Shin-Ei Fuzz-Wah - Univox/Unicord Super Fuzz - debug and verification (2/4)

The Fuzz-Wah pedal must first be powered with an external +9V DC power supply (AC/DC adapter or 9V battery). If an external AC/DC adapter is used tip is connected to - and sleeve to +.
See this blog entry for additional information on debugging instruments used (multimeter, signal generator and oscilloscope)
The first stage is the verification of DC voltages with a multimeter, +9V DC input is doubled by U1 charge pump regulator ICL7660S. The voltage at the output of the dual diode D2 is actually closer to +16V due to the diodes drop.
We will compare the results of the real pedal with the LTSpice simulation.

Fuzz-Wah pedal schematics

LTSpice Schematics

The next step is to use a signal generator and check the signals on the different test points with an oscilloscope. I used a 2.5 kHz 0.3V peak-to-peak sinewave at the input. See figure 1.

Fig 1. 2.5 kHz 0.3 V peak-to-peak sinewave at the input (TP1)

Q1 and Q2 amplify the input signal and define the maximum gain of the signal. Gain potentiometer R9 allows reducing the level of the amplified signal.

The signal at TP2 after a two transistors (Q1, Q2) amplifier is amplified by 15.4 with a voltage amplitude of 4.63V peak-to-peak

Fig 2. 2.5 kHz 4.63 V peak-to-peak sinewave after Q1-Q2 amplifier (TP2)

Fig 2.1. LTSpice simulation at TP2

Q3 is a transistor amplifier. Signals at the collector and emitter are used as input  to the octave doubler Q4, Q5. Gain potentiometer R9 is set to maximum. The signal at TP3 at the base of Q5 and connected to the emitter of Q3 through a resistor in series with a capacitor has an amplitude of 4.19 Vpp and appears slightly clipped on the positive cycles.

Fig 3. 2.5 kHz 4.19 Vpp clipped sinewave after Q3 emitter at Q5 base (TP3)

Fig 3.1. LTSpice simulation at TP3

The signal at TP4 at the base of Q4 and connected to the collector of Q3 through a resistor in series with a capacitor has an amplitude of 5.66 Vpp and appears clipped on the positive cycle and sharped on the negative cycle.

Fig 4. 2.5 kHz 5.66 Vpp clipped and distorted sinewave after Q3 collector and Q4 base (TP4)

Fig 4.1. LTSpice simulation at TP4

Q4 and Q5 implement the octave doubler, the signal at TP5 at Q4-Q5 collectors has a frequency of 5 kHz, double of the input signal, with an amplitude of 0.77 Vpp. Both cycles are strongly clipped.

Fig 5. 5kHz 0.77 Vpp frequency doubled and clipped signal at Q4-Q5 collector (TP5)

Fig 5.1. LTSPice simulation at TP5

Dual Schottky diode D1 in series with R22 (100 ohms) clips the signal in a similar fashion to a germanium diode (see this blog entry for additional information on the use of Schottky diodes to replace germanium diodes). Signal on TP9 is very similar to the signal at TP5, since between them there is only a 10uF AC coupling capacitor to remove DC biasing.

Fig 6. 5kHz 0.78 Vpp frequency doubled and clipped signal at R22 + D1 clipping diode (TP9)

Fig 6.1. LTSpice simulation at TP9

R25, R26, C12, C13 implement the tone cut filter, R23, R24 implement a voltage divider so that the signal level at TP10 is similar when cut tone is used or bypassed by S2 switch.
Signal at TP6 is TP9 clipped and frequency doubled signal after the tone cut filter. It has an amplitude of 0.61 Vpp.

Fig 7. Signal after Tone cut filter 0.61Vpp (TP6)

Fig 7.1. LTSpice simulation at TP6

Signal at TP8 is TP9 clipped and frequency doubled signal after voltage divider and has an amplitude of 124 mVpp. R27 is the main volume potentiometer and is set to maximum.

Fig 8. Signal after voltage divider 0.124 Vpp (TP8)

Fig 8.1 LTSPice simulation at TP8

Q6 is another transistor amplifier which is the output of the fuzz effect without wah. Fuzz signal at TP12 when tone cut filter is bypassed has an amplitude of 0.53 Vpp

Fig 9. Signal at the output of Fuzz effect with Tone Cut filter bypassed (TP12)

Fig 9.1. LTSpice simulation at TP12

Signal at the output of the fuzz effect (TP12 ) when tone cut filter is on has an amplitude of 1.28 Vpp.

Fig 10. Signal at the output of Fuzz effect with Tone Cut filter on (TP12)

Fig 10.1 LTSpice simulation at TP12

Q7, Q8, Q9 transistors implement the Wah effect. R43 is the Wah potentiometer that tunes the Wah band-pass filter frequency.

Signal at Q7 collector (TP13) with Tone cut filter off and wah at potentiometer at 0 has an amplitude of 2 Vpp.

Fig 11. Signal at Q7 collector with Tone cut filter off and Wah pot at 0 (TP13)

Fig 11.1. LTSpice simulation at TP13

Signal at Q7 collector (TP13) with Tone cut filter off and wah at potentiometer at 10 has an amplitude of 99 mVpp. 

Fig 12. Signal at Q7 collector with Tone cut filter off and Wah pot at 10 (TP13)

Fig 12.1. LTSpice simulation at TP13

Signal at the output of Wah effect (TP15) with Tone cut filter on and wah pot at 0 has an amplitude of 2.9 Vpp.

Fig 13. Signal at Wah output with Tone cut filter on and Wah pot at 0 (TP15)

Fig 13.1 LTSpice simulation at TP15