Help with submarine device (EQD the Depths by PCB Guitar Mania)

[neiltheseal]

Hello again.

With this forum's help with my previous posts I have successfully completed and/or troubleshooted quite a few pedals.

I was feeling confident however, I have run into an issue with the submarine device by PCB Guitar Mania. I hope that someone can assist with this as I'm a little stumped.

The problem

The pedal sort of works.

• Works in bypass and when engaged.
• The volume and voice pot affect the signal as does the switch. I can see the little LED change colour and intensity when the switch/voice pot is adjusted.
• The switch and voice pots audibly affect the signal
• However the LED does not throb as I assume it should. The rate, intense and throb pots do not affect the signal

I have checked all of the parts (resistors, capacitors and electro capacitors) and they are correct. ICs and transistors are correct and are placed correctly per the provided build docs. This is assuming the letters at the end of IC names mean nothing. I have TL074CN instead of TL074 and TL072CP instead of TL072.

Pics of both sides of the PCB and my audio probe can be found here (https://imgur.com/gallery/xdP3fTa)

There seem to be a lot of gaps in the audio probe:
- R3, 4, 7, 8, 10 and 11 and C2, 6 and 7 only had sound on one side.
- VR 2 had audio on one leg but none of the other light dependent resistors had an audio signal. Is this normal?
- The signal seems to come out of pin 1 on IC 2 but I cannot see where it goes in.
- The bottom half of the schematic does not have an audio signal at all but this seems correct.
- I can get the voice pot to adjust the brightness of the red/green LED and the switch changes the colour but I can't get it to throb.

I will reflow solder on all parts tomorrow and see how that goes.

Thanks in advance!

Link to build docs:

https://pcbguitarmania.com/wp-content/uploads/2019/11/Submarine-Device-Building-docs.pdf

Voltage (not sure if it helps):

IC1 - TL072CP
1 - 4.61
2 - 4.61
3 - 2.16
4 - 0
5 - 4.02
6 - 4.61
7 - 4.63
8 - 9.28

IC2 (TL074CN)

1 -  4.84    8 - 4.59
2 -  4.64    9 -  4.61
3 -  4.59    10 - 4.58
4 -  9.28    11 - 0
5 -  3.58    12 - 4.58
6 -  4.61    13 - 4.61
7 - 4.62     14 - 4.61

Q1 (MPSA18)

C - 2.08
B - 0.59
E - 0

Q2 (MPSA18)

C - 1.48
B - 0.89
E - 0.23

[mauman]

The issue is in your LFO (low frequency oscillator) section, shown in this clip.  Since you've verified all the component values, I'd resolder these components, starting with R22 and the other parts right around Q1.

The LFO generates a low frequency waveform that drives the brightness of that two-color LED, which slowly changes the resistances of the light-dependent resistors.  It's way below audio frequency, max is several oscillations per second.  Your goal is to get that LED pulsing, with the pulse rate changing when you crank the Rate knob.

You can't hear an audio signal at one end of R3, R4 etc because the inverting inputs of the op amps are at virtual ground.  No worries there.

[NorthCoast]

It looks like you used a pair of B2K pots where there should be B25K pots.

Take a look at the intensity and throb controls.


Also, be careful bending the legs of the pots, after 5-10 wiggles back and forth, they'll break.

[neiltheseal]

Damn you are completely right! I checked everything else but missed the pots. Hopefully I have some 25k around.

Thanks for that.

Yes I did realise after bending for the picture that it looked a little unstable.

[neiltheseal]

It looks like you used a pair of B2K pots where there should be B25K pots.

Take a look at the intensity and throb controls.


Also, be careful bending the legs of the pots, after 5-10 wiggles back and forth, they'll break.

Hey as an update. You were completely right, it was the pots that were the issue.

I put a 25kb in the throb control and a 50kb in the intensity and it works great. Thanks for picking this up. I can't believe I missed it.

I know the build docs say 25kb for intensity but I only had a 50kb. It sounds pretty good when I tested it so figured I'd leave it. Tacked a 50k resistor from lugs 1 and 3 (which I think should bring the pot down to close enough to 20k) but I couldn't really tell the difference so just went with the 50k pot.

[NorthCoast]

That's great! You're welcome. I'm happy to help and glad that you got it working. Would be great to see a build report!