GGG ITS8 - Max gain level is unity - help please

Started by prskier17, December 30, 2009, 01:37:15 PM

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prskier17

I just finished building the General Guitar Gadgets ITS8.... a tube screamer kit purchased from GGG.  Everything is stock built with the kit components and I built with assymetrical clipping diodes (included in the kit).  When tested with a guitar, all of the controls seem to function as they should but the level control only provides about unity gain when fully clockwise regardless of the how the other controlls are set.  I know this pedal should be able to substantially boost the signal so something is wrong.  Also note that I started testing with a brand new duracell battery that tested over 9V and it has already dropped to 8.5V and is warm.  My instinct tells me there must be a short somewhere but I cannot find one and my input resistance between the +/- battery terminals (with the battery removed and instrument cable inserted to complete circuit) stabilizes around 19.5k ohms.  I'm not sure what that means entirely.  Follow is the debugging info.  I would greatly appreciate any help!

1.What does it do, not do, and sound like? Sounds like it's supposed to except for there is no gain.
2.Name of the circuit =  General Guitar Gadgets ITS8 Kit
3.Source of the circuit:
http://www.generalguitargadgets.com/index.php?page=shop.product_details&flypage=shop.flypage&product_id=22&category_id=7&option=com_virtuemart&Itemid=45
All of the schematics and diagrams are linked from the above page.....
4.Any modifications to the circuit? No, except the assymetrical clipping which is included with the kit.
5.Any parts substitutions? No
6.Positive ground to negative ground conversion? No
7.What is the out of circuit battery voltage? => 8.6
Voltage at the circuit board end of the red battery lead = 8.6
Voltage at the circuit board end of the black battery lead = 0

Q1
C = 8.5
B = 3.3
E = 2.8

Q2
C = 8.5
B = 3.5
E = 3.1

IC1
P1 = 4.3
P2 = 4.3
P3 = 4.3
P4 = 0
P5 = 4.3
P6 = 4.3
P7 = 4.3
P8 = 8.5

D1
A = 4.3
K = 3.86

D2A
A = 4.1
K = 4.3

D2B
A = 3.86
K = 4.1

D3 LED
A = 4.3
K = 3.08

D4 LED
A = 3.08
K = 4.3

D5
A = 8.7
K = 0

petemoore

  Use the audio probe.
  1:1 gain between buffer B/E pins.
  First opamp should be quite gainy with 500k feedback resistor.
  Read GEO, ''technology of the Tubescreamer''.
Convention creates following, following creates convention.

krelly5

I have exactly the same problem. Did you figure out what the solution is?

GibsonGM

Probably a solder short between IC pins or something like that. Good call on the effect of the battery's voltage dropping rapidly and getting hot...that is a major indicator that there's a short! 
+1, you should have a lot of gain at the 1st opamp output (pin 1).
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jkokura

You power might be leaking to ground. I would check and make sure you diodes are orientated correctly, because one of them might have been put in backwards and blown. Use your DMM and see if you 9V and Ground pads are connected with a beep test on your DMM.

I had a tubescreamer clone from tonepad give me low volume, turned out I had put in a wrong resistor at the end - it was supposed to be 100 ohm and I had used a 100k resistor. Check for that as well.

Sorry if neither of my suggestions help, but those are the things I think of immediately.

Jacob

GibsonGM

Yes, if a diode had shorted, there would be a short there, draining battery power!  Check those components - you should either have a low, high, or infinite (out of range) resistance, but not Zero resistance! 
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MXR Dist +, TS9/808, Easyvibe, Big Muff Pi, Blues Breaker, Guv'nor.  MOSFace, MOS Boost,  BJT boosts - LPB-2, buffers, Phuncgnosis, FF, Orange Sunshine & others, Bazz Fuss, Tonemender, Little Gem, Orange Squeezer, Ruby Tuby, filters, octaves, trems...