Green Bean Casserole

[saxoftenest]

I've been trying to debug my Green Bean board for the past few days without any improvement. After I populated the board, I hooked it up to my breadboard. Boy does it rock! Too much, in fact. I seem to be at full gain, regardless of where the pot is set. First thought was that it's a bad pot. Before desoldering, I checked the resistance over the sweep of the gain pot. As expected, it was wide open regardless of the setting of the pot. I pulled the pot, but out of the board it's functioning properly. I tested another gain pot to make sure it was good and installed it. Same problem - it also reads like it's turned fully clockwise, no matter where I turn it. After that, here's what I've tried:

Confirmed all components are correct values
The only substitute is a 2kB pot for tone
Reflowed all joints and checked for any solder bridges
Tried different ICs

While checked continuity and voltages, a clipping LED lights when I touch pin 2 of the IC and various other components (only when the clipping switch is set to the LED side).

Here are the voltages I'm getting from the IC and transistors:

JRC4558D
1.84
4.56
.40
0
.94
2.01
2.01
8.91

Q1
1.55
0
8.9

Q2
1.77
0
8.88

Power provided by a 1spot supplying 9.53v.

The volume and tone pots are functioning properly. Anybody have some pointers?

[LaceSensor]

Ill be able to post my voltages if noone else does but not til end tommorow earliest.
My green bean works 100% out the gate fwiw

[saxoftenest]

Yeah, this one's got to be on me somewhere, just not sure how to troubleshoot this particular problem.

[Scruffie]

Keep checking values and looking for solder bridges, your IC voltages are pretty off... you should expect to see half supply on Pins 1,2,3,5 & 7.

The bases on your transistors are wrong too... something's up.

[saxoftenest]

So here's an update. I am fairly confident that all components are correct (as confident as one can be when it's broken), unless something's mislabeled. I've spent a lot of time with my DMM looking for continuity where there shouldn't be, but everything seems to check out.

I forgot that I subbed a 1n4002 for D6, so that and the 2kB tone pot are the only subs on the board. The transistors are socketed, and I've swapped out a few just to see if there's any difference but nope. Any chance the problem is this 2kB tone pot?

Any suggestions for tracking down where my voltages are getting screwy?

[jkokura]

Well, usually at this point you're looking for connections where there shouldn't be, and making sure that there are connections where there should be! If you use your DMM in continuity mode, I'd start walking through the schematic one part at a time, checking things off as you verify them. At the same time, you can reverify the parts for the correct values as well.

An Audio probe might help you too.

Jacob

[bobertperry]

+1 to an audio probe. 

Image from http://www.diystompboxes.com/pedals/debug.html:

[saxoftenest]

I really appreciate all the help! I have my audio probe ready to go. I've put together a quick trace of where I think I should be checking. Can anyone confirm if I've missed something and what I should be listening for along the way?

[LaceSensor]

As promised, if a little late, here are some reference voltages. I hope they are useful.
Readings taken with all pots at noon, if it matters.
Vc = 9.00v

IC1 - JRC4558D

1- 4.35
2- 4.52
3- 4.28
4- 0
5- 4.35
6- 4.35
7- 4.35
8- 8.78

Q1 - 2N5088
3.47
2.81
8.77

Q2 - 2N5088
3.47
2.82
8.77

Regards

LS

[jkokura]

Actually, in this case I wouldn't recommend using the audio probe first. The reason is that it sounds like it's working already except that your gain section seems to be 'stuck on'.

Using Lace's voltages (very helpful by the way Ian), you can see for sure that you've got a problem with your voltages. If that's the case, a multimeter would be the tool I would use. I would first start by using the schematic and walking though making continuity tests - test to see if things are connecting to places they are supposed to, and that they are NOT connecting to places that they aren't supposed to.

Next, I would start to zoom in on specific problems. For example, you have .94V on pin 5 of your IC, but on Pin 1 you have 1.84. That's way messed up, because they should both be approximately 1/2 of Vc (or about 4.5V). If that's the case, you will likely have a problem with your Vd supply, which means that either there's a cold joint or incorrect part or backwards part in your power section (R9, R17, R18, R19, C11, C12, D6).

That should get you started. An audio probe won't help you find issues in your power section really.

Jacob

[saxoftenest]

Sweet, thanks!