Rat Crackle

[jcuempire]

Hello.

I just built a ProCo Rat clone and I must say, I like the sound alot.  My question regards some crackle as a chord or note decays - right at the end as it's losing the most volume.  Is there a way to combat that? 

Thanks

Keith

[Processaurus]

Something's busted.  Check the bias on the opamp input and the JFET output buffer, maybe.

[jcuempire]

I'll try that.  Thanks

[jcuempire]

as soon as I figure out how.  There must be a newbie guide somewhere, I'll wager.  I'll check all the usual suspects...

[Hides-His-Eyes]

Step 1: voltages!

[jcuempire]

Okay, with the aron's article, I was finally able to check the IC voltages.  At first it was really weird because I had the battery leads reversed (I use a power supply for playing - so I never noticed it) but then it became slightly less weird, but still wrong:

1 - should be 8.4 but is 7.6
2 - should be 4.5 but is 4.14
3 - should be 4.1 but is 2.08
4 - should be 0 and is 0 -hooray, got one right!
5 - should be 0 but is 4.19 - elation was short-lived
6 - should be 4.5 but is 8.34
7 - should be 9 but is 4.5
8 - should be 4.6 but is 0

I assume that 1 & 2 are close (close enough?) but the others are grossly wrong.

The only change I made was C4 was supposed to be 30pf and all I could find was 33pf.  But that cap is directly connected to the IC.

Does anyone have any ideas?


Thanks

[Processaurus]

Are you numbering the opamp's pins correctly?  Looking from the top, you start by the dot and go counter clockwise, down the left side, up the right.

  (dot)
1  o         8
2             7
3             6
4             5

[jcuempire]

Ahh!  No, I did left top down and then right top down so,

1 - should be 8.4 but is 7.6
2 - should be 4.5 but is 4.14
3 - should be 4.1 but is 2.08
4 - should be 0 and is 0
5 - should be 0 and is 0
6 - should be 4.5 and is 4.5
7 - should be 9 and is 8.34
8 - should be 4.6 and is 4.19

Not nearly as bad.  Are these values close enough?  Other than 3?

Thanks for the revelation, Processaurus!

Keith

[Hides-His-Eyes]

Using a dead battery?

[jcuempire]

nope.  I made sure the battery was brand new

[StereoKills]

I'm not sure what layout you're using, but I would check the Vb trace (what tonepad calls it) and the connected components. The pair of 100k resistors that form that voltage divider would be where I would start. Make sure those are actually 100k.

[jcuempire]

It's the GGG board.  Where would I find the VB trace?  I will check out tonepad's layout.  thanks

[StereoKills]

Looking at the GGG schem, they are 10k resistors, R15 and R16. The Vb trace runs from between those resistors to R1.

[Processaurus]

Yes, pin 3 being 2 volts is a problem.  The voltage there should be 4.5v, which is provided through R1, the 1 Meg resistor, from the voltage divider made by R15 and R16.  Check the values of those resistors, and their soldering.  The voltage where they meet should be half the power supply voltage, so that the opamp can amplify your guitar signal in the middle of its power supply range.  This is reffered to as biasing.  The opamp can't amplify outside of the window of voltage it is supplied with (actually it craps out within a volt or so of 0 volts and 9 volts), so if it is misbiased, like with 2 volts in this case, it might be cutting off the bottom of the signal, causing the sputtery sound.

Same story for R13 and R12, they are making a 4.5v bias supply for the FET buffer near the output of the pedal.

Schematic referred to:
http://generalguitargadgets.com/pdf/ggg_rodent_sc.pdf?phpMyAdmin=78482479fd7e7fc3768044a841b3e85a

[jcuempire]

I really appreciate all the information.  So I checked the values of R15, 16, 12, 1 and they are all ok.  So I resoldered all the questionable points on the board and I got a 3.3 on pin 3, so I closed it up to listen to it through the amp and I did something bad.  It sounds like hell and only the heavy strummed sounds come out of it and then crackle away after a second.  Oh well, time to open it back up.  But there are Christmas gifts to wrap and kids' shows to attend so it'll have to wait until late tonight

Keith

[jcuempire]

Well, thanks to all your great advice, I have, after a number of desolders and resolders as well threatened use of a large ball peen hammer, a nice sounding pedal.  There was one cap (C4 - directly ahead of the IC) that was the wrong value.  I looked at it a dozen times before it dawned on me.  I still have trouble reading cap values, but this one was way off.  Amazing what the right parts will do.  But I learned alot and that's the important thing.  Thanks again.

Keith