A colleague gave me a BOSS ME-33 multi-effects pedal that he had previously bought as a faulty unit. The pedal was apparently working, but the sound was badly distorted, not all the time.
This turned out to be a nice little troubleshooting exercise, because the initial symptom suggested an analog audio problem, while the real cause was actually related to the power supply.
The main symptom was a heavily distorted output signal. It didn't sound like a normal overdrive or distortion effect. Instead, it sounded more like the signal was being clipped somewhere in the analog circuitry. The interesting part was that the problem was intermittent: depending on how hard I played the guitar, the distortion could appear or disappear. That immediately made me suspicious of the signal level somewhere in the input or analog processing stages.
My first thought was that the problem could be caused by the first operational amplifier after the guitar input. Since an op-amp failure can certainly produce strange distortion, I replaced it. Unfortunately, it made no difference at all.
If an analog circuit clips when the input signal gets larger, one possible cause is that the circuit simply doesn't have enough voltage headroom. So I started checking the internal power rails.
While measuring the different supply voltages, I noticed something suspicious: the +4V rail was lower than it should have been.
In other words, the circuit can reach its voltage limits much earlier than it should. This also explained the strange behavior I was seeing. When I played softly, the signal remained within the available voltage range and everything could sound reasonably normal. When I played harder, the signal exceeded the reduced headroom and was clipped badly.
The next step was to find out what was pulling the +4 V rail down. After tracing the circuit and checking the components connected to the supply, I eventually identified the culprit as a 74HC4052 analog multiplexer. The 74HC4052 is a dual 4-channel analog multiplexer/demultiplexer. Although it isn't an obvious suspect when dealing with an audio distortion problem, a faulty device can cause problems beyond simply affecting the audio signal itself. I replaced the 74HC4052.
After replacing the multiplexer, I powered the ME-33 up again and measured the +4 V rail. This time it was stable and remained at approximately 4 V, as expected. I then connected the guitar and tested the unit again. The distortion was gone.
Notes
- read risk disclaimer
- excuse my bad english