What's misbiasing my Guv'nor?

[Herr Masel]

I built the guv'nor according to the GGG schematic http://www.generalguitargadgets.com/diagrams/guvnor_sc.gif except that I ommitted the effects loop and connected the node of R2 and C4 straight to the +9v, and I left out the LED and 4k7 resistor.. The battery I'm using is a little on the low side (8.07v) because it's all I have right now but it should be enough shouldn't it?

After getting very low output (though disotrted) and very fast decay I measured the voltages.

At +9v/pin 8 of the IC:       6.68v
At pin 3:    1.68
At pin 5:    3.38v

What could be causing this problem, and where could I stick a trimpot if it occurs right at the source (battery)?

I've checked the circuit for mistakes a thousand times, though my experience has taught me never to say I'm positive that I did it right. Still, I can't find the problem.

Thanks. Yaniv.

[MartyMart]

I had a similar problem with the Boss AC-2 clone, my VB was being drained so :
Check your voltage divider and all connections/ traces from 4.5V.
Pin 5 looks right ( from your low battery ) - though I'd get a fresh one anyway !
Check all opamp pins for solder blobs and all values .... you'd be surprized how
a 47k can look like a 4k7 for instance

MM.

[slacker]

The junction of R2 and C4 shouldn't be connected to anything. If you connect it to 9 volts it will mess up the bias and you won't get any sound out of the pedal.

[Mike Burgundy]

What that node and the switching as shown does is clamp the input of the opamp to ground when the pedal is off. It is not connected when the pedal is on.
If you use a different (true-bypass anyone?) switching scheme, don't connect that node to anything as it would be withe the pedal on and the original wiring.
Clamping it to 9V will result in no signal (V+ is the same as ground as far as the signal is concerned) but shouldn't influence bias - it's blocked on both ends by a cap.

[MartyMart]

The junction of R2 and C4 shouldn't be connected to anything. If you connect it to 9 volts it will mess up the bias and you won't get any sound out of the pedal.

Very good point, no 9v required there !

MM/

[Herr Masel]

Thanks for the answers but like I siad it does actually work with the 9v connected there (just sounds bad), and I only connected it recently because I thought I had made a mistake in leaving it out. That makes me think that maybe I did miss a connection somewhere if you all say it shouldn't be working. Before that I had the exact same problem, perhaps the voltages were a bit lower. I am in a hurry now so I don't have time to look at the pedal but I'll try again and let you know. I did however check the voltage divider and measured the resistors and all's good there.

Thanks, Yaniv.

[Mike Burgundy]

Simply remove the 9V connection and debug from there.

[Herr Masel]

I put the guvnor aside for a few days and with new energies I came back to it. I'm still making stupid mistakes coming from lack of patience, I really need to learn to deal with this. Anyway, I bought a new battery and with a fresh mind came back to the board with an audio probe. The things I changed are adding a LED (and wiring the switch correctly), and changing lugs 1 and 3 on the gain pot, which was wired backwards. The thing is I now have NO sound when the pedal is on .

The voltages are:

Pin 8:   8.8v
Pin 3:   2.15

All the rest are 4.4, other than pin 4 which is grounded.I went over the circuit with an audio probe, it sounds good till the output of the drive pot, when I get to pin 6 the sound disappears (is this because it is the inverting input or should this not be happening?), only to come out again from pin 7, high frequencies very slightly attenuated, no additional gain or volume....

Please try and help. I am so impatient and plain bad at this I want to cry.

[Gus]

first measure the voltage at the top of R4(battery voltage) and then at the junction of R4 and R5

if the voltage is not 1/2 the battery voltage at the R4, R5 node you might have the wrong values at R4 or R5

Look for solder shorts and open or bad connections

pin 3 and pin 5 and the outputs  pin 1 and pin 7 (set gain at min) should be the same voltage as the R4 R5 node

Get that fixed and then work on the rest.

[Herr Masel]

Pin 3 has a 1M resistor between it and the voltage divider, so I don't think it should have the same voltage as pin 5 (it's about half of it). Like I said, 1, 2, 5 and 7 are all 4.4v. What else should I be looking for?

[Gus]

Pin 3 should be the same voltage as 5, 1 and 7.  If pin 1 is 4.4VDC then maybe the meter is loading down pin 3.
What type of meter are you using older analog or DMM?
  What is the loading caused by the meter?

 

[Herr Masel]

Pin 3 should be the same voltage as 5, 1 and 7.  If pin 1 is 4.4VDC then maybe the meter is loading down pin 3.
What type of meter are you using older analog or DMM?
  What is the loading caused by the meter?

I'm using a cheap digital multimeter. I'm sorry, but I don't understand why pin 3 should be the same voltage as pin 5 if it is connected to the voltage divider through a 1M resistor, unless it was getting voltage from inside the IC (pin , which would give it the full 9v.

Also, I'm sorry but I don't understand what you mean when you ask "What is the loading caused by the meter?"