Burnt 9V input section on guitar pedal

Potato Cakes

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Hello, everyone,

A friend of mine had the 9V input section burn up when using a 9V battery and gave it to me to use for parts but it seems that it can be repaired. The problem is the area of concern burned the PCB bad enough to destroy some of the traces. I took a working pedal that I owned of the same brand and uses the same enclosure to compare circuits but the 9V input section are not the same. I have parts to fix the burnt pedal but I am not sure about the component arrangement. I am also confused by the orientation of the diodes. I would thing they would be used for circuit protection but to me it seems the way they are placed would cause a short between V+/- when voltage is applied.

Here is the burnt PCB.
 

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Potato Cakes

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Here is the 9V input section of the pedal that is working. I've removed the 9V connector on both pedals to better see the traces that pass underneath.
 

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Potato Cakes

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And here are the schematics as for both sections based on doing some continuity metering. For the burnt schematic, I've circled what I think are the connections that need to be made but I do not know as those points are in the area that has the burned traces. I added the connection from the SS14 anode to the mystery capacitor (C39) as that connection is still intact. Would there be any harm in just using the power section schematic from the working pedal? And could I use something like a 1 ohm carbon resistor for the fuse?

Thanks!

Paul
 

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radardoug

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Its very common in pedals to have a diode across the 9 volts in to short the supply to ground if you apply the wrong polarity supply. Yes its a stupid idea, especially as you could use a series diode and not do any damage.
But I think they do it because they dont want .6 volts of diode drop, and they dont care if you burn the pedal up.
The way you have drawn it, the diode could be backwards, but also check what polarity is on the input connector. Some stuff uses positive centre pin, some negative. And some even use a.c. Not yours though!
Rewire it with a series diode.
 

Khron

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In guitar pedals, it's usually the barrel (outside) of the DC jack that's positive. The normally-closed contact within that connects the battery to the circuit, when no DC Jack is inserted.

You might want to revisit those schematics in this (new) context.
 

CJ

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shrink the pics to 600 x 800 so everybody can see them, this will also help keep the upload folder from filling up,

what kind of effect is it?

you can build anything you want on the cheap, check this action>

https://aionfx.com/

this is a good source for DIY parts>

http://smallbear-electronics.mybigcommerce.com/


 

Potato Cakes

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radardoug said:
Its very common in pedals to have a diode across the 9 volts in to short the supply to ground if you apply the wrong polarity supply. Yes its a stupid idea, especially as you could use a series diode and not do any damage.
But I think they do it because they dont want .6 volts of diode drop, and they dont care if you burn the pedal up.
The way you have drawn it, the diode could be backwards, but also check what polarity is on the input connector. Some stuff uses positive centre pin, some negative. And some even use a.c. Not yours though!
Rewire it with a series diode.

Thanks for the reply. It's definitely center pin negative as that was the first thing that crossed my mind but the label on the pedal at the 9V connector confirms this.

Thanks!

Paul
 

Potato Cakes

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Khron said:
In guitar pedals, it's usually the barrel (outside) of the DC jack that's positive. The normally-closed contact within that connects the battery to the circuit, when no DC Jack is inserted.

You might want to revisit those schematics in this (new) context.

Well, it looks like somebody can't use a multimeter. That or I confused the beeping noise in my head with my meter. Here is the actual circuit of the working pedal's 9V input section. I'm not certain what the value of C20 is and since it's SMD I don't want to take it out of circuit to measure it and risk destroying it. I imagine a 0.1uF ceramic would be fine when applying this to the burnt out guy.

I tried to make the images smaller but they blow up real big in the thread. Downloading my be best for viewing.

Thanks!

Paul
 

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Potato Cakes

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CJ said:
shrink the pics to 600 x 800 so everybody can see them, this will also help keep the upload folder from filling up,

what kind of effect is it?

you can build anything you want on the cheap, check this action>

https://aionfx.com/

this is a good source for DIY parts>

http://smallbear-electronics.mybigcommerce.com/

The burnt out guy is an EHX Memory Boy and the working one is an EHX Pulsar (tremolo).

I have seem that DIY pedal company and I do occasionally use small bear for harder to find parts.

Thanks!

Paul
 

rock soderstrom

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Photo PCB

It seems that just the + rail on the PCB and FB5 are burned. Bridge it and with a little luck you are back in business....
 

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Potato Cakes

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rock soderstrom said:
Photo PCB

It seems that just the + rail on the PCB and FB5 are burned. Bridge it and with a little luck you are back in business....

Oh, sweet! That is really helpful! Can you confirm that C38 is indeed connecting the V- to V+ before the fuse? It looks that way in your photo and I think that is how it was meant to be, but I have no way of confirming this with the unit I have as those traces are destroyed.

Thanks!

Paul
 

rock soderstrom

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Potato Cakes said:
Oh, sweet! That is really helpful! Can you confirm that C38 is indeed connecting the V- to V+ before the fuse? It looks that way in your photo and I think that is how it was meant to be, but I have no way of confirming this with the unit I have as those traces are destroyed.

Thanks!

Paul

Hard to tell, I only have this photo from the internet. I think the pedal will work without this (filter) capacitor. I would first replace the trace between the 9V connector and C39 temporarily, see if the pedal has function again.
 

Potato Cakes

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rock soderstrom said:
Hard to tell, I only have this photo from the internet. I think the pedal will work without this (filter) capacitor. I would first replace the trace between the 9V connector and C39 temporarily, see if the pedal has function again.

Got it. I found that same capacitor with the same connection points on my other pedal it was just located further away. I've attached how the power input seems to be connected on both of these EHX pedals.

It's also alive! At least it powers on and passes signal on a voltmeter. I was having a bit of trouble after rebuilding the power section as I wasn't getting V+ into the circuit. I failed to realize that the DC connector switches between the two V+ contacts to protect the pedal by preventing voltage from the battery and a power supply from being connected at the same time. I had to short connection points as the trace is burned and I never plan to use this with a battery.

Thanks again for sending that photo. It was super helpful.

Thanks!

Paul
 

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rock soderstrom

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Potato Cakes said:
Got it. I found that same capacitor with the same connection points on my other pedal it was just located further away. I've attached how the power input seems to be connected on both of these EHX pedals.

It's also alive! At least it powers on and passes signal on a voltmeter. I was having a bit of trouble after rebuilding the power section as I wasn't getting V+ into the circuit. I failed to realize that the DC connector switches between the two V+ contacts to protect the pedal by preventing voltage from the battery and a power supply from being connected at the same time. I had to short connection points as the trace is burned and I never plan to use this with a battery.

Thanks again for sending that photo. It was super helpful.

Thanks!

Paul

👍
 

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