Tuesday, February 07, 2017

Classe Power Amplifier - Classe CA-150-151 – How to test the output transistor – Circuit block diagram - Calibration and idle current setting

How to test the output transistor 
You should check all pin combinations, as there is a chance that only one side of the transistor is short. When replacing outputs transistors, should first disconnect both outputs and main board to check if the blown outputs were caused by a faulty component on the amplifier’s base. Blown output transistors should  be replaced as kit, not individually. These kits should contains matched transistors for both rails of one channel. When replacing blown output transistors, you must also replace positive and negative Mosfet drivers and bias transistor. Carefully inspect 4.7 Ohms resistor and 10 Ohms soft start resistor, and replace if necessary. Replace any burnt component or board.
Pins :
1 Base
2 Collector
3 Emitter
You should have these results, if the transistor is good
2SC3263 (NPN)
Positive lead
Negative lead
Result
B
C
Conductive
B
E
Conductive
C
B
Infinite
C
E
Infinite
E
B
Infinite
E
C
Infinite
2SA1294 (PNP)
Positive lead
Negative lead
Result
B
C
Infinite
B
E
Infinite
C
B
Conductive
C
E
Infinite
E
B
Conductive
E
C
Infinite
Calibrations and Adjustments
Bias adjustment:
Note: Bias adjustment should be made without any load connected to the amplifier’s output, and no signal connected to the amplifier’s input.
1. Connect a multimeter to R137 leads and set the multimeter to mV scale.
2. Turn trimpot RV102 counterclockwise until it reaches 0Ohm. A click should be heard.
3. Turn on amplifier, and let it warm up for 15 minutes on idle.
4. Adjust trimpot RV102 to get a 14mV reading.
5. Let the amplifier warm up for a 24 hours period with signal and load.
6. Remove signal and load, and reconnect the multimeter, setted to mV scale, to R120 leads. Note the reading.
7. Connect the multimeter to R137, R138 and R139 leads. Note the readings.
8. Input the results into this formula:
((72 – mVR137 – mVR138 – mVR139) /3) + mVR137
9. Connect the multimeter to R137 leads, and adjust RV102 to match the reading to the formula result.
10. Results should be around 24mV.
DC offset:
Note: DC offset adjustment should be made without any load connected to the amplifier’s output, and no signal connected to the amplifier’s input.
1. Connect a multimeter to the output binding posts and set the multimeter to mV scale.
2. Adjust trimpot RV101 in order to reach 0mV. It is easier to turn the trimpot 1/2 turn at a time, and let DC offset stabilize in order to get an accurate reading.
3. A tolerance of +/-1mV is acceptable.
BLOCK DIAGRAM 
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