Speaker Protection; Fan Speed Control - Alesis MATICA 500 Service Manual

through the bias sense transistor (Q18) making the current in the sense transistor relatively
constant. Q17 is the pass transistor of the regulator. Note that any time the output transistors
are replaced, the pass and bias transistors must also be replaced. Failure to do so will probably
result in the output transistors failing again.

1.60 Speaker Protection

There are two sections to the speaker protection circuit. A section made up of a quad
comparator with a low pass filter before it and a discrete bipolar transistor circuit that controls the
two speaker relays. The transistor circuit and +15 volt three terminal regulator make up the
power up delay circuit and "instant off" power down circuit. When the unit is turned on the 15 volt
regulator supplies power to the discrete circuit. Q24 controls the speaker relays and is held off
during power up by Q23. During power up C24 is not charged and must be charged before Q23
will turn off. This delays the turn on of the output relays to protect against any transients that
may occur at power up. Once on, there are four ways the relays can be opened again.
The first is if the heat sink gets to 80 deg. C. then TH1 will open and release the relays.
Second is by shut down of the amplifier. Upon the removal of AC to the amplifier, the three
terminal regulator will fall out of regulation that forces Q21 on because C23 acts like a battery
and momentarily turns on Q21. This pulse will cause Q22 to dump the charge on C24,
turning on Q23.
The third way is if the line voltage get low (about 90 volts in the 120v unit). At this time the
regulator will fall out of regulation and the first pulse from the 50 or 60 Hz that gets through
the regulator will cause C24 to be dumped again and the 3-5 second charge time will occur
again.
The fourth way is to have the circuit be triggered by the comparator circuit.
The comparator is DC coupled to the output of the amplifier before the speaker relays.
They are set up with each one of their inputs tied to a +/- 1.2v reference derived from CR7, CR8,
CR9, and CR10. The low pass filter is comprised of R64, R65, R66, R67, C21, and C22. When
the output of the amplifier is driven to full output at or below 5Hz, or 1.2v of DC offset appears at
the output of the amplifier, it will trigger one of the comparators which in turn opens the output
relays. When this happens the same 3 to 5 second period must occur before the relays will
engage again. If the fault condition persists then the relays will remain open.
The 18 volt AC winding which runs this circuit is rectified by a half wave rectifier. This is
allows one side of the winding to be grounded. The 18 volt winding is also provided at the Alink
connector to facilitate generation a +/- 15 volt supply to run auxiliary accessories requiring
phantom power.

1.70 Fan Speed Control

The fan circuit has three modes of operation in which it varies the speed of the fan
depending on demand or condition. At power up of the amplifier Q25 and Q26 will be saturated
until the relay circuit enables the speaker relays. This condition lasts for 3 to 5 seconds. During
this time Q27 is also saturated. This forces the fan to run at an elevated speed momentarily.
After this time only R85 will be delivering current to the fan. The fan will be running at a very
slow speed. The reason for the accelerated speed of the fan at turn on is that the current
delivered by R85 may not be sufficient to start the fan, especially at low power line levels.
Signals from both channels are detected by Q25 and Q26 as the amplifier is driven harder. They
act as rectifiers of the signal, and when they conduct the 40
more current to the fan motor, causing the fan speed to increase with the drive of the amplifier.
The fan motor itself acts as a flywheel or filter to smooth the modulation and the speed of the fan
is proportional to the average of the drive to the amplifier. If the heat sink gets hotter than 55
deg.C. then an additional 20
Alesis A4/A8 Amplifiers Service Manual------
resistor is switched in to deliver more current to the fan. The fan
resistor connected to Q27 delivers
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