Angry Charles Bass Pot

B

bobthebuilder

New member
Hey everyone - new pedal builder here.

I've recently built an angry charles and everything seems to work except the bass control. After 12 O'clock on the knob the volume cuts off suddenly. Before 12 O'clock the volume increases as the knob is turned CCW. I've given a preliminary look over all the solder joints, components, and correct orientation of the potentiometers, and nothing seems obviously out of order. Any thoughts on what could be causing this strange behavior?
 
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Can't say for sure of course, but it almost sounds like pin 2 of the bass pot is grounded... For troubleshooting, it's nice that you have the issue isolated to one area of the circuit. Try to follow the schematic in that area and determine if all the bass pot pins and surrounding components are connected to the right place, and not going somewhere they shouldn't (use a multi-meter to check continuity).

You could try to post some clear close-up pics, which might allow someone to see if there appears to be a mistake.

If it gets to the point that you need to de-solder components, I recently saw some tips about desoldering on the build reports - Chela Overdrive thread that might be helpful.
 
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Wish I could help, I took a look at your pics and nothing jumped out. If it were me I'd start with a multimeter and check for continuity between all the connections surrounding that bass knob, checking the resistance of those resistors, as well as the resistance across the pot lugs.
 
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I would ground the black lead on your DMM to something grounded to your board like the sleeve on the audio in or auido out jack, set the DMM to volts, and go ahead and record the voltage on all the pins on the ICs and let's see what you get.
 
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also -- since you are running wires to your pots, make sure the three connections are correct for your bass pot (middle lug connects to R15 and then goest to IC2, pin 2, etc)
 
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I hope these sketches are legible. My multimeter is analog and probably not very accurate so I'm not sure how helpful these results are.

The second picture represents the paths I've traced and verified that they are connected.

The resistance values for the bass pot between lugs 2 and 3 vary from 0-15k. Zero resistance when the knob is turned fully CW.
 

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To confirm -- the mid and treble pots work for their full range.
Bass pot works until the rotation is about halfway. Then does all of the sound stop when you rotate past that point? Or do you lose the bass but everything else is still working? I read your question as saying all the sound stops at that point and not just the bass.

turn the treble and mid controls up all the way, and then turn the bass pot up to the point where the sound stops. what happens if you then turn down the mid and treble? nothing? or does the sound come back? I'm asking to see if there is a chance that the input on the IC is messing up somehow if the signal level gets "too high"

Can you swap out another TL072 chip and see if it makes a difference?
 
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Mid and treble work for their full range. Their position does not affect when the bass pot cuts out. For example, when they are both cranked, the bass pot cuts sound at around 2 O'clock. When the mid and treble are turned down, the bass pot still cuts sound at the same position.

(Mid and treble both increase volume as they are turned up and decrease volume as they are turned down. I'm not sure if that is normal behavior for this pedal.)

Volume and gain pots work as expected.

Bass pot acts like a reverse volume control up until 2 O'clock when it rapidly cuts the output sound to barely audible volume. From 2 O'clock to fully CW, I don't notice anything changing at all. The bass pot increases the volume dramatically when it is fully CCW.

It seems that all the EQ controls feed into the same pin on IC2 so it seems strange that only the bass control is having problems. I don't have another TL072 to swap, unfortunately.

If C13 was damaged, or if the bass bot was faulty, could this behavior be attributed to that?
 
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Maybe you are describing a linear pot for bass that is wired in "backwards" since you describe it as a reverse volume control. when you note the volume changes dramatically at one end, that could be because it is a linear taper and if you used an audio taper it would give you more control over changes in sound level at that end.

So long as mid and treb are not also configured as reverse volume controls, I would swap the wires on the 1 and 3 lugs on the bass pot to also make it perform like a normal volume control. The build doc calls for a 25K linear taper pot for the bass, and your pot could be an audio taper that would have exaggerated what was happening at each end or the rotation. After you switch the wires, see if it changes the behavior you were hearing. If it is a linear pot you will have changed the direction of the control but the effects at each end will still be the same. If it was an audio taper pot, you will probably have more control over the part of the range that previously had the dramatic volume change.
 
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I swapped the wires on the 1 and lugs on the bass pot and it behaves like a linear pot, basically the control is exactly reversed from before.

I'm not sure where to go from here. I'm confused because the mid and treble pots both work, yet everything feeds into pin 2 on the TL072. This makes me think that the IC is okay. The resistor values in the bass control circuit are continuous and they read the correct values, so the only thing I can think of is C13. Any further help is appreciated!
 
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I was wondering if your bass pot is bad, but that does not seem to be the case. when you turn the bass knob, does it only affect the volume? or is it also increasing the bass as well as the volume?
 
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I can't really tell if the bass is being affected because the volume adjustment is so dramatic. I will look at that article, sounds like it will be a good read, thanks!

I will check the orientation of the capacitors, that would be a nice easy fix if that turns out to be the problem!
 
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