Not the usual fare, but someone might find it interesting.
My brother brought me an old Ibanez flanger guitar pedal and asked if I could fix it. I warned him that a flanger was about as complex as old analog pedals got, but I'd give it a try. It was totally dead - no flanging, no change when the switch was pushed. I found a photocopy of a photocopy of a schematic that was more or less readable. The footswitch was a momentary switch that toggled a discrete transistor flip-flop. When the effect was off, the guitar just ran through a couple of op amp buffers. When on, the flip-flop drove an indicator LED, and also enabled (via a diode) a FET switch that mixed the delayed sound with the clean sound to create the flanging effect.
I took it apart and cleaned things up a little, and realised that the plastic footswitch surround was cracked, and the momentary switch was permanently held down. Fixed the mounting with a generous helping of superglue, and tried again. Success! The LED turned on and off when the footswitch was pressed. Plugged in a guitar and turned it on - yay! Flanging! Pressed the footswitch again - not so yay! The LED was now off, but the pedal was still flanging!
At this point it seemed clear - the flip-flop was working, so either the FET or the diode must be broken. The FET was some number I'd never heard of, and the diode looked ordinary, so I desoldered both (with quite a bit of effort) and tacked a replacement 5457 FET and 4148 diode on the back of the board to test. No change, the pedal was still permanently on.
I started looking more carefully at the circuit and measuring some voltages. The op amps used a standard 4.5V virtual ground, using two 10k resistors to 9V and 0V, and an electrolytic capacitor to keep things smooth. The same virtual ground was also connected to the source and drain of the FET, so that the flip-flop could pull the gate down to 0V (plus a couple of diode drops) to switch it. Except ... the virtual ground measured a little over 1V, low enough to stop the FET switching, but just high enough to keep the op amps working. Hah! Obviously the electrolytic capacitor had died, and was pulling the virtual ground down. More desoldering, leave it empty for now, and - no difference.
Weird. There didn't seem to be anything else likely to fail. I finally tested the two 10k resistors - one was right, the other was way off. I don't think I've ever seen a resistor fail outside of a power supply before, but there it was. Grabbed a new 10k off the shelf, replaced all the original components, screwed everything back together, and told my brother his pedal was fixed.
My brother brought me an old Ibanez flanger guitar pedal and asked if I could fix it. I warned him that a flanger was about as complex as old analog pedals got, but I'd give it a try. It was totally dead - no flanging, no change when the switch was pushed. I found a photocopy of a photocopy of a schematic that was more or less readable. The footswitch was a momentary switch that toggled a discrete transistor flip-flop. When the effect was off, the guitar just ran through a couple of op amp buffers. When on, the flip-flop drove an indicator LED, and also enabled (via a diode) a FET switch that mixed the delayed sound with the clean sound to create the flanging effect.
I took it apart and cleaned things up a little, and realised that the plastic footswitch surround was cracked, and the momentary switch was permanently held down. Fixed the mounting with a generous helping of superglue, and tried again. Success! The LED turned on and off when the footswitch was pressed. Plugged in a guitar and turned it on - yay! Flanging! Pressed the footswitch again - not so yay! The LED was now off, but the pedal was still flanging!
At this point it seemed clear - the flip-flop was working, so either the FET or the diode must be broken. The FET was some number I'd never heard of, and the diode looked ordinary, so I desoldered both (with quite a bit of effort) and tacked a replacement 5457 FET and 4148 diode on the back of the board to test. No change, the pedal was still permanently on.
I started looking more carefully at the circuit and measuring some voltages. The op amps used a standard 4.5V virtual ground, using two 10k resistors to 9V and 0V, and an electrolytic capacitor to keep things smooth. The same virtual ground was also connected to the source and drain of the FET, so that the flip-flop could pull the gate down to 0V (plus a couple of diode drops) to switch it. Except ... the virtual ground measured a little over 1V, low enough to stop the FET switching, but just high enough to keep the op amps working. Hah! Obviously the electrolytic capacitor had died, and was pulling the virtual ground down. More desoldering, leave it empty for now, and - no difference.
Weird. There didn't seem to be anything else likely to fail. I finally tested the two 10k resistors - one was right, the other was way off. I don't think I've ever seen a resistor fail outside of a power supply before, but there it was. Grabbed a new 10k off the shelf, replaced all the original components, screwed everything back together, and told my brother his pedal was fixed.
Statistics: Posted by nigel — Wed Dec 18, 2024 12:02 am — Replies 3 — Views 110