BOSS car amp

[Franc Zabkar]

I've just repaired a BOSS AVA-450 160W+160W "MOSFET" car amp with a
protection fault. As this is the first amp of this kind that I've
worked on, I'd just like to make a few observations.

(1) The amp uses a PWM regulator IC (KIA494 = TL494) to step up the
+12V supply and generate positive (V+) and negative (V-) supply rails.
Strangely, the V+ and V- outputs appear to be unregulated, ie the
TL494 appears to free-run at a fixed duty cycle. Is this normal?

(2) One of the error amps of the TL494 is dedicated to sensing the
heatsink temperature via a thermistor. It shuts down the TL494 in the
event of excessive temperature, but does not appear to turn on the
protection LED.

(3) The second error amp of the TL494 indirectly senses the voltage
across the 0.3 ohm 5W emitter resistor of one of the four output
transistors. If the DC level of the emitter current exceeds 2A for a
certain length of time (set by an RC combo), a protection fault is
triggered. The TL494 is then latched off until the power is cycled.
The other three transistors are not monitored. Why?

(4) The only MOSFETs in this "MOSFET" amp are the two IRFZ44Ns in the
switchmode power supply. The audio transistors are BJTs. Doesn't this
amount to deceptive labelling?


-- Franc Zabkar

Please remove one 's' from my address when replying by email.

 

[Franc Zabkar]

** The current in them is similar ???

The amp can be connected in two channel mode, or in bridged (single
channel) mode. In the case of two channels and two speakers, there is
no relationship between each NPN-PNP transistor pair.

As it was, the amp was connected in bridged mode to a sub woofer. The
actual fault was an open emitter resistor in the unmonitored pair.
This resistor (a vertical mounting type) had a fractured pin, probably
as a result of vibration. I don't think the protection fault would
have been detected in two channel mode, although the sound of the
affected speaker would have been very distorted.


-- Franc Zabkar

Please remove one 's' from my address when replying by email.