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Toa 912A PA Amplifier protection circuitry locked on. Why?

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  • notlih Dec 09, 2008

    This could be on the right track as I have found a transistor and adjacent resistor getting hot very quickly. They are on the main board so I shall remove it for inspection.

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Hi,
W-912A Mixer Power Amplifiers have a built-in
one-octave graphic equalizer with 9 bands to tailor sound
system frequency response to room acoustics, reducing
feedback tendencies and improving intelligibility. A built-in
compressor circuit is provided to protect the output level from
distortion as a result of excessive input and keep it constant.
The compressor’s threshold level can be adjusted. The W-912A Mixer Power Amplifiers have output terminals to
match 4- or 8-ohm speaker systems, or a speaker distribution
system may be connected to the 25 or 70-volt terminals. User’s
(blank) space is provided for additional functions to be made
by the customers. The expanding kit (optional) may be loaded
in this space when the number of inputs is increased.
Protection against tampering is assured by means of a keylocked hinged door. The W-912A can be
mounted in any four-inch wall with use of the BX-9F Flush
Mounting Back Box. Wall mounting is also possible with use of
the BX-9S Surface Mounting Box.

Good luck

Posted on Dec 09, 2008

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There is a short circut in the unit check wires and solder joints! somthing is touching somthing it should not be!

Posted on Dec 09, 2008

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hi,

AV receivers shows protect mode , when there is a hardware fault in side it. Its a warning that some thing is wrong. Is there is any standby light. If not check the AC fuse.
Normally AV receivers show this when any of the output amplifier get some problem. there are five amplifiers in this AV receiver. May occur when the output diver transistors of any of the amplifier become shorted. if the output transistor get shorted this will lead to flow of current through the speakers. To prevent this there is a protection circuit for each channels. One combined output of this protection circuit is connected to the system control. when the any of the output amplifier fails, the protection circuit detects it and switch of the speaker and sent data to the system control. when the system control get this data it will give a warning to the customer through display that PROTECT.

First of all remove all speaker connections from the amplifier and powerup the amplifier. If the amplifier turns ON then, then any of the speaker have some problem.
If not You have to check the amplifier.
for that you need digital multimeter,soledring iron screwdrivers etc
open the amplifier , then power it on. connect the negative terminal of the multimeter any where in the chase( metal body of the amplifer). then connect the positive terminal to all the output transistors emitter pin one by one . the voltage should be bellow 0.5V. If any of the emitter of the transistor shows voltage more than 0.5V., then that amplifier stage is faulty. Remove the transistors and check it. if it is shot then replace with a new one.
after replacing the transistors do not power up the amplifier directly from the main. You have to put a bulb of 60 watts in serial.
You can do this easily remove the mains fuse. put the bulb in a holder. now you have to wire from the holder. connect one of it to the one end of the fuse holder and the other wire to the other end of the fuse holder. Why using a bulb is, if there is any further problem in the amplifier stage on which the transistors replaced, there is a possibility of the new transistor get damage. if we use a bulb in serial with its mains supply ,when any overload in the amplifier occur the bulb will drop the current, by lighting itself.
After connecting the bulb in serious power on the amplifier. watch the bulb. It should light for one or two second and go dim . if so check the display, is there is protect mode is on or not . if not and come back to normal operation ,then remove the power , remove the bulb insert fuse and check
connect the power and check.
If the bulb shows bright light then there is some component also faulty in the same amplifier. after removing the power you have to check again the amplifier stage of which the transistor is replaced, for any other faulty components.
ok

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Hi and welcome to FixYa,

Initially, your concern would be indicating two (2) possibilities:
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