ALESIS
RA100 Amplifier
(PA)
Service Manual
P/N: 8-31-0020-A
Preface
This document is intended to assist the service technician in the operation, maintenance and repair of the Alesis device. Together with the User Reference
Manual, this document provides a complete description of the functionality and serviceability of the Device. Any comments or suggestions you may have pertaining to the document are welcome and encouraged.
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In addition to any purchase price that Alesis may charge as consideration for Alesis selling or otherwise transferring this service manual (“Manual”) to you, if you are not a service and repair facility (“Service Center”) authorized by Alesis in writing to be an authorized Service Center, Alesis sells or transfers the Manual to you on the following terms and conditions:
Only Service Centers authorized by Alesis in writing are authorized to perform service and repairs covered by an Alesis warranty (if any), and transfer of the Manual to you does not authorize you to be an authorized Service Center. Therefore, if you perform, or if the Manual is used to perform, any service or repairs on any Alesis product or part thereof, any and all warranties of Alesis as to that product and any service contract with Alesis for that product shall be voided and shall no longer apply for such product, even if your services or repairs were done in accordance with the Manual.
All service or repairs done by you or with reference to the Manual shall be solely your responsibility, and Alesis shall have no liability for any such repairs or service work. All such service or repairs are performed at the sole risk of the person performing the service or repairs. You agree that all such work will be performed in a competent, professional and safe manner at all times and to indemnify and fully hold Alesis and its successors and assigns harmless in the event of any failure to so perform.
Your purchase of the Manual shall be for your own ultimate use and shall not be for purposes of resale or other transfer.
As the owner of the copyright to the Manual, Alesis does not give you the right to copy the Manual, and you agree not to copy the Manual without the written authorization of Alesis. Alesis has no obligation to provide to you any correction of, or supplement to, the Manual, or any new or superseding version thereof.
Alesis shall have the right to refuse to sell or otherwise transfer repair parts or materials to you in its sole discretion. You shall not use, sell or otherwise transfer spare or replacement parts supplied by Alesis to you (i) to repair or be used in products manufactured for or by third parties or (ii) to any third parties for any purpose.
You shall not make any warranties or guarantees with respect to the products of Alesis or the use thereof on behalf of Alesis or in your own name.
The foregoing describes the entire understanding related to sale or transfer of the Manual to you, and no other terms shall apply unless in a writing signed by an authorized representative of Alesis.
All Trademarks are property of their respective companies.
Warnings
TO REDUCE THE RISK OF ELECTRIC SHOCK OR FIRE, DO NOT EXPOSE THIS PRODUCT TO WATER OR MOISTURE.
The arrowhead symbol on a lightning flash inside a triangle is intended to alert the user to the presence of un-insulated "dangerous voltage" within the enclosed product which may be of sufficient magnitude to constitute a risk of electric shock to persons.
The exclamation point inside a triangle is intended to alert the user to the presence of important operating, maintenance and servicing instructions in the literature which accompanies the product.
REPAIR BY ANY PERSON OR ENTITY OTHER THAN AN AUTHORIZED ALESIS SERVICE CENTER WILL VOID THE ALESIS WARRANTY.
PROVISION OF THIS MANUAL DOES NOT AUTHORIZE THE RECIPIENT TO COMPETE WITH ANY ALESIS DISTRIBUTOR OR AUTHORIZED REPAIR SERVICE CENTER IN THE PROVISION OF REPAIR SERVICES OR TO BE OR MAKE REPAIRS AS AN AUTHORIZED SERVICE CENTER.
ALL REPAIRS DONE BY ANY ENTITY OTHER THAN AN AUTHORIZED ALESIS SERVICE CENTER SHALL BE SOLELY THE RESPONSIBILITY OF THAT ENTITY, AND ALESIS SHALL HAVE NO LIABILITY TO THAT ENTITY OR TO ANY OTHER PARTY FOR ANY REPAIRS BY THAT ENTITY.
Regarding the Power Supply Fuse
CAUTION: The product under service may employ the use of a replaceable fuse. Danger of fire or electrocution if fuse is incorrectly replaced. Replace with only the same type or equivalent type recommended by the equipment manufacturer.
Regarding the Internal Battery
CAUTION: The product under service may employ the use of a internal battery. Danger of explosion if battery is incorrectly replaced. Replace only with the same or equivalent type recommended by the manufacturer. Dispose of used batteries according to the manufacturer's instruction.
Safety Instructions
Carefully read the applicable items of the operating instructions and these safety suggestions before using this product. Use extra care to follow the warnings written on the product itself and in the operating instructions. Keep the operating instructions and safety suggestions for reference in the future.
1.Power Source. The product should only be connected to a power supply which is described either in the operating instructions or in markings on the product.
2.Power Cord Protection. AC power supply cords should be placed such that no one is likely to step on the cords and such that nothing will be placed on or against them.
3.Periods of Non-use. If the product is not used for any significant period of time, the product's AC power supply cord should be unplugged from the AC outlet.
4.Foreign Objects and Liquids. Take care not to allow liquids to spill or objects to fall into any openings of the product.
5.Water or Moisture. The product should not be used near any water or in moisture.
6.Heat. Do not place the product near heat sources such as stoves, heat registers, radiators or other heat producing equipment.
7.Ventilation. When installing the product, make sure that the product has adequate ventilation. Improperly ventilating the product may cause overheating, which may damage the product.
8.Mounting. The product should only be used with a rack which the manufacturer recommends. The combination of the product and rack should be moved carefully. Quick movements, excessive force or uneven surfaces may overturn the combination which may damage the product and rack combination.
9.Cleaning. The product should only be cleaned as the manufacturer recommends.
10.Service. The user should only attempt the limited service or upkeep specifically described in the operating instructions for the user. For any other service required, the product should be taken to an authorized service center as described in the operating instructions.
11.Damage to the Product. Qualified service personnel should service the unit in certain situations including without limitation when:
a.Liquid has spilled or objects have fallen into the product,
b.The product is exposed to water or excessive moisture,
c.The AC power supply plug or cord is damaged,
d.The product shows an inappropriate change in performance or does not operate normally, or
e.The enclosure of the product has been damaged.
The input signal drives QX15 (one half of a diff amp pair; consisting of transistors QX15 and QX16). QX16 is driven by a feedback signal from the output section through a 39K resistor (RX30). This 39K resistor is used to match the impedance seen at the base of QX15 so that the voltage at the base of both transistors is approximately 0.1V. This insures consistent operation of the input diff amp.
The output of the first diff amp is used to drive a second diff amp pair (QX12 and QX13) where the actual gain of the amplifier is set up. this is done with a Vbe multiplier (QX04) connected to the second diff amp pair that can be
used to adjust the amount of current that the output section will deliver to the load
(more about this in section XX).
The output of the second diff amp pair drives the output stage that consists of two pairs of driver and power transistors (QX05/QX07 and
QX01/QX03). This stage of the circuit is where the current to the output is amplified.
The input diff amp (QX15 and QX16) is protected from high input voltages with a limiter made up of two diodes (DX08 and DX09) connected in parallel (with reverse polarities) directly across the bases of both transistors. This limiter is used to keep the input voltages at or below 0.7V.
`The mute fast supply circuit is also connected into the input stage to eliminate any voltage surges that could be caused during power up or power down that could damage circuit components or speakers. This circuit consists of a JFET (QX11) connected across the supply storage capacitor (CX08) for the input diff amp and an RC time constant that charges the gate of the JFET. As the unit is powered up, the JFET acts like a 30 resistor short to ground around the supply storage cap until a -10V supply from the units power supply is applied to the gate of the JFET through the RC time constant (RX43 and CX09). Once
CX09 is charged and applied to the gate, the JFET turns off and allows CX08 to charge and supply voltage for the input stage. As the unit is powered down, CX09 quickly discharges through DX13 and C4 turning the JFET back on and shorting any leftover supply voltage to ground immediately.
The thermal shutdown circuit is also connected into the input stage. This circuit is similar to the mute fast supply in that a transistor (QX10) is connected across the supply storage capacitor top the input stage and, when turned on, shorts the supply voltage to ground. The biasing voltage for QX10 is determined by a transistor (QX09 -placed in the heat sink) in parallel with a voltage divider at the base of QX10. The base of QX09 is left open so that it acts like a zener diode and can drop about 7.7V across it. This voltage is applied to RX39 and RX40 and keeps about 0.48V at the base of QX10. As temperature increases in the heat sink, the voltage across QX09 begins to increase, causing a larger voltage to be seen at the base of QX10. As this voltage increases, QX10 begins to turn on and short some of the supply voltage to the input stage to ground. This causes a reduction in output current and allows the unit to cool down.
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