Help with fault diagnosing, Boss MT-2

Thread Starter

gface83

Joined Jul 16, 2016
83
H i Guys

I'm hoping someone can point me in the right direction with diagnosing a fault on a guitar pedal. The pedal is working as far as I can tell, but, the output is barely audible. I have measured some voltages on the board which are correct, i.e. the supply voltage to the chips etc, however I have noticed that the negative supply to the op amps are not right. When the pedal is plugged in and powered up both negative and positive supplies to the chip are there, but the negative supply is showing a positive voltage. when I then connect the input jack I don't get any voltage on the negative pin of all the op amps. Can someone please take a look at the schematic and tell if this seems correct. I have checked the datasheet for the op amp and it says it can be run as low as +-2v and up to +-7v, I am reading 8.7v on the power pins off a 9v power supply. I don't know if these readings are anything to do with the issue of the output been barely audible.
So my questions are as follows:

1. Where in the circuit is the negative supply being generated?
2. Does the issue of it being barely audible suggest an input amplification stage or an output stage? and could you point my in the direction of where this is likely to be in the circuit. I'm guessing its an op amp issue, but could be transistor problem.

Also the schematic has labeled the op amps as 1/2M5218AL, but my pedal is using NJM14558L and NJM4558LD. Maybe they updated them at some point.

Any help would be greatly appreciated.

Thanks
 

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ArakelTheDragon

Joined Nov 18, 2016
1,327
It will take time to review your circuit(normally circuit reviewing without a description and proper documentation is a NO).

The problem comes from the output voltage or current being lowered by something. If its the voltage, then start by investigating the opamps, if its the current start by investigating the transistors. The opamp can sustain "+-7V" or "0VDC" and "14VDC". Also how did you find out which pin is the negative power supply? Is it possible to trace the voltage from the output to the power supply?

Is it possible to measure the voltage on pins U+, U-, Power+, Power- and Out for every opamp under question?
Right it like this please:
U+ = +5VDC
U- = -5VDC
Power+ = +5VDC
Power- = -5VDC
Uout = +15VDC

How do you measure the voltage, with a multimeter or osciloscope?
 

Thread Starter

gface83

Joined Jul 16, 2016
83
Hi Thanks for the reply

To Start off, the circuit is a distortion pedal for a guitar. Its not my design, its by boss (Roland). I acquired the pedal with the fault and decided to try and fix it, but its a bit beyond my experience, hence the post.

How can I determine if its voltage or current. All voltages I have measured so far are correct except for the negative supply voltages on the op amps. They are 0V. I found out the negative supply pin on the op amp by downloading datasheet. Its pin 4. I probably could trace the voltage from the output to supply but will take some time to do as everything is tightly packed on the board. I will have a go.

Yes, I will have a go at measuring the pins and get back to you. When you say U+ and U-, what do you mean? I have measured the power pins on each op amp and all are the same, I mentioned the power pins and voltages in the first post. I haven't measured the outputs yet, but is it worth it without a signal going through the circuit?

I used a multimeter to measure the voltages but I have got an oscilloscope to.

Thanks
 

Dodgydave

Joined Jun 22, 2012
8,867
Your circuit uses one 9V battery, and has a split 4.5V rail for the op amps, your signal in and out uses the full supply ground.
 

Thread Starter

gface83

Joined Jul 16, 2016
83
Hi, Thanks for the reply. Ah right OK, I have measured the point at which the the supply is divided (if I recall correct, its R56 and R57), but That only seems to be +4.5V. Where is the negative supply being generated? Are you saying that full signal out should be 9V peak to peak with -4.5V, Gnd, +4.5V?

Thanks
 

Thread Starter

gface83

Joined Jul 16, 2016
83
Hmmm, I don't think I fully understand what your'e asking. Ive been testing the circuit with a multimeter. Black lead goes on the ground of the power supply and I'm using the red prob to test the voltages. The results I got were in the original post. Do you mean switch the leads around?
 

ArakelTheDragon

Joined Nov 18, 2016
1,327
Hmmm, I don't think I fully understand what your'e asking. Ive been testing the circuit with a multimeter. Black lead goes on the ground of the power supply and I'm using the red prob to test the voltages. The results I got were in the original post. Do you mean switch the leads around?
If you have a "9VDC" on the input, it can be split inside of the circuit to "-4.5 and +4.5" in order to generate an AC signal(audio sugnal) with a maximum peak to peak amplitude of "-4.5VDC to +4.5VDC".

Measuring the output of the amplifier right before the output connection(wire, jack or whatever it is) will give you the output voltage and current. If you want to measure the current, you must connect the osciloscope in series, if you want to measure the voltage, you need to connect the osciloscope to the point you want to measure and ground(parallel).

You said the amplifier is weak, that means from it at the output you get a weak signal, measure its voltage and current that goes to the next device on the line and see whats going on. Are you sure this is not just a pre-amp that requires a second amplifier to make the signal stronger?

EDIT:
The opamps amplify the voltage, if you want to amplify the current you need transistors. After measuring the output voltage and current you will know which one is lower than expected and if its not the voltage than the problem does not come from the opamps.
 

Thread Starter

gface83

Joined Jul 16, 2016
83
Sorry for the delay had a busy day yesterday.

Ok, I am aware of everything you're saying in your first paragraph. Ive built a few analogue synthesisers that run off a dual supply that use an AC wall wart. That AC signal then gets rectified to provide both negative and positive DC voltages. When I look at the schematic for this circuit I can see where 9v starts, I can also see where that 9V signal gets halved by the voltage divider, providing 4.5V. Both of these voltages are represented by a black or white triangle connection point in the schematic. What I can't see anywhere is the -4.5V. I don't think this is anything to do with the issue of it being a quiet output signal, but, I wanted to know where I can actually measure the -4.5V in the circuit. Preferably at its source, but, I cant see where that is in the circuit.

Ok I will measure the output signal as you have instructed and report back with the results.

Also, yes, this is only a pre amp circuit and requires an guitar amplifier on the output which is what I have been doing.

Thanks
 

Thread Starter

gface83

Joined Jul 16, 2016
83
Ok. Ive just noticed that pin 4 of the op amps on the schematic are connected to ground. So can someone explain why these op amps don't need a negative supply? I know some op amps only require a single rail but the datasheet for these says its needs a negative supply.
 

Thread Starter

gface83

Joined Jul 16, 2016
83
Right, Ive measured both input and output voltages and the results are:

Input; approx 9V PP
Output; approx 100mV PP

So, it seems it's definitely a Voltage issue. Ive measured the power pins of all the op amps and they are correct. V+ = 8.9V and V- = 0v.
Shall I just replace all the op amps or is there a particular one on the schematic that's likely to be the issue?

Thanks
 

ArakelTheDragon

Joined Nov 18, 2016
1,327
Its not possible all the opamps to be defective at the same time. Its possible the voltage is halved for some reason and they only use "4.5VDC" as the intended output.

If this is a pre-amp shouldn't it require a second amplifier in order to be not quiet?

Please measure only the output pin of the opamps. All opamps have at least 5 pins(2 power supply, 2 input, 1 output), sometimes in 1 IC all the power supplies are connected so there are 2 power supply pins for the whole IC.
 

Thread Starter

gface83

Joined Jul 16, 2016
83
Yes, I agree its probably not all the op amps, I just thought it might be easier to replace them all as there is only 4 of them.

If you re-read what I said about the pre amp, I said that I have been connecting it to a guitar amp after the pedal, so that is not the issue.

I will check the output pins and report back.
 
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