741 Op Amp Wrong Voltages in Inputs

[exztinct01]

I built an MXR Dist+ and read the voltages in the pins of the 741. All pins are good except pin 2 (1.5v) and pin 3 (0.5v) instead of 4.5v and 3.6v respectively. Anyone might care to tell me why is that?

[feddozz]

Yes.

It is because the voltage difference of those pins with your reference point is respectively 1.5V and 0.5V.

[slacker]

You probably have a problem with the circuit around pin 3, once pin 3 is wrong pins 2 and 6 will be affected and will probably be wrong as well.
Give us a link to the schematic or layout and we can tell you where to start looking.

[exztinct01]

i used the mxr dist+ project in http://www.tonepad.com

[antonis]

If your 9V power supply is OK, something wrong happens with the resistances of Vref voltage divider (R2/R3)..

It seems to me that the R3 resistor is 32% lower than it's original value (i.e. 680k instead of 1M)

[R.G.]

Yes to the advice above.

Also remember that the meter you use to probe the circuit has an input impedance. Today's meters often have an input resistance of 10M or so, but some of them are only in the 1M range, and that is very significant where you're probing a point that is biased with ~1M resistors.

Analog meters from the past forced users to learn this, because they had input resistances that were very, very much lower.

Meter resistance may not be your problem, but  I don't see anything ruling it out.

[antonis]

Just what R.G. said (as allways!!)

Measure the voltage between R2's upper leg (connection to +9V) and GND and if it's OK measure then the voltage between R2's lower leg (junction between R2/R3/R4) and GND.
If it's OK (4.5V) then check R5..
(but it should be around 20GΩ to dominate Vref..)

[exztinct01]

it's 11:00 p.m. here so I think I'll do that tomorrow, I'm already in bed, haha
anyway, i already measured the leg of R2 connected with the +9v and it's good. The junction between the three, not yet. Pin 3, 0.5 volts.
If there really is a problem in that junction, could it be the reason I'm not getting much gain?

[exztinct01]

I fixed this problem already. However, there's another bit of a problem. It's so noisy. When I plug my guitar to the amp, there's a small "zzzzzzzzz" sound coming out of it. I don't know if that's what you call a hiss or a hum. When I use the pedal in bypass mode, the same sound can be heard. When I touch the strings with my bare hand and my bare feet is on the floor, that noise disappears. When I lift my feet from the ground, the noise comes back again. When I turn on the pedal, the noise is amplified. Is it a grounding issue with my guitar?

[exztinct01]

another thing, when i touch the strings with my feet on the ground, that noise is just minimized but does not disappear completely. It's still so audible when the effect is on. i know the 741 is a noisy chip but I have heard some ds+ in youtube without this issue.

[PRR]

Perhaps "buzz"? Hiss has NO pitch. Hum has strong pitch with low timbre. Buzz is "fuzz-distorted hum"; it has pitch, but timbre is high not low.

Sure sounds like a poor ground in guitar.

Possibly picking-up a crappy power supply or an un-shielded case.

Health Tip: a floor which conducts enough to damp buzz will conduct enough to electrocute you. One reason I've lived too long is rubber shoes on wood floors.

[exztinct01]

oh sorry I forgot to mention my effect is still on a breadboard since I was testing it before soldering. I desoldered my earlier ds+ pedal because the pcb has bad traces so I was currently testing different clipping diodes and configuration. Maybe, the fact that it was still on a breadboard was the reason of that buzz.
I'll solder this then and try again after putting it in a case 😊

[bluebunny]

Maybe, the fact that it was still on a breadboard was the reason of that buzz.

Yes, likely.

and try again after putting it in a case

...and after putting your (rubber) shoes on.