Scream-less ITS8

[djp8djp]

I'm having trouble with a GGG ITS8.

Actually, it's mostly great. It powers up fine, and I've got Volume (output), Tone, and Gain (drive). I have the power LED, and the bypass works as it should.

But it won't scream. Meaning, I get no clipping at all. I read through the ITS8 posts here, and didn't see this problem, so I'm starting a thread for it.

Toggling the clipping selector makes no difference - it's all just clean boost. I've tried the silicon diodes and the LEDs. For now I'm debugging with just the LEDs, and have swapped in another new pair with the same results. I've checked they're polarities, and they are opposite. They don't light up at all, no matter the settings or how hard I strum. The clipping selector switch appears to be OK, because when I select the silicon side (now that they're out), it goes silent.

Here's my config and results of my testing... it all specs out fine.
IC is TI RC4558P      
Q1 and Q2 are 2N4401      
LEDs are red 3mm      
      
IC   1   4.5
   2   4.6
   3   4.4
   4   0
   5   4.5
   6   4.5
   7   4.5
   8   9.1
Q1   C   9.1
   B   2.7
   E   3.3
Q2   C   9.1
   B   3.8
   E   3.4
C12 (100u)+   9.1
   -   0
C7 (47u)   +   4.5
   -   0
C11 (10u)   +   3.4
   -   0
LED1   CAT   0
   AN   0
LED2   CAT   0
   AN   0

I'm guessing that I'm supposed to have some signal at the diode leads, but can't figure out why I don't see any. PCB photo attached

Hmm. Maybe not. How's that done?

[Govmnt_Lacky]

Something doesn't look quite right with Q1.

Your Base voltage should be about 0.5V higher than the emitter.

Re-check all of your component values around this area 

Also... could you provide a schematic for reference?

[djp8djp]

Hi, Thanks, I'll look at that. Just on my way out the door, but I'll test as soon as I get back in.

I'd attach the schematic, but I haven't yet figured out how to attach docs or photos (or I would have above). But here's a link to their page with all the info.

http://www.generalguitargadgets.com/projects/17-distortion/82-its8-tube-screamer-808-replica-project?phpMyAdmin=78482479fd7e7fc3768044a841b3e85a
http://www.generalguitargadgets.com/projects/17-distortion/82-its8-tube-screamer-808-replica-project?phpMyAdmin=78482479fd7e7fc3768044a841b3e85a

[Govmnt_Lacky]

Start with R2 (should be 510K) and work out from there. Focus on the transistor's Base connections as that is what seems to be low.

Good Luck

[PRR]

Check pinouts on Q1 Q2. You *may* have Emitter and Collector swapped.

[djp8djp]

All right, new report. But same results. I'm fuddled.

PRR, I checked the pinouts on Q1 (again), and I have CBE correct per the schematic and datasheets, and incidentally, also oriented as in the GGG "example" photo. I re-checked the voltages on Q1. I had a new 2N3904, so I tried that. With input voltage at 8.73, I got C8.7 B2.49, E2.86. I also tried a new 2N5088. With input at 8.67, I got C8.67, B2.69, E3.37. So I'm not seeing the B>E by 0.5V that Govmnt_Lacky posted. I checked all the resistance values, first just across each of the resistors to see if they were in spec, and then across the board... trying to go as far back as the +ve supply line or ground. Except for not being able to test across the capacitors, everything checks OK.

I jumpered the clipping switch points on the board, to avoid the switch. Not much hope there anyway, but getting desperate. I tried the LEDs in both clipping circuits, and checked them again for functionality and polarity. Because it was the only part of the IC feedback loop I couldn't test, I pulled the 51p cap (I despise de-soldering!) and replaced it with a new one. Went over the PCB with a fine-toothed comb, found no interrupted traces or solder bridges or cold solder points (excepting the new 51p cap which look a tad grey, but I'm gonna leave those pads alone for now). I also swapped out the IC for a TL072, which sounds nice but made no difference.

Still have lots and lots of boost, all pots work as they should. But no light from the LEDs and no overdrive. Except when I max it all out and overdrive the amp itself.

Can't figure out the CBE voltage weirdness. And while it's a nice booster, it's supposed to be an overdrive. Now what?

ps - my fear is growing that it's going to turn out to be something monumentally simple that I'm missing completely

[djp8djp]

By the way, one particular thing that doesn't add up for me... the gain pot works, adding/cutting volume across its full range, so it must be modulating the voltage through the IC feedback loop. And if that loop is getting juice, how come the diodes don't show it?

[Govmnt_Lacky]

Pictures of the build might help! 

Good, clear images of the component and trace sides. As well as off board wiring.

[samhay]

The Tubescreamer should have a gain of ~100. If you feed it with any reasonable guitar signal and remove the clipping diodes, then you should hear a signficant amount of clipping from the op-amp. If at max gain the signal is still clean (no clipping) then there is a problem with the gain of the 1st op-amp and/or the buffer preceding it (Q1) is only letting a fraction of the signal through (which is consistent with the odd voltages you posted). You can check the buffer with an audio probe - http://www.diystompboxes.com/pedals/debug.html
If you use LEDs as the clipping diodes, you may not see them light up when they are clipping.