Tube Screamer troubles

[mark p]

I need some help with a noisy Tube Screamer.

I built JC Maillet's "SRV Special" from the schematic at http://www.lynx.net/~jc/SRVspecialSchematic.gif but there is a background hum/hiss, especially when the tone pot is on full treble.

The noise is present whether I use an adapter or battery.  I'm fairly certain it's not a grounding issue, as my guitar is properly grounded and touching the strings doesn't affect the hum.  Also, the IC is in proper working condition.

I have done several of the common mods to the circuit but don't expect those to be causing the hum.  Besides that, I substituted a couple parts for ones I didn't have.  These include a 470k resistor in the input buffer instead of 510k, and 9014b's instead of the 2n5210 transistors.  Something I found strange is that I couldn't use a 10k resistor off of the first transistor because it would make a brief crackly sound, but only if I strummed really hard.  I resorted to using a 100k which seems to work fine.

Any suggestions?

[richon]

change the first transistor , maybe it is bad or not in the right position.

[mark p]

You're right.  It turns out one of the times I switched out transistors I put it in backwards, so now the 10k resistor works but there still is the same background hum...

[mark p]

I don't know if this will help but here are some of the voltages:

TL072
P1- 2.64v
P2- 2.79v
P3- 2.66v
P4- 0v
P5- 2.75v
P6- 2.75v
P7- 2.75v
P8- 8.88v

Q1                     Q2
C- 8.88v             C- 8.87v
B- 2.11v              B- 2.77v
E- 2.66v              E- 2.19v

[Projectile]

Those voltages don't look right. You should be getting something around 4.4v for your reference voltage, so all of the pins that show 2.6-2.8 volts are way too low. Check the resistor values that create the +4.5v reference from the 9v power supply. Something is wrong there.

[mark p]

I checked the parts and all of them were the right value, but +4.5v was just under 4v.  After trying some different things I added a 1uf capacitor and a 10k resistor connected to +4.5v just before the op amp.  This was not included in the "SRV Special" schematic but seems to straighten out the voltage issue:

Bias = 4.35v

TL072
P1- 4.21v
P2- 4.62v
P3- 4.29v
P4- 0v
P5- 4.21v
P6- 4.22v
P7- 4.22v
P8- 8.87v

Q1                     Q2
C- 8.87v             C- 8.87v
B- 2.97v              B- 4.21v
E- 3.85v              E- 3.62v

Adding those two parts also reduced the hum slightly, but it still is there...