Proco Rat Clone problem

[gtudoran]

Hello guys couple of years ago i build a rat from tonepad, this schematic http://www.tonepad.com/getFile.asp?id=89 I finished another one today but i do have some problems with it. The only mods that i've done are:

- all pots 100kB (dont have 100kA at the moment)
- ruets mod (replaced the 560 and 47 ohm rezistors with 2 x 1kB pots)
- 30p between pin 1 and 8 i used a 46p (didn't had a 30p ceramic cap)
- insted of 2n5458 i used a 2n5457

And that is all .. now the problem is this: the effect is working only form 2% to 10% max. from the gain pot any value below or above i have to strike very hard on the strings to have some output.

The thing that i observe is that on the pin3 of the IC i have only 2.36V - but the bias voltage (in Vb point) is 4.48v - could this be the problem?
I cheked and recheked parts values - no problem there ... also no problem with solder bridges ...

I would appreciate a kick in the esse to go forward

Thank you in advance guys.

Best regards,
Gabriel Tudoran
Analog Sound

[aron]

>above i have to strike very hard on the strings to have some output.

This sounds like the classic mis-biased circuit syndrome.

Maybe your problem is not with the IC but at the output section? If you had an audio probe (debugging page), you could listen to the circuit at pin 3 and 6.

[K Zustang]

Hey Guys,

I have just been experiencing the same kind of problem. That is, The Rat Clone only outputs sound on the low side of the Drive knob.
Anything above 10-15% CC gets muted dramatically.
If that is a bias problem as Aron states. what should be checked with an audioprobe?
If the problem is on the 2N5458 side? How is this bias controlled which leg should get a different resistor?

Thanks
Doron

[knurra]

missing a dc blocking cap?

[PRR]

> If that is a bias problem as Aron states. what should be checked with an audioprobe?

Bias problems are checked with a DC volt-meter.

[K Zustang]

Well I started with the Audio Probe and it seems everything is quite OK on the input side of the LM308N (leg 3), but raising GAIN pot to 20% or so and the sound on the output leg (6) mutes also on the audio probe. So I guess the problem is somewhere around the LM308 but all the voltages look right:
the voltages when gain is 0 on the chip legs are as follows:
1. 8.3V
2. 4.45V
3. 4V
4. 0V
5. 0V
6. 4.45V
7. 8.9V
8. 5V

Does these voltages look OK?

When the Gain is raised to max:
1. 8.25V
2. 2V
3. 2.5V
4. 0V
5. 0V
6. 8.2V
7. 9V
8. 8.8V

Is that strange or OK?
I changed the chip and got more or less similar results.

Thanks.
Reallly want to get this pedals done already.

Zustang

[K Zustang]

I forgot to mention that the DC caps I am using for the 4.5V (the 1uF) is a non polarized cap.
Is it a must using polarized caps? I just used what I have and I have some 1uF Sprague non polarized caps laying around.

Zus

[ashcat_lt]

Something going on with the caps to ground in the feedback section.  The pots you put in are wired as variable resistors with no lugs grounded (purposely or accidentally)?

[PRR]

> when gain is 0
    1. comp
in- 2. 4.45V
in+ 3. 4V
V-  4. 0V
    5. nc
out 6. 4.45V
V+  7. 8.9V
    8. comp
> When the Gain is raised to max:
    1. comp
in- 2. 2V
in+ 3. 2.5V
V-  4. 0V
    5. nc
out 6. 8.2V
V+  7. 9V
    8. comp

What ashcat says. Acts like the 4.7u and 2.2u capacitors are shorting to ground. (And somehow when that action slams output to the + supply, it also sucks excess input current.) Could this be a mistake in the "ruets mod"? You need cap(s) there.

> the 1uF is a non polarized cap.

OK. (Costs more, but since it was laying around...)

> 30p between pin 1 and 8 i used a 46p

Not today's problem. Eventually you should get a 30pFd. When working (correctly) at MAX gain, the 30p slows the opamp, a 47p slows it more. This means less extreme treble. It may sound fine with 47pFd, and if you play mellow you might like 100pFd. Values less than 30pFd make it a little brighter but it may go-crazy at minimum gain.

[K Zustang]

Wow guys, thanks for the help.
As a matter of fact the two capacitors are also non polarized. Could this be it? I started witth the Ruetz mod but went back to original when the problem persisted?
The two caps are supposed to be connected to ground. Do you mean that it sounds as if there is a short to ground before the caps?

The thing is that I had a few old electronic boards filled with caps and what not and I ripped it apart and now I have a lot of non polarized caps and all my polarized 1, 2.2, 3.3, 4.7 uF are actually those.

should I replace the ones on the LM308N leg 2 with polarized ones?

Zus

[ashcat_lt]

They don't need to be polarized.  I think that they're shown as polarized because high values like this are easier to find (and smaller in physical size?) as electrlytics.  

It seems to me (and PRR seems to agree) that there really has to be some DC path to ground in the feedback loop to cause the symptoms you've described.  You should measure DC resistance to ground on either side of the gain pot with it rotated both ways.

Have you actually tested these salvaged caps?  If they've failed (mostly) shorted, or very leaky (?) it could cause the problem.  It can't be a dead short here, or a complete bypass of the caps because even 560R/100560R x 9V < 2V.  But there's something funny happening in that feedback loop!

[K Zustang]

Guys....Ashcat, PRR...you solved it!

It works!!!!!!!!!!    

I replaced those two salvaged caps with regular crappy polarized caps and it works like a charm.
Measured the resistances as you said afterwards and all checked out OK - infinite resistance between gain lugs and ground for both ends of the dial.

I will put the Ruetz mod back with the diode section switch and the overdrive and start boxing it.
Your help was simply super.

Thanks again,
Zus

[ashcat_lt]

Glad I could actually be of assistance in a troubleshooting question.  I don't know much about this stuff, but when it comes to non-inverting opamp circuits...

...I've still got a lot to learn.

[seedlings]

Guys....Ashcat, PRR...you solved it!

It works!!!!!!!!!!    

I replaced those two salvaged caps with regular crappy polarized caps and it works like a charm.
Measured the resistances as you said afterwards and all checked out OK - infinite resistance between gain lugs and ground for both ends of the dial.

I will put the Ruetz mod back with the diode section switch and the overdrive and start boxing it.
Your help was simply super.

Thanks again,
Zus

Hey!  I appreciate you taking the time to let us know what the trouble was, and that it's working.  There are a lot of threads where troubleshooting is done and we're all left hanging.

CHAD