General Description; Power Supply; Battery Backup; Analog Signal Paths - Alesis MIDIVERB IV Service Manual

1.00 General Description

The MidiVerb IV is a continuation of the Alesis line of quality studio signal processors. As
is the tradition at Alesis, much of unit's complex functions are incorporated into an LSI ASIC,
reducing parts count and cost, as well as increasing reliability and serviceability. The MidiVerb
IV pushes the concept of integration further than any previous processors, adding hardware to
the ASIC that was previously discreet components.

2.00 Power Supply

The M4 power supply begins with the 9 Volt A.C. adapter (Alesis P3 type). Input from J8
is R.F. filtered by C35 before on/off switch SW1. From there it is split for the +12V, -12V, and
+5V rails. The +12V rail consists of a voltage doubler (C11, C18, and D3, D5), a 7812 regulator,
and a filter capacitor (C16). The -12V rail is a "mirror" of the +12V rail, consisting of voltage
doubler (C12, C19, and D4, D6), a 7912 regulator, and a filter capacitor (C20). The +5V rail
consists of a rectifier diode (D2), an input filter capacitor (C5), a 7805 regulator, an output filter
capacitor (C10) and a multitude of 0.1 F bypass capacitors. Note that the raw +5V line used by
the microprocessor reset, and the SRAM power supply, and is located at the input to the 7805
regulator. Likewise, the +12 V and -12V raw supply lines are available for use by the LCD.

2.10 Battery Backup

Battery backup is actually more complicated than it might first appear, as it
depends on a good system reset (see section 5.10 for details) in order to function properly. The
actual backup circuit consists of a battery (3V Lithium), a 10K resistor (R21) for checking
standby current (see below), a "steering" diode (D8), a filter capacitor (C32), and a
transistor/resistor/diode combination that acts as a steering diode, while maintaining a high
enough supply voltage to prevent SRAM data corruption.
SRAM standby current should always be checked. While the unit is off, check the voltage
across R21. If the voltage is higher than 80mV (specification, although a 1 to 20mV range is
more normal) then a problem exists. Usually it indicates a bad (or simply wrong) SRAM, or a
short, somewhere along the SRAM V cc line.
WARNING: Danger of explosion exists if the battery is incorrectly replaced. Replace only with
the same type or equivalent type recommended by the manufacturer.

3.00 Analog Signal Paths

3.10 Input

The signal(s) from input jacks J3 (right) and J4 (left/mono) are current limited by R20,
R26, and have their impedances set at 1M
These signals are then buffered by U8C and U8D respectively, before being sent to the input
DigPots (U6B, U6A) via A.C. coupling capacitors C4 and C2. After a small gain stage (U8B,
U8A) the signals are passed along to the ADC for further processing.
Alesis MidiVerb IV Service Manual'
Battery Manufacturer: Panasonic/Matsushita
Type: BR2325
Rating: 3V, 5mA
(stereo), 500K (mono) by resistors R12 and R27.
1 03/05/99