Alesis Midiverb II Repair

Overview I bought a couple of MidiVerb II’s in May 2020, both were listed for repair and at a super low cost of £30. Although there is no service manual or schematics online, some information can be gleaned from the MidiVerb III service manual, and I was hopeful that they would be simple repairs. Typical issues are voltage regulators going out of spec, or the crystal oscillators fail. The rest of the analog and digital circuits seem reliable and robust.

Midiverb II – S/N 91606 The first one I bought has seen a lot of use, the rack ears have been cut off and the lettering was rubbing off. Powering it up initially proved it was working and the effects were really quite good and the noise floor low (its uses TL084 chips, the LF354’s in early units are noisy). I went ahead and recapped the power supply and replaced the voltage regulators. The PCB is a later version and has separate oscillators for the 8031 microprocessor and the DSP chip. The OS is MVII.OBJ dated 4/10/87.

Midiverb II – S/N 39303 The second one I bought was sold as broken and the seller stated it was non functional with all LED’s on at power up. Once in the studio I switched it on and it was working ok, so I went ahead and recapped the power supply and replaced the voltage regulators (as they were below specification). I noticed there were some PCB cuts and the timing capacitor for the 16 MHz oscillator was not connected. Powering back on resulted in failure, the most common error code was all LED’s on, sometimes OVLD off. This error code means the microprocessor has not started.

Further investigation shows a 12 MHz crystal and timing capacitor added onto the rear of the PCB for the 8031 microprocessor and the PCB cuts were to disable the divide down clock from the 16 MHz oscillator. This looks like a factory modification to increase the clock speed for the microprocessor from 8 to 12 MHz, as newer models had two clocks. I checked that both clocks were working and found the RESET signal for the 8031 was high. This is why it was bricked, it should be zero.

The reset circuit for the MV2 is very simple; a resistor and capacitor charge up after power on. This is not 100% reliable and the Midiverb III changed it during production to a more reliable transistor based circuit. When I replaced the 4.7uF capacitor I had shorted it out on top of the PCB, so hidden from view, this shorted the RESET pin to +5V. I re-soldered a new cap and added in a missing 10k resistor. Power back on and the MV2 now works perfectly.

Power On Patch One of the differences I noticed between the two MidVerbs is the patch that they initialized to on power up is different. With no battery backed RAM it wasn’t until I looked at the Xicor X2444P chip near the 8031 that I realised there was a very small amount of on board NVRAM. Very small is just 32 bytes, but it does last 100 years! This is where the MIDI program change message (01-32) mapping to the patch number (00-99) is stored and retained during power off. It also stores the MIDI control channel number (0-16) and at power on the first program (01) in the table is loaded.

This explains why the different MidiVerbs power on with different patches. It is because the mapping tables are different, they can of course be configured to get any patch (00-99) at power on. Mystery solved!

Mixer Pots The front panel potentiometers get a lot of use over the years especially the MIX. This is a dual 10kB PCB mounted potentiometer, looks like an easy to find Alpha pot.

Outcomes  I sold the Midiverb II’s in 2021 as I have too many FX!  I bought some Behringer 100M replica modules instead.