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Help debugging analog delay effect
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lorenzo.leo



Joined: Apr 16, 2021
Posts: 4
Location: Italy

PostPosted: Fri Apr 16, 2021 12:47 pm    Post subject: Help debugging analog delay effect
Subject description: Seeking help debugging an analog delay for guitar
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Hi! Thanks in advance for your help. I'm new to this forum and to electronics in general, so please bear with me and help me improve.

I'm working on a clone of the B.Y.O.C. "clone" of the Boss DM-2 analog delay. I didn't want to buy their kit because I figured the best way to learn was to fiddle with the circuit on the breadboard, experiment to understand how it works and try to get some experience designing a PCB with EasyEDA. My main goal is not necessarily to have the most reliable guitar pedal but to have the most fun in the process. As I said, I'm pretty new to electronics, and my very shallow basics are pretty rusty.

Now, onto my problem. The pedal works great. When it works. Unfortunately, when you turn it on, it (usually) doesn't. The signal reaches the MN3205, but the only thing getting out is the clock signal. However, while disconnecting and reconnecting the 9V battery, I noticed that if I erroneously touch the pins with the wrong polarity for a fraction of a second, and then connect the battery properly, the pedal works. If I unplug and replug shortly after it still works, a few minutes has to pass before the pedal gets silent again*. While scratching my head, I thought that maybe in that way I was discharging the capacitors, and that solved the problem. So the next time it got silent I tried discharging C1, C2, and C3 with a screwdriver (I know, not the best way, but it didn't seem bad with these voltages). And then when I replugged the battery the pedal worked fine.

So, now...
1. Is it really possible that it's the capacitors?
2. Why?!?!
3. I thought D1 prevented a battery connected with the wrong polarity from doing anything. How is that discharging the capacitors?
4. How should I fix this?

I know I don't have a lot of information to share about the problem, but I have never debugged a circuit before and all of this is very confusing to me.

From my small researches (on this same forum) I learned that the MN3102 would be happier with 5V, but I looked at the datasheet and it seems it accepts up to 10V. Also sometimes the pedal works perfectly, so I don't think that should be a problem(?)

*with "silent" I mean that there are no repetitions, the dry signal gets through fine

Also, yeah, I bought the cheapest components from China on ebay. They might be misbehaving Sad

Thank you all for your help and your time Very Happy

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Grumble



Joined: Nov 23, 2015
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Location: Netherlands
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PostPosted: Sat Apr 17, 2021 10:06 am    Post subject: Reply with quote  Mark this post and the followings unread

I see that in the datasheet the C’s at pins 1 and 6 are 2u2 and in your schematic it says 0.22uF.
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lorenzo.leo



Joined: Apr 16, 2021
Posts: 4
Location: Italy

PostPosted: Sat Apr 17, 2021 10:50 am    Post subject: Reply with quote  Mark this post and the followings unread

Grumble wrote:
I see that in the datasheet the C’s at pins 1 and 6 are 2u2 and in your schematic it says 0.22uF.


I see.. I took my values from BYOC's schematics which should work though...
How could this affect the circuit and cause the problem?
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Grumble



Joined: Nov 23, 2015
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Audio files: 30

PostPosted: Sun Apr 18, 2021 1:43 am    Post subject: Reply with quote  Mark this post and the followings unread

Dunno, just noticed the difference between this and the datasheet
Do you have a clock signal coming from the MN3102 at pin 2 and 4?

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lorenzo.leo



Joined: Apr 16, 2021
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PostPosted: Sun Apr 18, 2021 2:24 am    Post subject: Reply with quote  Mark this post and the followings unread

Yes, clock signal seems fine. Audio signal, on the other hand, reaches pin 13 of the MN3205 just fine, but doesn't get out. From the output pin you only get clock noise.
Basically it looks (sounds) like wrong bias, but there is no position on the bias trim pot which lets the audio through, not until I "reset" the capacitors.
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AlanP



Joined: Mar 11, 2014
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PostPosted: Sun Apr 18, 2021 7:34 pm    Post subject: Reply with quote  Mark this post and the followings unread

Try lower values for the resistors on either leg of your bias trim (R27 and R28). Your BBD may need a bias outside the range that they put the trimpot into.
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lorenzo.leo



Joined: Apr 16, 2021
Posts: 4
Location: Italy

PostPosted: Mon Apr 19, 2021 1:52 am    Post subject: Reply with quote  Mark this post and the followings unread

AlanP wrote:
Try lower values for the resistors on either leg of your bias trim (R27 and R28). Your BBD may need a bias outside the range that they put the trimpot into.


I thought about that, but wouldn't this reverse the problem? If I change the bias the situation when now works wouldn't work anymore. I mean, statistically it would probably work more often, but still not perfect
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