Alesis Midiverb II
This time, a legendary Alesis Midiverb II effects processor came in for repair. The issue was a complete lack of audio signal at the output. After an initial inspection, it turned out the unit had already been serviced before – unfortunately not only unsuccessfully, but also quite carelessly, as some of the PCB traces had been damaged.
As it soon became clear, the fault rendering the device inoperative was fairly serious. First of all, the input power supply was faulty – it failed to provide the +12V rail, while the digital section voltage was elevated to as much as 5.3V. This module has a rather unfortunate and failure-prone design, as it is based on a dual voltage-doubler topology. This stems from the fact that the Midiverb is powered by a 9V AC adapter, yet is expected – by design – to generate regulated ±12V and +5V rails, which is quite a risky design choice.
After replacing all voltage regulators, rectifier diodes, and the full set of electrolytic capacitors, the power supply came back to life (although it would occasionally fail to start). However, there was still no audio signal at the output. It turned out that the CD4053 analog switch at the DAC output was heating up significantly, which was caused by a shorted TL074 operational amplifier (located in the section of the PCB that had been previously repaired and damaged by the former technician).
As a result, the op-amp, the CD4053 switch, and four capacitors that had been scorched during the previous repair were replaced. The makeshift PCB repairs were also corrected, and some solder joints were reflowed. As a preventive measure, I also replaced the electrolytic capacitors in the audio path with high-quality components. In the end, the processor was successfully brought back to life, although it required quite an extensive repair.
