8
b. If the optocoupler’s LED is lighting up then you know the problem is with the analog
part of the circuit.
i. Make sure the jumpers are soldered properly
ii. Check for bridged or bad solder joints on all the components on the non-working
side of the circuit. For the HPF side check: R17-R20, R4, S6, and C2. For the LPF
side check: R21-R24, R5, S5, and C3. The PCB gets kind of crowded around the
DIP switches and all the stand-up resistors...It’s easy to bridge solder joints around
this area.
iii. Check TP1-TP4 for ~4.5V.
iv. Check all the pins on U2 for proper voltage...See the U2 VOLTAGES section.
v. Test for continuity across the pins of the individual DIP switches that are turned ON.
APENDIX B: U2 VOLTAGES
The IC U2 is a quad opamp...Meaning there are 4 separate amplifier sections in it. These amplifiers
are called “operational amplifiers” or “opamps” for short. Each amplifier section has 3 pins associated
with it. Here is a list of the pins in each amplifier section and their associated parts.
Opamp A: Pins 1,2,3; with associated parts C3, R21-24
Opamp B: Pins 5,6,7; with associated parts C1, R2, R3, R15
Opamp C: Pins 8,9,10; with associated parts R4, R5, R6, VR10, C4
Opamp D: Pins 12,13,14; with associated parts C2, R17-R20
Each pin of these individual opamps should be set to the bias voltage (given by R10) which should
be about half of the voltage supplied to the pedal (9V)...so the bias voltage should be around 4.5V. If
you notice the bias voltage is off by over a volt or 2 then there is a problem somewhere. Most of the
time it means there is a short (solder bridge) between 2 pads somewhere causing the bias voltage to
get shunted to ground, but on a very rare occasion it could mean that the IC has been damaged. If the
voltage of one of the these opamp sections is off, check all of it’s associated parts for bad solder joints
or bridges. If the bias voltage is off on all of them check the voltages on R10. Keep in mind, opamps A
and B share a bias voltage, so if there is a problem with one of those 6 pins, the voltage will probably
be off on all of them.
Summary of Contents for QUAK
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