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I received a guitar pedal in poor condition and looking to repair. Checking through the schematic, it appeared the electrolytic caps were installed backwards. All other picture I could find online had the caps orientation opposite what was in this pedal (as per schematic). Looking at the schematic, I'm pretty sure the ones in this pedal were installed backwards. I did not attempt to power on the pedal due to the received condition and was just checking through to make sure things were ok. The light bulb is a 387 bulb as stated in one of the answers (28v 40ma). So I think there is an error on this schematic and for now let's assume there is a proper ground in the power loop.

Is the resistance value for the 387 bulb correct? I've seen values ranging from 150, 200, or 220 ohms for this resistor.

Second question is will this still work correctly if all of the capacitors were reversed polarity? In other words if the + in the schematic was oriented to the bottom vice top would this cause any issues? I assume this would mess with all the output voltages?

Trying to learn something here.!

Pedal

Image source: Schematic Heaven - Tel-Ray/Morley PWB Wah/boost

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  • \$\begingroup\$ Where did this schematic come from? Please link the source. Is 387 the resistance of the bulb? A part number? Polarized capacitors are polarized for a reason. Flipping them around will destroy them. \$\endgroup\$
    – vir
    Commented Feb 18 at 2:02
  • \$\begingroup\$ A 387 incandescent bulb is rated for 28 volts, 40 mA and would not normally be used with a series resistor. In that circuit there is apparentlu no path for current through the lamp so the lamp and resistor are irrelevant. \$\endgroup\$
    – Peter Bennett
    Commented Feb 18 at 2:07
  • \$\begingroup\$ Seeing the full schematic, it appears that the 387 bulb is relevant. I assume that the bulb shines on the WAH LDR at the top of the schematic. The bulb brightness will depend on the current drawn by the circuit, and will affect the resistance of the WAH LDR. \$\endgroup\$
    – Peter Bennett
    Commented Feb 18 at 2:57
  • \$\begingroup\$ Yes! There is a curtain attached to the pedal that covers/uncovers the LDR's. The bulb's brightness remains constant. At least it appears not to dim when signal is applied through the pedal. \$\endgroup\$
    – Glenn
    Commented Feb 18 at 3:00
  • \$\begingroup\$ Thanks for adding the link to the source of the schematic, as required by the rules here. \$\endgroup\$
    – SamGibson ♦
    Commented Feb 18 at 3:20

1 Answer 1

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  1. The resistor is irrelevant, since there is no other connection to ground, and there is no current flowing through that part of the circuit.
  2. Electrolytic capacitors are polarized, meaning when connected backwards, the thin film of oxide is removed and the cap dies. poof

Learn about the function of diodes.

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