First pedal no output

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Valerio313
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Post by Valerio313 »

Hello everyone,

I'm new to the world of DIY pedals and just built my first kit "Musikding: The May"

The pedal works perfectly for the first 5 to 10 minutes, after which it starts buzzing intermittently until the sound is totally lost or extremely reduced. It feels like it stops working as it "warms up".

Switching the pedal off makes the effect true bypass and that's working perfectly, with clean sound from guitar to amp.

At the moment the back is open and the PCB is not in contact with anything.

How can I try and find where the problem can be?

Thanks

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Manfred
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Post by Manfred »

Check the solder joints.
Measure the DC voltages at the transistor terminals when the device is working and when the error occurs.
Here is the circuit diagram and the wiring diagram.
mayschematic.pdf
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maywire.pdf
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Valerio313
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Post by Valerio313 »

Thanks for the answer,

I'll check everything later today.

With DC at the transistor terminals, you mean Voltages between Base/Collector, Base/Emitter? How can I know the kind of voltages I should expect?

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Post by Manfred »

With DC at the transistor terminals, you mean Voltages between Base/Collector, Base/Emitter? How can I know the kind of voltages I should expect?
No, the voltages on the transistor pins in respect to ground.
In the case of the working circuit:
U_Base ~ 1.6V
U_Emitter ~ 1 V
U_Collector ~ 6V

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Valerio313
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Post by Valerio313 »

I did check voltages and they are correct with the pedal "cold", I strum my guitar for few minutes and when the sound disappears the reading are:
U_Base - 2.54V
U_Emitter - 1.89V
U_Collector - 2.88V

Checking the joints later and let you know if I get any change!

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Valerio313
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Post by Valerio313 »

I just checked all joints and they look "properly" done.

I checked all resistors during the error and R2 seems to drop to 18K, is it possible that this happen and could create this kind of problem?

I also replaced the transistor but even with the new one it seems to have the same problem...

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Post by Manfred »

I checked all resistors during the error and R2 seems to drop to 18K, is it possible that this happen and could create this kind of problem?
Have you measured the resistors under voltage with the ohmmeter ?
This is not possible because the values will be measured wrong.
Does the transistor get warm in the 5 to 10 minutes when you play the guitar?

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Post by Manfred »

Some further questions.
Does the error set in abruptly or is the intermittent buzzing get worse over time?
Where does the operating voltage come from, battery, plug-in power supply, power plant or other power supply unit?
Did you build the circuit on the PCB from Musikding, on a stripboard or point to point?
It could also be that the circuit starts to oscillate.

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Valerio313
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Post by Valerio313 »

Manfred wrote: 25 Apr 2021, 23:14
I checked all resistors during the error and R2 seems to drop to 18K, is it possible that this happen and could create this kind of problem?
Have you measured the resistors under voltage with the ohmmeter ?
This is not possible because the values will be measured wrong.
Does the transistor get warm in the 5 to 10 minutes when you play the guitar?
I did check all resistor right before start playing and as soon I have the error. I just unplugged everything and test. I looks like that R2 value is fluctuation between 15K and 150K. As soon as the value stabilize at 100K, I can plug in and the pedal is temporary working again. I will try replacing that resistor today to see what happens.

The transistor is staying cold for the whole time, and as mentioned, I've already checked with a new one, same problem.

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Valerio313
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Post by Valerio313 »

Manfred wrote: 26 Apr 2021, 07:55 Some further questions.
Does the error set in abruptly or is the intermittent buzzing get worse over time?
The sound is perfect for few minutes, then it starts making some noise (the one you have when you use a loose jack plug and you move the cable: I don't know how to call it in English), at this point the sound fades out and it never comes back. Most of the times the "noise" stays when the pedal is turned on; if turned off at this point, the true bypass is working perfectly and I have sound out of the amp.
Manfred wrote: 26 Apr 2021, 07:55 Where does the operating voltage come from, battery, plug-in power supply, power plant or other power supply unit?
At the moment I'm using a 9V 1A power supply which I've always used with all my pedals in the past. I know it's not a proper one, but it always did the job at home. I also tried a second power supply with slight less amperage and 9V battery. Same problem with all of them.
Manfred wrote: 26 Apr 2021, 07:55 Did you build the circuit on the PCB from Musikding, on a stripboard or point to point?
It could also be that the circuit starts to oscillate.
I bought the one from Musikding with the PCB. About the oscillating part, I don't know how I could check that...

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Post by Manfred »

About the oscillating part, I don't know how I could check that...
To see this you need an oscilloscope.
Where do the 2N5088 transistors come from, possibly from China?
I have already bought there transistors where it was not what is written on it, the values and the PIN assignment were wrong.
I could prove this with my semiconductor analyzer.
There are also good dealers but also black sheep.

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Post by Manfred »

Make a try and solder out the electrolytic capacitor and see what happens then.
You should hear a slightly higher volume than in bypass mode.

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Post by Valerio313 »

At this stage I removed almost everything... it's only the PCB, Jacks and power connector (tried both battery and power supply).
I removed the electrolytic capacitor C4 and also C2. I also replaced R2 with a new one and checked with a second transistor. Still the same problem.
I have nothing left to change basically...

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Post by Manfred »

Such a simple circuit and no solution to the problem this is unsatisfactory
Please post a photo of both sides of the board including wiring maybe you overlooked something.

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Post by Valerio313 »

Here's a pic of both sides of the PCB, for being my very first, it doesn't look too bad I guess 😅

Jacks are connected directly to the board with no potentiometer for now.

I keep finding strange that when the error occurs, R2 value readings cahnge that much
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20210429_183752.jpg
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Post by Manfred »

The potentiometer or a 100k resistor to ground at the output is needed for a trial.

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Post by idy »

When they say "check solder joints" they don't mean just look, but go over them again with the iron, reheat to make sure everything is connected. (not too long near the transistor, but your is socketed, sockets are often not good connections. Make sure it is firmly in place.) Often I've had things that look fine but a "cold solder" joint makes a bad connection.

You can't measure the value of a resistor when the power is on! And it is difficult to measure it "in circuit," but you can get some results, you have to look out for parallel R's or even a transistor that may act as a diode.

I would also, power off, measure for continuity between all the places that should be connected by the traces on the PCB. Sometimes a trace gets lifted or otherwise spoiled on a PCB, and although things look good, a connection is "open."



Not sure why you took out C2.

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Post by Manfred »

I had an idea and ran a simulaltion in which I changed the 2N5088 model.
I gave the transistor a larger base-collector reverse current of about 50 Mircoampere and I reached the approximate voltage values measured in the fault case.
The behavior in this order of magnitude is known only from the germanium transistors, there this reverse current increases with the heating in the time.
In this case the transistor must be faulty, how crazy that would be. :scratch:
Try some other transistor type and pay attention to the pin assignment.
Last edited by Manfred on 30 Apr 2021, 17:01, edited 1 time in total.

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Post by Manfred »

I forgot to mention that this reverse current or also called leakage current flows to ground via the R5 = 22k and thus increases the voltage drop across the resistor, which causes a shift of the bias point.

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Valerio313
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Post by Valerio313 »

Manfred wrote: 29 Apr 2021, 18:50 The potentiometer or a 100k resistor to ground at the output is needed for a trial.
ops... I'll put it back.

In any case, the circuit was working exactly in the same way, even without it...

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