Alesis RS-150 Reference Amplifier

[thedefog]

Hello everyone!

New here, hoping someone can help me out with my repair here. I'm currently working on an RS-150 reference amplifier by Alesis. When I received it, there were a couple of charred 1watt resistors and a blown capacitor on the power board. I replaced those, along with the other components in the vicinity.

This is a stereo amplifier that has two separate boards for each channel. One of the channels is working perfectly, loud and clear. The other is about 1/4 of the max volume of the working channel. I've probed voltages on all of the regulators on both boards, and they match. I've also checked all caps (both electrolytic and poly/ceramic) for shorts and leakage with both the multimeter and scope. All transistors (including the discrete opamp triple-darlington transistors) check out okay. Diodes check out okay. Visual inspection of all other components checks out okay, as well as level pots. Input jacks are good, I even replaced the NE5532 input opamp on the bad channel, thinking it had something to do with the input stage.

I cannot source a schematic for this amp, and my only reference is the owners manual on specs.

I swapped boards with both channels on the power board, and it is definitely the one board that is faulty. I wanted to swap the discrete opamp boards to see if that was the issue, but one is specifically populated with components for bridging the amp.

Thanks,
Chuck

[TinyTool]

Hi Chuck,

It sounds like you've done a thorough job of troubleshooting so far. Given that one channel works perfectly and the other is at reduced volume, you’re likely dealing with an issue specific to the problematic channel rather than the power supply or overall design.

Since you’ve already replaced components in the vicinity of the charred resistors and blown capacitor, and confirmed the functionality of the regulators, transistors, and other components, here are a few additional things you might check:

1. **Signal Path Analysis:** Even though you’ve swapped boards and ruled out the power supply, try injecting a known good signal into the input of the problematic channel and trace it through the signal path. Use an oscilloscope to see where the signal level drops off. This could help identify where the signal is being attenuated or lost.

2. **Output Stage Components:** If the signal path is fine but the output is weak, the issue might be in the output stage. Check the output transistors and associated components. Sometimes, even if the transistors test okay in isolation, they can still cause issues when under load or at operating temperatures.

3. **Biasing and Offset:** Verify the biasing of the output stage. Improper biasing can lead to reduced performance. Although you mentioned checking voltages, make sure the idle bias current is set correctly according to the design specs. If there are trimmers or adjustment points, ensure they are set properly.

4. **Connections and Soldering:** Inspect all solder joints and connections on the faulty board. A cold solder joint or poor connection could cause intermittent issues or reduced performance. Resoldering joints or inspecting them under a magnifying glass might reveal hidden problems.

5. **Component Values:** Double-check the component values of resistors and capacitors on the faulty board against those on the working board. Even if they appear correct, there might be differences in tolerances or types that could affect performance.

6. **Thermal Issues:** Ensure that there are no thermal issues with the board. Sometimes, components that appear fine can fail under load due to overheating or poor thermal contact. Ensure that any thermal paste or heatsinks are properly applied and functioning.

Let us know if you need any further information.

Good luck with your repair!