Service Center Training Guide
Protection Circuitry Servicing
&
Theory of Operation
In the event of a fault, an HK AVR will go into Protection. Protection is a condition where the
AVR removes the main power from itself and returns to the standby state (amber indicator). The
AVR does this in order to prevent further damage to the AVR or your loudspeakers. Protection
related issues can happen at initial turn on or intermittently after hours of use. Sometimes the
front display on the AVR will say the word “Protect” before returning to standby if there is a
fault. However, it is possible for the AVR to go into protect without saying so on the front
display. There are other possible reasons for an AVR to shut itself besides protection issues, but
it is usually a primary suspect.
There are typically 4 fault detection mechanisms which are used. These mechanisms are
Overload detection, thermal/temperature detection, DC detection and power failure (P-Fail)
detection. In a typical protection scheme, the output of all 4 fault detection mechanisms combine
to form the Protect line. The Protect line is monitored by the microcontroller. If the
microcontroller detects a fault condition on the protect line, it will remove main power from the
AVR which causes the AVR to go back into standby.
It helps to keep in mind that all transistors in the protect circuitry behave like a switch. They
only work in an ON/OFF, Open/Close manner. The collector and emitter are like switch contacts.
When the base receives a signal, the contacts close (i.e. short together).
Some faults will keep the protect line in a steady state fault condition. However, many other
faults will only create a momentary pulse on the protect line. When checking the protect line for
a fault, you should use an oscilloscope and keep a lookout for pulses as well as the DC steady
state voltage. Storage Oscilloscopes and latching capable logic probes are very useful for
detecting an infrequently occurring fault condition on the protect line.
Now that you have an idea of the protection circuitry topology, we will take a closer look at
each of the 4 fault detection mechanisms.
1
Over-Load Detector
Overload detection detects if the amp is excessively loaded. It protects the amplifier from
potential damage if its output sees impedance which is too low, or its output amplitude is too
high. The circuit works by sensing the amount of voltage drop across the amplifiers output
resistors. The more current flows through the amplifiers output, the larger the voltage that
develops across the emitter resistors. Once enough voltage develops, it turns on the detect circuit.
Possible reasons for it to trip while playing are:
1) Volume to high.
2) Impedance too low.
3) Bad/shorted speaker wiring connected to speaker terminals.
Possible reasons for it to trip at turn on, with no load attached
1) An output stage is blown, shorted output transistors and/or open amplifier emitter
resistors.
2) One of the detector circuits has been damaged as the result of a previously blown up
channel.
3) Faulty biasing component or bad wiring between amp and bias PCB. If the amplifiers bias
circuit opens, it will cause the idle current to be extremely high.
2
DC Detector
The DC Detector looks for excessive amounts of DC offset on the amplifiers speaker terminals.
This is a safety measure to protect the loudspeaker from damage if a fault should occur in the
amplifier. The input of the DC detector uses a low pass filter to block the audio signal, allowing
only DC to pass.
A positive DC offset will turn on the left side transistor, where as a negative DC offset will turn
on the right side transistor. When one of the transistors turns on, it will pull the output down to
ground potential.
Possible reasons for tripping while playing, but not at turn on
1) Excessive sub-sonic frequencies in audio content (playing a warped LP?)
2) Input filter component out of spec (most likely the cap)
3) Offset adjustment calibration needs to be made (on applicable models)
Possible reasons for tripping at turn on with no audio
1) Blown amplifier output stage.
2) Defective component in detector.
3) Offset adjustment calibration needs to be made (on applicable models)
3
Power Fail (P-Fail) Detector
This detection senses for faults in the AVR’s power supply. The detection is usually
accomplished by a resistor which pulls up one of the MCU’s pins to 5VD or 3.3V. On several
models, this resistor pulls up the protect line which is shared with the other protect mechanisms.
On some models “P-Fail” has it’s own line which is independent from the other protect
mechanisms.
Some AVR models also check for a failure in the amplifiers +/-VCC as part of their P-Fail
detection. It does so by using a center tapped voltage divider to check symmetry between the
+VCC and –VCC lines.
If the +/-VCC become imbalanced, it will cause a DC voltage to develop on the voltage dividers
node. This will turn on either the negative or positive offset detect transistors. This circuit
operates very similar to the DC Detector mechanism mentioned earlier.
Possible reasons for tripping
1) Defective component in the detected power supply.
2) Blown amplifier output stage (in models which check for VCC symmetry).
3) Defective main bridge rectifier (in models which check for VCC symmetry).
4) Defective component in VCC symmetry detection (if applicable).
4
Thermal Detector
The thermal protector detects the temperature of the amplifiers heat sink. If the temperature
becomes too high, the thermal detection will trip. The typical circuit uses a voltage divider with a
positive temperature coefficient on the ground side. As the temperature goes up, so does the
thermistor’s resistance. This causes a higher voltage to form at the voltage dividers output node.
Once this voltage becomes high enough it will turn on the transistor in this circuit.
AVR1xx/2xx/3xx use one thermal detector for all 7 channels. AVR4xx/6xx/7xx use one detector
for four channels and another detector for the other 3 channels.
Possible reasons for tripping while playing:
1) Fan is failing to turn on as unit heats up.
2) Inadequate ventilation.
3) Thermal instability in amp channel (idle current increasing with temperature).
4) One or more channels over-biased.
Possible reasons for tripping at turn on:
1) Open thermistor or thermistor wiring harness.
2) Loss of power to temp sensor (AVR445/645/745 only).
3) Other defective component in thermal detection circuitry.
5
Test Point Table
DC DET O ver L oa dP-Fail4CH ThermalProtect 3CH Thermal
Test Point O K Fault Tes t Point O K Fault Test Point OK Fault Test Point OK Fault Test Point O K Fault Test Point OK Fault
AV R130/230/330 CN80-5 0 V 15V Across R955 0 V .7 V Acr oss R955 0V .7V BN81-6 5V 0V Acr oss R940 0 V .7 V Acr oss R939 0V .7V
A VR430/630 N40 3-18 5V 0V N808-1 0V .7V N807-1 0V .7V A cros s R503 0 V . 7V P80 5-2 0 V .7 V P 805-1 0V .7V
AVR7300 P106-15 5V 0V R964 0V .7V R964 0V .7V - - - R969 0V .7V R961 0V .7V
A VR135/235/33 5 CN80-5 0V 1 5V A cros s R955 0 V .7 V Across R955 0V .7V BN81-6 5V 0V Across R940 0V .7V Across R939 0V .7V
A VR435/635 N 802-2 5V 0V N808-5 0V .7V N807-1 0V . 7V A cross R503 0 V .7V N8 08-2 0V .7 V N808-3 0V .7V
A VR140/240/34 0 CN80-5 0V 1 5V A cros s R955 0 V .7 V Across R955 0V .7V BN81-6 5V 0V Across R940 0V .7V Across R939 0V .7V
A VR145/245/34 5 CN11-7 0V 5V Acros s R955 0 V .7V Acr oss R955 0V .7V CN11-6 3.3V 0V A cros s R940 0 V .7 V Across R939 0V .7V
AVR445/645 /745 N702-2 5V 0V N702-6 ~1.2V 0V/5V N702-5 ~1.2V 0V/5V N702-13 15V 0V P812-10 0V 0.7V P812-11 0V 0.7V
A VR147/247/34 7 CN11-7 0V 5V Across R955 0 V .7 V Acr oss R955 0V .7V CN11-6 3.3V 0V A cros s R940 0 V .7 V Across R939 0V .7V
A VR154/254/35 4 CN11-7 0V 5V A cross R955 0 V .7 V Across R955 0V .7V CN11-6 3.3V 0V Across R94 0 0V .7V Acr oss R939 0V .7V
6
7
8
9
10
11
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