Tonepad Rat debug (filter control)

[himey77]

I built my Tonepad Rat and everything seems to work except for the filter control.

What should I look at as culprits??

[petemoore]

  The  schematic used [  ]? probably has no culprits in it.
  It is probably perfect, and comparing [under applied DMM/probe/debugging scrutiny] a RAT circuit to it should soon reveal any differences/problems.
  See "Debugging' thread.

[himey77]

I am using the schematic for the Landgraff MO'D.

I tried a couple of different 100k Audio pots to rule out a bad pot and rewired.

I checked for solder bridges and glaring errors. I'm not sure if the filter lugs should be receiving power or what the voltages should be. 

I will post pics of my board and list my voltages for the IC and FET soon.

[petemoore]

  It looks like the 100k is a series variable resistor in the signal path, and works with the .0033uf to rolloff HF's.
  So one end of it goes to the btb diodes and 4u7 cap from the opamp output, the other end to the 1k5..[and nowhere else].

[himey77]

I'm using the Tonepad Rat board







Here are my voltages:

Battery: 8.57

IC Pins
1  7.86
2  4.29
3  3.88
4  0.00
5  4.38
6  8.51
7  4.29
8  0.00

fet G 8.50  S 7.78  D 7.79

I'm all kinds of confused. In other Rat builds, the voltage for the gate on the Fet is 0.00   

[soulsonic]

It looks like the JFET is backwards.

[himey77]

Fixed the FET. I must have left it in backwards during my "debugging by experimentation" phase.  I have been getting sound and there is no output with the FET in backwards.

Voltages for the FET

G - 0.00
S- 8.44
S- 8.48

There is barely any output when the volume knob is at 100% and there is severe buzzing when the volume knob is set to anything other than 100%. Plus, the Filter control is still inactive.

[himey77]

Still no solution after several more hours of tinkering. I replaced the diodes and corrected the backwards FET.

The distortion pot is the only pot that seems to function properly. The filter control does not work at all, and the volume pot puts out very little output only when it is fully on.

I also tried replacing the IC with TL071 and another LM308. I still have the same symptoms.

This is one of my first builds so any help would be appreciated. Please tell me what information to provide.

[himey77]

Several days later and I still haven't given up or figured out my problem. This is my first build outside of a kit and I've read so much in the last month that I think my head will explode. Desperately seeking help.

I created an audio probe and I'm not sure how to decipher the results. I'm not familiar with how the IC functions in this circuit.  I downloaded the datasheet for LM308N, but again it's not much help to me.

I see that that output is at PIN 6. Using the audio probe, at this point in the signal chain the volume is extremely low compared to the input of the circuit.

Using a multimeter I see that only the Distortion pot is getting voltage. Should the Filter pot and Volume pot be getting power?

PIN 5 connects to the positive lead of the 4.7uf cap. Using a multimeter, I see that the positive lead of this cap gets 4.24V (with battery at 8.48V), however, the negative lead of this cap is at 0.0V. Is this the correct voltage for the negative lead of this cap?  I can't understand why it's grounded. Using the audio probe in the portion of the circuit. The positive lead creates louder-than-input, distorted signal. The signal stays loud and distorted until it reaches the leg the 1k resistor connected to lug 2 of the filter pot. This is where the signal significantly weakens and does not seem to recover. If the circuit relies on the JFET to recover the volume, it is not happening.

Thanks in advance for any assistance.