I am testing an amplifier module that is failing the series light bulb test. I know of two failure modes which can fail the series light bulb test. These are a shorted power transistor in the output stage or an incorrect VBE multiplier bias. Are there other amplifier faults which fail the test?
1- Shorted Output Transistors: This is the most common problem in amplifiers. Output transistors (both power transistors and their drivers) can short, leading to high current draw as soon as the amplifier is powered on.
2- Damaged Diodes or Rectifiers: The rectifier diodes in the power supply can fail or short. If any of these diodes are shorted, AC voltage may reach the output stage without being properly rectified, leading to excessive current draw.
3- Shorted Electrolytic Capacitors: Capacitors in the power supply or amplifier stage can also short. The filter capacitors in the power supply are common suspects in cases like these.
4- Power Supply Issues: If the amplifier uses a transformer, a winding could be damaged or shorted. If it uses a switching power supply, any issues with high-power transistors or diodes could cause it to fail the light bulb test.
5- Bias or Polarization Circuit Issues: An incorrect adjustment or a fault in the bias circuit may cause the output transistors to conduct excessively, leading to high current draw and failure in the test. Some amplifiers have trimmers to adjust the bias current; if one of these is damaged or misadjusted, it could overload the circuit.
6- Protection Circuit Activating Incorrectly: Some integrated amplifiers include circuits for short-circuit or overload protection. If these fail, they may activate and cause unexpected current draw.
7- Shorts in PCB Traces or Internal Connections: A physical short in the PCB traces or poor solder connections could also cause the amplifier to fail this test. This often happens in older amplifiers with aging solder joints.
8- Shorted Power IC or Driver IC: In some modern or compact amplifiers, a single IC handles power amplification. If this fails, it’s very likely to cause the amplifier to fail the light bulb test.
If you're trying to diagnose the fault, start by checking the output transistors and rectifier diodes in the power supply, as they’re often the most common causes of light bulb test failure. Hope it helps. Cheers.
2- Damaged Diodes or Rectifiers: The rectifier diodes in the power supply can fail or short. If any of these diodes are shorted, AC voltage may reach the output stage without being properly rectified, leading to excessive current draw.
3- Shorted Electrolytic Capacitors: Capacitors in the power supply or amplifier stage can also short. The filter capacitors in the power supply are common suspects in cases like these.
4- Power Supply Issues: If the amplifier uses a transformer, a winding could be damaged or shorted. If it uses a switching power supply, any issues with high-power transistors or diodes could cause it to fail the light bulb test.
5- Bias or Polarization Circuit Issues: An incorrect adjustment or a fault in the bias circuit may cause the output transistors to conduct excessively, leading to high current draw and failure in the test. Some amplifiers have trimmers to adjust the bias current; if one of these is damaged or misadjusted, it could overload the circuit.
6- Protection Circuit Activating Incorrectly: Some integrated amplifiers include circuits for short-circuit or overload protection. If these fail, they may activate and cause unexpected current draw.
7- Shorts in PCB Traces or Internal Connections: A physical short in the PCB traces or poor solder connections could also cause the amplifier to fail this test. This often happens in older amplifiers with aging solder joints.
8- Shorted Power IC or Driver IC: In some modern or compact amplifiers, a single IC handles power amplification. If this fails, it’s very likely to cause the amplifier to fail the light bulb test.
If you're trying to diagnose the fault, start by checking the output transistors and rectifier diodes in the power supply, as they’re often the most common causes of light bulb test failure. Hope it helps. Cheers.
I disconnected the amplifier module just after the bridge rectifier. This time the series light bulb did not light.This means the mains transformer and bridge rectifiers are not the culprit. The rectified DC smoothing capacitors are on the amplifier module.
Try shorting the vbe multiplier entirely. Base of driver to base of driver, directly.
One can also verify vbe multiplier operation, off line, before another attempt. Inject a couple of mA, using a 9V battery and 4.7k series resistor, with the amp unpowered and filter caps temporarily shorted. Verify that you can vary it, and that it sits at the proper end of rotation to put the bias LOW when firing back up.
One can also verify vbe multiplier operation, off line, before another attempt. Inject a couple of mA, using a 9V battery and 4.7k series resistor, with the amp unpowered and filter caps temporarily shorted. Verify that you can vary it, and that it sits at the proper end of rotation to put the bias LOW when firing back up.
One power supply out of the two power supplies, (+/-), has a shunting resistance of 22 Ohms. This resistance may drop even lower for the rated voltages. I will remove the smoothing capacitors of the affected power supply to measure their resistance out of circuit.
Finally, I found the fault which was a short circuit from the base of the output transistors to ground. Removing the main heatsink and installing it back resolved the short circuit. Now, the amplifier passes the series bulb test.