Problem with a Rat.

[supernaut]

It's been a while since I've posted here; as after my first build, which was a success (in the end) I stopped messing with electronics for a while for various reasons. I got back into it recently with making a ProCo Rat clone. The board layout I used is here: http://tagboardeffects.blogspot.com.au/2012/01/proco-rat.html
The only change I made to the circuit was a 33pf capacitor instead of the 30pf one.

You'll see I've already posted in the comments on that site asking for help. Before I try a complete rebuild, I'd love to actually know what the problem is.

Essentially my problem is that when bypassed everything is fine, but when engaged I get no sound at all. What I have since discovered is I get a very distorted/crackly noise at high volume (and with the distortion on the amp turned up). But aside from that nothing. I have no idea what's causing it.

Since posting in the comments for the layout, I replaced the JFET to see if it changed anything (it didn't) so I did another voltage reading.
They are:

Battery: 9.22v

Maximum distortion:

IC:

1 = 8.29
2 = 8.08
3 = 4.58
4 = 0.00
5 = 0.00
6 = 8.56
7 = 9.19
8 = 8.94

JFet:

D = 9.19
s = 1.43
G = 0.00

Minimum distortion:

IC:

1 = 8.26
2 = 8.51
3 = 4.56
4 = 0.00
5 = 0.00
6 = 8.55
7 = 9.19
8 = 8.94

JFet:

D = 9.18
S = 1.43
G = 0.00

I posted photos on the tagboard site, but you can find newer (and better looking) ones here: http://www.mediafire.com/?3u276n31jtqkoto

That link also has a short clip I recorded with my phone. To begin with it is bypassed, then I engage the pedal. Most of the time, you'll be hearing the guitar unamplified until the sound from the pedal kicks in.
The reason it is distorted in bypass is that it's almost impossible to hear the crackling when I have the amp clean. Also, my playing in that recording is horrendous, so please ignore that 

[ashcat_lt]

Looks like one or both of the caps from the feedback to ground is shorted.  Check the 2u2 and 4u7 on the left side of the layout.

Edit - boy, I've been back and forth on this one!  Check to make sure there isn't a little solder bridge between the 47R and the 1M directly below it.

[supernaut]

Will do. How do I know if they are shorted?

Thanks for the fast reply.

[ashcat_lt]

You ninja'd my edit, dude!

It kinda doesn't look like its dropping enough voltage for a short across those caps, but with the 1M to ground it makes a lot more sense.

[supernaut]

Well, there was definitely a solder bridge. Well spotted. I've scored between the tracks, and now there is no more bridge.

How did you know that was shorted?

Unfortunately though, my problem remains. Although it does seem like more sound is getting through when engaged. That could be my imagination.

[ashcat_lt]

Where was the short?

Pins 2, 3, and 6 should all be very close to one another, and all right around half the supply voltage.  Yours showed that the output was being pushed as close to the positive rail as the opamp will go, with a little bit being dropped in the feedback loop.  This means you're getting DC gain, which means there must be a DC path to ground in the loop somewhere.

Post new voltages, please.

[reveal]

Check that the node between the 33k resistors is providing 4.5 volts. 

[supernaut]

The short was between the 1M and the 47R. At least I think there was one 

I scored between the tracks and there definitely isn't one now though. I'll have a look for some more when I get home from work this afternoon. Seems my soldering skills might need some improvement haha.


Reveal, there aren't any 33k resistors. Do you mean the 100k resistors?

[reveal]

If the 100k resistors are where the reference (4.5 volts) voltage is then yes.   Sorry, I was looking at this schematic for the rat http://www.diystompboxes.com/pedals/PCRAT1.GIF  and it was close enough to confuse me.  The schematic at GGG shows them as 10k. At Tonepads they are 100k. 

[supernaut]

I'm not real sure where the reference voltage is; I'm still very new to the theory aspect of electronics. However I measured the voltage between those two 100k resistors and got 9v.

[ashcat_lt]

That doesn't make any damn sense at all.  Your original list of voltages shows 4.58V at pin 3.  That V comes via the 1M resistor closer to the opamp from the junction of those 100Ks.  Unless you've messed something else up in trying to fix your original issue, I think you've somehow measured wrong.

Please post new voltages.  Also, turn the Distortion knob all the way up, remove power (unplug the battery or whatever) and then measure resistance across the two caps please.

[supernaut]

I must be measuring wrong. Because pin 3 still shows 4.5v

When I measured between the resistors I just stuck the positive lead on the multimeter at the end of each of the resistors (the end where they join). The negative lead was attached to ground.

I'll do completely new voltages when I get back from work tonight. Also which two caps do you mean?

[ashcat_lt]

Sounds like you measured where you were supposed to, but I didn't see what you did, so...  shrug it off.  The voltage at pin3 is within an RCH of the "reference voltage" between those two resistors.  You asked earlier - the opamp can only supply output between 0 and 9V*, but the input comes in swinging above and below 0V - Sometimes it's negative.  The opamp can't give negative output, though.  So we add 4.5V to the input signal.  Now it swings around the middle of the supply, and the opamp can amplify the bottom half of the wavefirm.  After the opamp that reference voltage is blocked by a capacitor so that the output swings around 0V again.

The caps which block DC in the feedback loop are the 2u2 and 4u7 on the left side of the layout.  It doesn't quite seem right to me that these would be shorted.  Seems like pin2's V should be much lower if that were the case, but it's worth a shot.

[reveal]

The voltage between the two resistors should be around 4.5 volts.  Check to see if one of the 100k resistor is attached to ground.  While you are at it, make sure all the ground point are attached to ground by checking continuity of each one with the main ground point. 

[ashcat_lt]

Dude!  Pin 3 shows us the voltage between those resistors!  The bias voltage is fine.

[reveal]

You're right, I was looking at the wrong one again.     I'll go back to sleep now.  Though shouldn't pin 2 be close to 4.5 volts also?

[supernaut]

Sounds like you measured where you were supposed to, but I didn't see what you did, so...  shrug it off.  The voltage at pin3 is within an RCH of the "reference voltage" between those two resistors.  You asked earlier - the opamp can only supply output between 0 and 9V*, but the input comes in swinging above and below 0V - Sometimes it's negative.  The opamp can't give negative output, though.  So we add 4.5V to the input signal.  Now it swings around the middle of the supply, and the opamp can amplify the bottom half of the wavefirm.  After the opamp that reference voltage is blocked by a capacitor so that the output swings around 0V again.

The caps which block DC in the feedback loop are the 2u2 and 4u7 on the left side of the layout.  It doesn't quite seem right to me that these would be shorted.  Seems like pin2's V should be much lower if that were the case, but it's worth a shot.

I'm getting 605 ohms across the two caps.

I've also done another voltage reading. They are

(New) Battery: 9.40v

Maximum distortion:

IC:

1 = 8.41
2 = 8.20
3 = 4.63
4 = 0.00
5 = 0.00
6 = 8.69
7 = 9.32
8 = 8.93

JFet:

D = 9.33
s = 1.43
G = 0.00

Minimum distortion:

IC:

1 = 8.42
2 = 8.68
3 = 4.65
4 = 0.00
5 = 0.00
6 = 8.69
7 = 9.33
8 = 9.08

JFet:

D = 9.33
S = 1.43
G = 0.00


Thanks so much for helping me out with this.

[ashcat_lt]

I'm getting 605 ohms across the two caps.

What did you do to get this reading?  I was hoping you would turn the D pot to max and then put one probe on each leg of one cap, measure, and then repeat with the other.

I've also done another voltage reading. They are

(New) Battery: 9.40v

Maximum distortion:

IC:

1 = 8.41
2 = 8.20
3 = 4.63
4 = 0.00
5 = 0.00
6 = 8.69
7 = 9.32
8 = 8.93

JFet:

D = 9.33
s = 1.43
G = 0.00

Minimum distortion:

IC:

1 = 8.42
2 = 8.68
3 = 4.65
4 = 0.00
5 = 0.00
6 = 8.69
7 = 9.33
8 = 9.08

JFet:

D = 9.33
S = 1.43
G = 0.00


Thanks so much for helping me out with this.

Pin 6 is still pulling up to the positive rail and taking pin 2 with it.  You should really run that knife between all of the vero rows.  Go through the whole thing with a magnifier and make sure that nothing is contacting anything it shouldn't, and that all of your track cuts are actually cut all the way.

This can be frustrating, but with patience you'll get it!  It seems to be pretty close.

[supernaut]

What did you do to get this reading?  I was hoping you would turn the D pot to max and then put one probe on each leg of one cap, measure, and then repeat with the other.

Whoops. I measured across both caps at the same time.

The 4.7uf is 558 ohms and the 2.2 uf is 922 ohms.

I'll cut some tracks and get back to you.

[ashcat_lt]

Yeah, run the knife and get back to us. 

It hurts my head trying to find the path that gives those numbers.  It should be wide open.  The 605 makes sense, but sort of shoots holes in a number of theories for the others.  Maybe somebody with fresh eyes will look at it?