Debugging noisy tonepad Rat

Started by Bassco, May 18, 2007, 06:22:12 AM

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Bassco

Hey guys,

I have build the Rat clone from tonepad.com. It is very noisy when not playing.
Could someone help me pinpoint the problem? Below is the list with measurements
and descriptions.

Thanks a lot.


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

There comes a sound of it. When playing I have got a distorted sound.
This distoreted sound is not that great sounding. When I remove the clipping
diodes the sound gets a little bit louder.

When not playing there is a lot of noise. This happens when I turn up
the volume of the guitar. When the guitar volume is off there is no
noise anymore. I tried 2 different guitars.

The circuit behaves the same with the LM308N and the TL071CP IC's.

I have created an audio probe and found out that the noise starts somewhere
with the 27pF (30 originally) ceramic capacitor. At pin 1 of the IC there is no humming.
After the capacitor at pin 8 there is the noise. Could it be the wrong value or
just a broken capicitor?


2.Name of the circuit =

La Rata
Ordered PCB from tonepad

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

http://tonepad.com/project.asp?id=45

4.Any modifications to the circuit? Y or N

No

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

Yes: the 30pF capacitor is replaced by a 27pF ceramic one

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? =>

I am using an adapter: voltage: 9.3
The adapter is working with the CE-2 I built, I also tried the battery and it had the same effect.

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 = 9.3

Voltage at the circuit board end of the black battery lead = 0

IC1 -> LM308:
P1: 8.52
P2: 4.64
P3: 2.26
P4: 0
P5: 0
P6: 4.62
P7: 9.28
P8: 4.58

Q2 -> 2N5458:
C(D) = 9.29
B(S) = 1.54
E(G) = 0

D1 -> 1N914:
K = 0.05 (Changes)
A = -0.00

D2 -> 1N914:
K = -0.00
A = 0.06 (Changes)

D3 -> 1N4002
A = 0.00
K = 9.25

GibsonGM

Hi Bassco,

Good job posting all of the information!  It is frustrating when something has a problem, but I'll try to help. I haven't built the BSIAB yet, but have some experience that might be useful for you.  About 98% of the time, a soldering error causes these issues; so, a very close inspection of the circuit is in order to eliminate that!   This is an ambitious build, so mistakes are to be expected.

I do not think pins 1 & 8 are the problem...your capacitor change to 22pF should not cause any issues, and they are only compensation pins.  It's worth noting that caps in the signal path should be polyester or any other material than ceramic or tantalum, since they are noisy...but probably are not the cause of your issues here since you say the problem is extreme. 

Your opamp voltages look OK...not sure about those at the 2N since I don't use them very much.  The FET is a booster here, anyway, so the signal should be nice but lower in volume before it.  One source of trouble could be that it is in backwards (?).

Double check pin 3, the 1M to Vb; that voltage seems a little low, maybe should be about 2V higher.   In fact, please post the voltage at the 1uF capacitor at your biasing network just for the heck of it... 

I'm concerned about the very low voltage on D1 & D2, I think there should be something more there! You post 4.62V on the output, pin 6...some of that should show up as AC at the junction of D1 and the 1K.   So maybe a short somewhere before them.   How does the output sound at pin 6 with the audio probe, while playing a chord?

This should get you started.  I'm going out of town for the weekend, but someone is sure to see your post and will help you more. 
Good luck!

Mike  :o)
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Bassco

I have done some remeasurements, here are the results:

I have checked the FET. It is is not in backwards. I tried it the other way and the result was that there was no output at all.

Pin 3 is really 2.26 volts.

1M to Vb on Vb side = 4.42
1M to Vb on 0.22 side = 2.25

Voltage at the 1 uF capacitor = 4.41

Voltage at the 4.7uF Capacitor + after pin 6 = 4.63
Voltage at the 4.7uF Capacitor - after pin 6 = 0.00 (When I touch the top of the cap the value rises)

If I short the pins of this cap the noise is gone and the resulting sound is softer. Maybe the cap has got a bad solder joint or it might be broken.

The output at pin 6 sounds less distorted but when not playing it is still noisy.

MAybe the capacitor after pin 6 is broken or not soldered allright,  will try to resolder the 4.7uF capacitor after pin 6.

Bassco

Well, I resoldered the capacitor and it is still very noisy.

Could it be that the circuit is supposed to be noisy or that it is making the noise because it still isn't in a box?

Auke Haarsma

Quote from: Bassco on May 20, 2007, 05:06:19 AM
(...) or that it is making the noise because it still isn't in a box?

Boxing up a OD/Distortion FX can make a huge difference to noise. The Alu box shields it from interference and the like.

Could you btw describe the noise? Is it humming? Or hissing? or etc?

Bassco

I finally boxed the effect and this made a big difference. It was a kind of humming noise and would get stronger when I would touch some of the wires. Thanks.

oldrocker

Maybe a grounding issue?  Loose wire. Cold solder joint.