B3K Clone Troubleshooting Help

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forgotmyeeclass
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Post by forgotmyeeclass »

Hi folks,

I recently put together my first ever build, the PCB Guitar Mania "Black Mirror" which is a B3K clone. Schematics and layout can be found here: https://pcbguitarmania.com/wp-content/u ... mirror.pdf
This circuit also has a section to insert an EQ circuit. I wired up the "Falcon EQ" circuit, schematics and layout are here: https://pcbguitarmania.com/wp-content/u ... g-docs.pdf

Initially, I was getting a signal through the pedal although none of the pots seemed to do anything, and now I am not getting any signal through at all. I traced the DC voltages which can be seen here:
https://drive.google.com/file/d/1gAc5ZZ ... sp=sharing

I also put together an audio probe, I get signal from the input up to Pin 3 of IC1 - but no signal coming out of Pin 1 of IC1. I reflowed the solder on Pins 1 & 2 of this IC, and no luck (I feel like this wasn't the problem anyway, since there is DC voltage at this pin?). I also do hear a very slight signal when probing the output jack but it's much quieter than the input.

If anyone can give me an idea of what next to check or fixes to attempt, I'd greatly appreciate it and I'm all ears!

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stolen
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Post by stolen »

Hey!

The first problem to look at is your bias voltage - if we read the sheet correctly, it's way too low at 2V, and curiously the input of IC1 pulls it even lower to 1V. It does sound like an accidential short somewhere; naturally it's hard to tell remotely where it is located. If the ICs are socketed we'd try removing them and see if VD rises to an appropriate ~4.5V. If that is not the case, look for shorts or wrong component values around the VD line; if it does, put in one IC after another to see which one pulls it down, and try replacing the culprit with another to see if it's a fried unit.

We'd recommend disconnecting the Falcon EQ for this. Also take care that that EQ has a VB referenced input and GND referenced output; we'd recommend omitting R9 and R12 or tying their GND side to VB in case it's wired up to the loop exactly as shown in both schematic. This is however not what's causing your primary problem.

Good luck!

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forgotmyeeclass
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Post by forgotmyeeclass »

Thanks for the advice. My ICs are not socketed but I'll try to pinpoint shorts where you suggested.

I don't think the IC is cooked (unless I did it when reflowing the solder on Pin 1, but it didn't feel like it got hot when I did that) since I did have audio through the pedal at one point.

But as I said I'm quite new to this stuff, is there any way I can check one way or the other, with a DMM and audio probe?

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Post by forgotmyeeclass »

Measuring R30 & R31 - they are reading about 5.8kOhm on my DMM. They're marked as 10k, but is this a sign the short is somewhere in here?

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Post by stolen »

forgotmyeeclass wrote: 08 Apr 2021, 21:18 But as I said I'm quite new to this stuff, is there any way I can check one way or the other, with a DMM and audio probe?
Unfortunately in this case no - in circuit your DMM will not show correct values.
forgotmyeeclass wrote: Measuring R30 & R31 - they are reading about 5.8kOhm on my DMM. They're marked as 10k, but is this a sign the short is somewhere in here?
This is normal. You could try lifting the connections to VD (drive pot, level pot, R2, R13, R21) and then connect one after another to see if it gets pulled down by any of those connections. Troubleshooting often involves desoldering (or intensely staring at traces for a while).

Did you build it on premanufactured PCBs or perfboard?

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Post by forgotmyeeclass »

stolen wrote: 09 Apr 2021, 09:33
Unfortunately in this case no - in circuit your DMM will not show correct values.

This is normal. You could try lifting the connections to VD (drive pot, level pot, R2, R13, R21) and then connect one after another to see if it gets pulled down by any of those connections. Troubleshooting often involves desoldering (or intensely staring at traces for a while).

Did you build it on premanufactured PCBs or perfboard?
I built it on the pre printed PCB I linked in the first post.

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