Sunking problems...

[pauloman80]

Finally got my 10kB pots in today so I put them in and fired this puppy up and got two undesired results:

First, got the high-pitched squealing a few other guys have been talking about, so I checked the charge pump IC, and it's the CPA rather than a SCPA.  That should be an easy fix once I can get to my local shop and find an SCPA (although I ended up with the CPA version the last time I went there). 

Second, the gain pot does nothing.  No change at all in the overdrive/clipping level when turned.  Tone and volume pots function as expected.  I did Madbean's Eleventy Bajillion Diodes mod (found here: http://www.madbeanpedals.com/forum/index.php?topic=1107.0), so I don't know whether I did something goofy or what.  Double-checked all my wiring, including the wiring going two and from the switches for the diodes mod.  Here's a couple shots of the guts:




Thanks in advance to any and all who have input and can help me get this puppy running! 

[madbean]

Gain pot does nothing: could mean a couple of things.

Gain pot wired wrong
Vb not correct
Diodes incorrect/bad or shorting out

First step to fix:
Temporarily disconnect wires to all clipping diode switches. Tuck the wires somewhere so the leads don't touch anything (or each other)

Does the gain pot do anything now?

[pauloman80]

Gain pot is definitely wired correctly.  I double & triple checked the lugs.  *CORRECTION TO ORIGINAL QUESTION: There is something going on when the gain pot is turned: there's an extremely slight difference in volume.  

What's the Vb?  Still noob-tastic here & there.  

It's certainly possible the diodes aren't set up right.  Does it look like there's possibility of shorting from the photo?  As hewn together as it may look, I took extreme care in preventing any shorts as that was a major problem with my very first build.  Warming up the soldering iron now, will disconnect the diode switches and see what's what, post again shortly...

BTW, this thing will at least look amazing.  The faceplate you sent me really sets it off.  I'll be that much more excited when it's working!

***UPDATE*** Same behavior as before when the diodes are removed from the signal path.  The gain knob still decreases the volume slightly when turned up, and the high-pitched whine (presumably from the charge pump IC) goes away when the gain pot is as 12 o'clock high but increases when turned in either of the other directions.

[madbean]

Vb is the bias voltage supplied to both the ICs and the gain pot. There is a voltage divider made of of two 27k resistors in the power section. This takes the supply and cut it in half. If there is a problem, or incorrect value in one of those resistors, it will throw the bias voltage off and that can tank the whole thing.

I would take some voltages with your DMM on all the IC pins and the 6 lugs of the gain pot and post them. That will help a lot. Although you do have the incorrect charge pump, the effect should still work.

What kind of power supply are you using? If using a wall wart, try battery. The typical Boss 9v WW puts out closer to 13v nominal, which could damage the charge pump.

[pauloman80]

Gotcha... I think.    Still learning.  I checked those two 27k resistors earlier today and they appear both to be the correct values.  I'll have to check the IC and the gain pot tomorrow when I get back to the office.  I sure hope it's not the gain pot, I installed the one you sent me, and I would hate that you accidentally grabbed a dud! 

My local building buddy thinks it's possible having the wrong charge pump would alter the amount of audio signal, but I can't remember exactly where along the signal chain he said it would be and how he described it.  Something about only half the audio signal going where it needs to go.  He's substantially brainier than I am at this, so I thought I'd pass his thoughts on to you and see what you think.

I'm using the typical Boss power supply, didn't wire up a battery connection.  Never do, batteries irritate me.  I've got a Voodoo Lab Pedal Power 2 Plus, would that be "safer" to power it with?  Ideally I'll be powering from that source anyway.

Got a guide or walkthrough on checking IC pins and the gain pot lugs?  Links are always good.  Thanks, Brian.  You're the man.

[pauloman80]

Okay, testing the IC's.

IC1 (TL072)
Pin 1: 4.78
Pin 2: 4.78
Pin 3: 1.25
Pin 4: 0.00
Pin 5: 4.72
Pin 6: 4.78
Pin 7: 4.78
Pin 8: 9.52

IC2 (TL072)
Pin 1: 4.77
Pin 2: 4.77
Pin 3: 4.72
Pin 4: 0.57
Pin 5: 4.72
Pin 6: 4.78
Pin 7: 4.79
Pin 8: 8.23

IC3 (ICL7660CPA) -should be SCPA, but like you said, it should still work.
Pin 1: 9.52
Pin 2: -0.02
Pin 3: 0.00
Pin 4: 0.46
Pin 5: 0.57
Pin 6: 2.62
Pin 7: 0.24
Pin 8: 9.52

Gain Pot (100kB dual)
**Top Row
Lug 1: 4.72
Lug 2: 4.72
Lug 3: 4.71
**Bottom Row
Lug 1: 4.71
Lug 2: 4.71
Lug 3: 4.71

Double-checked all my solder joints and points with a magnifying glass... easily the cleanest soldering job I've ever done.  Also double-checked the 27k resistors in the voltage divider section, both are spot on. 

Thoughts

[madbean]

On IC2, you should read -9v on pin4 and about 17.5 on pin8. On IC3 you should read -9v on pin5.

So, something is wrong at the charge pump. It could be a bad charge pump, or possibly one of the electrolytic caps is pointing the wrong way (C20 onward) or one of the 1N4001 is bad or pointing the wrong direction.

Can we get a pic of the topside?

[pauloman80]

Triple-checked all my caps, their polarity is good.  Also checked my 4001s, polarity is good there as well.  Just checked them both with the diode setting on my DMM, both are registering between 0.593 and 0.596... does that sound right?  

I suppose it's a good sign that only those three values are wacky and they appear to all be tied to the charge pump.  I just remembered that I have a couple more 7660s, I should try swapping it out for a new one.  Let me try that first, if it's still wacky I'll update this post with a shot of the topside and we'll go from there.

**EDIT**
Swapped out the charge pump, values still aren't matching what you said they should be, and they're different from what they were before.  IC2 Pin 4 now reads 5.81, Pin 8 reads 8.45.  IC3 Pin 5 reads 5.38.  Tried running my guitar through it to see if I could get sound.  Bypass signal is just fine, but the effect engaged now has a continuous digital "farting" sound, for lack of a better term.    Here's a shot of the topside:



I've got one more 7660, I'll try it, I suppose.  I'm starting to think I need to get a totally different IC though, either the SCPA version of the 7660 or a MAX1044.  

[gtr2]

I'm pretty sure IC3 needs to be an SCPA or you'll have problems.

josh

[pauloman80]

I went back and found this thread: http://www.madbeanpedals.com/forum/index.php?topic=81.0.  Says you can sub a MAX1044 for the 7660SCPA (which I can't find anywhere, not even futurelec).  Brian said in that thread that you can also use a TC1044... My local shop has LTC1044s which, after a bit of research, are LINEAR TC1044s, and "Linear" appears to refer to the manufacturer.  I just want to make sure it's work the trip before I drive on up to my local shop.

I did find the 7660SCPA IC at Mouser, they've got 'em pretty cheap (individuals at $1.08, between 10-25 qty. $.90, group buy, anyone??).  I'm trying to determine if I should order the SCPAs from Mouser or if I can just head up to my local shop and pick up those LTC1044s...

[gtr2]

It appears from comparing the datasheets that it may work...  It's worth a try at least.

You can buy the max1044 from small bear.

http://www.smallbearelec.com/Detail.bok?no=244

I'd love a "local" shop in my area.  There pretty non-existent in the Philly area. 

josh-

[pauloman80]

It appears from comparing the datasheets that it may work...  It's worth a try at least.

You can buy the max1044 from small bear.

http://www.smallbearelec.com/Detail.bok?no=244

I'd love a "local" shop in my area.  There pretty non-existent in the Philly area.  

josh-

Yeah, it's nice to have a local store, it's where I get most of my caps & resistors these days.  

I just got back from picking up the TC1044's.  Threw one in, still no change from the gain pot when plugged in, but I also don't have the diodes in there.  Going to check the values on the ICs again...

***EDIT*** Some success, the values are now where they should be.  But again, still not getting any change when turning the gain pot, which seems very strange to me.  Even without the diodes in there, methinks the gain pot should do something...

[pauloman80]

It appears from comparing the datasheets that it may work...  It's worth a try at least.

You can buy the max1044 from small bear.

http://www.smallbearelec.com/Detail.bok?no=244

I'd love a "local" shop in my area.  There pretty non-existent in the Philly area.  

josh-

Yeah, it's nice to have a local store, it's where I get most of my caps & resistors these days.  

I just got back from picking up the TC1044's.  Threw one in, still no change from the gain pot when plugged in, but I also don't have the diodes in there.  Going to check the values on the ICs again...

***EDIT*** Some success, the values are now where they should be on the ICs.  But again, still not getting any significant change when turning the gain pot, which seems very strange to me.  Even without the diodes in there, methinks the gain pot should do something...  Could it be the pot is bad?  It also seems to me that I should hear some kind of difference with the diodes back in there.

[gtr2]

An audio probe is simple to make and you most likely have the parts to make one.

http://diy-fever.com/misc/audio-probe/

I use the 1/4" jack socket version so I can use a longer cable if needed.

An audio probe is essential to debugging.  Follow along through the circuit with the schematic, it will help you locate where your having problems in the circuit.

josh-

[pauloman80]

An audio probe is simple to make and you most likely have the parts to make one.

http://diy-fever.com/misc/audio-probe/

I use the 1/4" jack socket version so I can use a longer cable if needed.

An audio probe is essential to debugging.  Follow along through the circuit with the schematic, it will help you locate where your having problems in the circuit.

josh-

Hmmm... does this mean there are things I can do with an audio probe that I can't with a DMM as far as troubleshooting and debugging?