Soldado / Active EQ from PCB Guitar Mania

[joelorigo]

I finished the Soldado & Active EQ combo from PCB Guitar Mania. The Treble pot acts more like a volume control with a really loud spot in the rotation. The Level control gets louder then quieter then louder again as it is rotated clockwise. In the attached video I have the Presence & Volume trimmers on the EQ board backed off a bit from full. (The Character trimmer was omitted & jumpered, and the 2 LEDs omitted with an added 1M resistor for one, as I didn't want/need any extra gain, based on advice from the Facebook PCB Guitar Mania group.)

 

[joelorigo]

The Soldado by itself works fine!

So this is the wiring I am woking on for the EQ board by itself. There doesn't look like any thing on the PCB for the LED wire on the foot switch, right? No LED with the EQ only?

 

[PJS]

That should work. You could even wire it without the switch for testing - Just put the in/out jacks direct to the board

 

[joelorigo]

Cool. Is this what that that would look like? It's just "on" as long as it's plugged in?

 

[PJS]

Correct - as long as you plug in to the correct socket you will get sound at the other one, you just won't be able to bypass it, but that will also eliminate one potential source of error - the switch.

 

[joelorigo]

I wired the EQ board as above and the pot still did the same thing. I removed the resistor from the diode/LED spot and put in 2 3mm LEDs. So now it's assembled per the build document. Now there is no sound going through.

 

[PJS]

No sound at all? OK, have you got an audio probe? If so, then probe through the circuit until the sound disappears. If not then look up how to make one.

 

[joelorigo]

I’ll make an audio probe but I thought of something. In the above photo of the EQ board, is it possible that I have the LEDs in reversed? I have the negative lead in the square pad. I’m just trying to wrap my head around why signal passed through the circuit when there was a resistor in one of the spots but removing it and adding the LEDs there isn’t.

 

[PJS]

The 2 LEDs go back to back, so if 1 is correct and the other the wrong way then that would affect it. If both are installed the wrong way then that would be the same as installing them both the correct way.

 

[joelorigo]

Someone on the PCB Guitar Mania Facebook group confirmed that I have them in correctly. Based on that they go back to back is it normal for the opposite LED light up when I use the DMM with black in positive lead and red on negative lead? Red on positive and black on negative lights up the one you're touching as expected.

 

[PJS]

The anode of one LED joins to the cathode of the other, so that is exactly correct. If you are using the multimeter in diode test mode then one LED will light up. Reverse the leads and the other will light up.

 

[joelorigo]

Thanks. Off to build an audio probe!!

 

[joelorigo]

I'm back! After building an audio probe I discovered it is actually passing signal. Just very quietly depending on where the Treble pot is set. Which means it is still acting more like a "volume" control with that loud, scratch part as seen in the video in the original post.

So to sum up, the Soldado Preamp works fine on it own (sounds good!), and the Active EQ is assembled per the original document, no more mods and the Treble pot is still wonky.

 

[PJS]

OK, looking at the schematic, the treble pot would be a volume control if C2 was a short. Have a look at the soldering around there - post a photo of the solder side of the board if you can. Most likely is that the C2 end of the treble pot is shorted to ground somewhere. You could probably check that with a multimeter. If the soldering is all good then replacing C2 is probably the next thing to try. If you have a capacitance meter you could test it first.

 

[joelorigo]

Here is the photo. I don't have a capacitance meter, but how do I check with the multimeter for shorting to ground? From the schematic it looks like something to do with lug 1, right?

 

[giovanni]

I would definitely clean those solder joints with alcohol and a toothbrush. Besides that, I don’t see suspicious joints.

 

[PJS]

Yes, lug 1 of the treble pot. It is hard to see on the photo of the component side, but looking at this solder side there is a lot of solder on lug 1 of that pot, and it is quite close to pin 4 (ground pin) of the op amp. Check both sides and make sure there is no short there. Maybe remove some of the excess solder. To check for short to ground from the treble pot you will need the power disconnected. If your multimeter has a continuity buzzer use that setting. If not then use resistance measurement. If your meter doesn't auto-range then choose the lowest range. Put one probe on any ground connection. Put the other probe on the pot lug. If the continuity buzzer beeps, or if the resistance is zero or close to then you have a short to ground.

 

[joelorigo]

There is no continuity witht one probe on ground and the other probe on lug 1. When you say check both sides and make sure there is no short you mean any solder from lug 1 touching pin 4 of the IC? There is none on either side. The big glob on lug one happened when I was removing the jumper or resistor earlier, so was not there initially when the pot had the issue. Here is a better photo of the top:

 

[PJS]

Double check the value of C2. It should read 4.7nF. Most likely it will be marked either 4n7 or 472. Do you have a spare for C2? Actually, just in case there is a broken trace, also check connectivity from Lug 1 Treble pot to C2, and from the other end of C2 to ground. Use your multimeter the same way as before.

 

[joelorigo]

The cap says 4700 100. Its a WIMA brand. I do have a spare 4.7n. There is continuity between treble pot lug 1 and one lead of C2 (the inner one) and continuity between ground and the other lead of C2 (the outer one).

 

[PJS]

Actually, while you are at it, what values are C1 and C6? They should be 10n, which I would normally expect to be the same size as the 4.7n. It is surprising that they are so much larger. Also, check the connection from Treble Lug 3 to C1, and also C1 to ground