EHX Soul Food low volume on output

[tegaki]

I have bought Soul Food with broken Drive knob (see pic.) and was able to mount shaft back to pot.

However upon trying the pedal I noticed very low output volume even with cranked Volume knob.

Checking with audio probe I found that U1 (TL072C) have low volume output at pin 1 and 2 (pin 3 is at normal volume).

Verifying with schematic I noticed there is Vb at the pin 3 and checked the voltage here to be 1.56V.

I figured it should be 4.5V as a reference voltage so I swapped R25, R26 and C19 but still measured the voltage as 1.56V.

I don't know what might be influencing the low voltage.

[stallik]

Check the MAX1044 / (7660) 

[GibsonGM]

Yes; what is the voltage on the TL072's pins 8? 

[PRR]

Check the MAX1044 / (7660) 

Vb does not come from the voltage converter.


I wonder if the battery could be weak (Mike's point). If down to 3.12V then the reading makes sense, even though not what we want.

1.56V is bizzarly spot-on for a single fresh C-cell.

[stallik]

Check the MAX1044 / (7660) 

Vb does not come from the voltage converter.

R25,R25, C19 have been replaced so I was wondering if a short elsewhere in the ps was causing the issue. Checking the pin voltages of the 7660 seemed sensible but then, I'd presumed the circuit was being run from a ps not a battery and it's never safe to assume.

Anyhow, I prefer your suggestion, 1.56v does seem rather coincidental

[tegaki]

Guys I @#$%ed up while trying to measure TC7660S IC my hand slipped and I shorted pin 3 and 4.

Fortunately I had measured U1 and U2 before and pins 1 to 4 in U3 (from datasheet pin 5, 6 is NC and pin 8 is 9.3V).

Probably I need to replace TC7660S but still I don't think Vb wrong voltage reference was caused from it.

Before incident:

U1:

• 1.1V
• 1.1V
• 1.37V
• 0V
• 1.5V
• 100mV then starts dropping
• 100mV then starts dropping
• 9.33V

U2:

• 2.45V
• 1.53V
• 1.53V
• -6.64V
• 1.52V
• 1.52V
• 618mV
• 15.18V

U3:

• 9.33V
• 5.72V
• 0V
• -2.72V
• (couldn't measure my hand slipped)
• (couldn't measure my hand slipped)
• (possibly 9.33V)

After incident:
U1:

• 4.36V
• 4.36V
• 4.86V
• 0V
• 5.34V
• 160mV then starts dropping
• 160mV then starts dropping
• 9.37V

U2:

• 9.88V
• 5.54V
• 5.50V
• 6.11V
• 5.53V
• 7.62V
• 8.88V
• 9.47V

U3:

• 9.37V
• 8.37V
• 0V
• 6.52V
• 8.25V
• 7.99V
• 9.37V

[tegaki]

Also I power the pedal with power supply, didn't try powering from battery.

[tootsMcgee]

Guys I @#$%ed up while trying to measure TC7660S IC my hand slipped and I shorted pin 3 and 4.

Ooof, yeah that might be replacement time. Dumb question though, have you pulled power and re-tested? It could've gotten stuck/latched in some weird state. (My background is digital stuff though so I'm talking out of my ass.)

For U1 and U2, it looks like they were previously getting the voltages they needed. U1 is connected to 9V V+ and ground. U2 is the one that gets a whopping 27V of headroom from 18V V2+ and -V. You can see that on pins 4 (ground) and 8 (V+) on U1 and U2. (Edit: actually -6V seems kinda wrong...it should be like -9V I think?)

Side quest question for the EE guys: one of the opamps is running with 27V of headroom but it looks like it's still biased at +4.5V? Does that mean huge signals will hit the negative rail sooner for some interesting asymmetry?

Back to this pedal.  One thing that comes to mind, you said the gain knob was broken? Could it be that it's busted internally? The gain pot on Klons is dual-gang, so there are actually two separate pots that get adjusted at the same time.

[ElectricDruid]

Side quest question for the EE guys: one of the opamps is running with 27V of headroom but it looks like it's still biased at +4.5V? Does that mean huge signals will hit the negative rail sooner for some interesting asymmetry?

I thought that too the first time I saw it, but think about it a minute. -9V to +4.5V is 13.5V. And +4.5V to +18V is...13.5V! So it's all symmetrical. It's just not referenced to ground.

[tootsMcgee]

Side quest question for the EE guys: one of the opamps is running with 27V of headroom but it looks like it's still biased at +4.5V? Does that mean huge signals will hit the negative rail sooner for some interesting asymmetry?

I thought that too the first time I saw it, but think about it a minute. -9V to +4.5V is 13.5V. And +4.5V to +18V is...13.5V! So it's all symmetrical. It's just not referenced to ground.

Durrrr math fail, I was subtracting from 27V instead of 18V. That makes a lot more sense. You're pulling GND down by 9V and pushing V+ up by 9V, so the bias point can stay centered at 4.5V. That's a ton of headroom. No wonder Klons are so damn loud haha.

As a side note, I have a Soul Food here that I repaired after someone plugged in the wrong power supply...it fried the polarity diode. Pretty easy to work on that part, but replacing the charge pump will be annoying though if you're not good with SMD soldering

[tegaki]

Dumb question though, have you pulled power and re-tested?

Yeah I re-powered several times but IC still gives weird voltages

Back to this pedal.  One thing that comes to mind, you said the gain knob was broken? Could it be that it's busted internally? The gain pot on Klons is dual-gang, so there are actually two separate pots that get adjusted at the same time.

Now that you said it I checked the schematic and noticed Gain pot is biased with Vb. Can load influence reference voltage?

replacing the charge pump will be annoying though if you're not good with SMD soldering

Soldering SMD is not problem, the problem is desoldering existing components. It is even harder because I only have soldering iron..