Two weeks ago, I volunteered to troubleshoot a power amplifier for Atlanta Chinese Christian Church ACCC. It was replaced because sometimes the sound disappeared. The model of this amplifier is Alesis RA300.
First, I need to find out what exactly the problem is with this amplifier. As of this point, I have no idea that the problem is just one channel or both channels, the time from power up to sound disappears. Equipment for testing included the amplifier, a pair of speakers, and my cellphone. My cellphone music output was used as the signal source, and was connected to the amplifier signal input, as shown in Picture 1.
First, I need to find out what exactly the problem is with this amplifier. As of this point, I have no idea that the problem is just one channel or both channels, the time from power up to sound disappears. Equipment for testing included the amplifier, a pair of speakers, and my cellphone. My cellphone music output was used as the signal source, and was connected to the amplifier signal input, as shown in Picture 1.
Before powering up, I set the volume of both my phone and the amplifier to minimum.
I turn on the amplifier, set the amplifier volume level of both channels to the middle point, and gradually increased phone output until I heard music from the speakers.
Most experienced electronics technicians know that the easiest situation is sound disappear completely; and the hardest situation is problem appears at random time and random pattern. Thank God. When I turned on the amplifier, the problem was immediately identified: the left channel was off most of the time; sometime sound appeared for 2-3 seconds, and disappeared. The right channel works fine.
Before working on the circuit board, I need the circuit diagram. Googling “Alesis RA300 pdf” helped me find the service manual on line, and an internet search tells me that a relay is the most common cause.
From RA300 POWER SUPPLY CIRCUIT, as shown on Picture 2, I found that J3-A connector pin 3 is the left channel power amplifier output. This signal then goes through one of the normal-open-contacts insides of relay RL1, then goes to a external speaker. The relay is controlled by the speaker protection circuit, which consists of U1 TA7317P and other components. When the amplifier is turn on, usually RL1 will delay for several seconds then the coil inside RL1 gets 12V voltage, the normal-open-contacts inside RL1 will close, and left channel signal is connected to J7, then to the speaker binding posts on the back of amplifier. The relay coil loop is:
+Vcc 54V-->R417//R418-->RL1 coil-->TA7317P pin 6-->GND
I turn on the amplifier, set the amplifier volume level of both channels to the middle point, and gradually increased phone output until I heard music from the speakers.
Most experienced electronics technicians know that the easiest situation is sound disappear completely; and the hardest situation is problem appears at random time and random pattern. Thank God. When I turned on the amplifier, the problem was immediately identified: the left channel was off most of the time; sometime sound appeared for 2-3 seconds, and disappeared. The right channel works fine.
Before working on the circuit board, I need the circuit diagram. Googling “Alesis RA300 pdf” helped me find the service manual on line, and an internet search tells me that a relay is the most common cause.
From RA300 POWER SUPPLY CIRCUIT, as shown on Picture 2, I found that J3-A connector pin 3 is the left channel power amplifier output. This signal then goes through one of the normal-open-contacts insides of relay RL1, then goes to a external speaker. The relay is controlled by the speaker protection circuit, which consists of U1 TA7317P and other components. When the amplifier is turn on, usually RL1 will delay for several seconds then the coil inside RL1 gets 12V voltage, the normal-open-contacts inside RL1 will close, and left channel signal is connected to J7, then to the speaker binding posts on the back of amplifier. The relay coil loop is:
+Vcc 54V-->R417//R418-->RL1 coil-->TA7317P pin 6-->GND
After carefully studying circuits in the service manual, for this kind of problem, the culprit could be the volume potentiometer, a component in preamplifier circuit board or power amplifier circuit board. I decided to start from the easiest part.
Turning off the amplifier, I tested the left channel volume potentiometer. The in-circuit-resistance of this potentiometer changed linearly when I adjust the left channel volume knob, and it tested the same as right channel volume potentiometer.
Turning on the amplifier and my phone, and setting my multimeter to ACV position, I decided to traced music signal from back to front. When multimeter probe was connect to power amplifier circuit output, which is J3 pin 3, I saw some voltage, but didn’t hear any sound from the left speaker. I started to suspect the relay.
To test the relay, I chose to measure AC voltage across the normal-open-contact. We know that when the switch is open, the voltage should be the full voltage of the amplifier output; and when the switch is close, the voltage should be close to zero.
I set my multimeter to AC voltage position. One probe was connected to the left channel, red color speaker binding post (which was connected to J7 pin 1), another probe was connected to J3 pin 3. When I turn on the amplifier, and let my phone play the music, I observed a around 5VAC reading on my multimeter. This confirmed that the contact inside the relay was bad.
At RadioShack I found a 12V relay with two normal-open-contacts. The physical size is the same, but the coil resistance different. The original Dong Woo DW323-D12s relay has a resistance at 270Ω, while my new relay has a coil resistance of 200Ω. This should not be a problem for the amplifier. After replacing the relay, the sound appear. And in 10minutes testing, both channel worked great.
I also measure the continuity of the original relay. The relay coil was connected to a external 12VDC power, and the contact resistance for one contact was 68Ω, and another about 0.6Ω. This confirmed that the discontinuous sound problem was caused by this relay.
I also measure the continuity of the original relay. The relay coil was connected to a external 12VDC power, and the contact resistance for one contact was 68Ω, and another about 0.6Ω. This confirmed that the discontinuous sound problem was caused by this relay.
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