss or other-brand pedal from deaths doors.  
Regards Dr. Bob |
Edited by - Dr. Bob on 03/08/2009 08:46:53 |
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Another MT-2 problem :( (updated)
