Switching between pairs of parallel diodes - stumped.

[Yeahno]

Shall do, Paul.

Thanks.

[Yeahno]

1.What does it do, not do, and sound like?  

Distortion pedal.  Works, distorts.  But when the original Si diodes (1N34) are swapped for Germanium, all I get is a a clean, loud signal.  Paul suggests that something's not right with the circuit.

2.Name of the circuit =

ROG Tube Reamer

3.Source of the circuit (URL of schematic or project) =

http://www.runoffgroove.com/tubereamer.html

4.Any modifications to the circuit? Y or N

Y.  Power jack, LED added, following this diagram:

http://www.generalguitargadgets.com/diagrams/switch_lo_3pdt_tb_dcjack.gif?phpMyAdmin=78482479fd7e7fc3768044a841b3e85a

5.Any parts substitutions? If yes, list them.

Yes.  

100k-A pot swapped for a 250k-A

6.Positive ground to negative ground conversion? Y or N

N.

7.Turn your meter on, set it to the 10V or 20V scale. Remove the battery from the battery clip. Probe the battery terminals with the meter leads before putting it in the clip. What is the out of circuit battery voltage? =>

8.45

Now insert the battery into the clip. If your effect is wired so that a plug must be in the input or output jack to turn the battery power on, insert one end of a cord into that jack. Connect the negative/black meter lead to signal ground by clipping the negative/black lead to the outer sleeve of the input or output jack, whichever does not have a plug in it. With the negative lead on signal ground, measure the following:

Voltage at the circuit board end of the red battery lead =

3.97

Voltage at the circuit board end of the black battery lead =

3.67

Now, using the original schematic as a reference for which part is which (that is, which transistor is Q1, Q2, etc. and which IC is IC1, IC2, C1, and so on) measure and list the voltage on each pin of every transistor and IC. Just keep the black lead on ground, and touch the pointed end of the red probe to each one in turn. Report the voltages as follows:

UA1 (which is an NE5532):

P1 = 3.95
P2 = 3.94
P3 = 3.58
P4 = 0.00
P5 = 3.94
P6 = 3.94
P7 = 3.96
P8 = 8.34

D1
A (anode, the non-band end) = 3.93
K (cathode, the banded end) = 3.92

D2
A = 3.92
K = 3.94

For extra credit, while you're waiting for someone to tell you what they see, probe the pins of each of the electrolytic caps, verifying that the voltage on the (+) pin is more positive than the voltage on the (-) pin. If it's not, that cap will eventually fail, whether it's the immediate cause of the thing not working or not.

The only polarised cap is reading = 0VDC at negative, 4.1V at positive

[PRR]

> Voltage at the circuit board end of the red battery lead = 3.97
Voltage at the circuit board end of the black battery lead = 3.67

From a 8.45V battery?? Something wrong there.

> UA1  P8 = 8.34

And how is there more voltage on the chip than on the battery lead??

However..... in this circuit, the diodes may not clip much. There's 1:6 gain after the diode stage, and the second stage will clip about 3V. So if diode voltage is greater than 0.5V, the output stage clips first and masks some of the diode clipping.

So it is a bit odd that it uses 0.6V Silicon diodes.

Just for curiosity: lift one end of the 100nF cap. Gain will drop, turn-up the output Volume. Now can you hear a difference Ge Si LED?

[Yeahno]

> Voltage at the circuit board end of the red battery lead = 3.97
Voltage at the circuit board end of the black battery lead = 3.67

From a 8.45V battery?? Something wrong there.

Yes.  I am an idiot. 

It's 8.34V.

> UA1  P8 = 8.34

And how is there more voltage on the chip than on the battery lead??

Now that I've metered it properly, there isn't.

However..... in this circuit, the diodes may not clip much. There's 1:6 gain after the diode stage, and the second stage will clip about 3V. So if diode voltage is greater than 0.5V, the output stage clips first and masks some of the diode clipping.

So it is a bit odd that it uses 0.6V Silicon diodes.

So you're saying that you'd want diodes with a lower voltage than .5v or they won't do anything?  This would be the forward voltage, right?

Just for curiosity: lift one end of the 100nF cap. Gain will drop, turn-up the output Volume. Now can you hear a difference Ge Si LED?

I lifted the 100n.  Gain dropped.  I turned up the volume and was still able to hear the switch between diode pairs.  But this wasn't at issue before.

Again, to be clear: I *do* hear a difference when I switch between diode combinations (including diode combinations that include one germanium diode).  But combine two Ge diodes and the circuit reverts to the perfect, loud, crystaline sound described above - there is no distortion whatsoever.  Any other combination (Ge/Si, Si/LED, LED/Ge) gives me lovely, crunchy distortion.  It's only the Ge/Ge combination that, mystifyingly, doesn't.

Thanks for your patience with me as I fumble around.