OCL power amplifier circuit principle and maintenance method

Due to the superior performance, high stability and reliability of OCL amplifier circuit, it has been widely used by various manufacturers for a long time. However, due to various reasons, the problem of burning power amplifier tubes, composite tubes and resistors often occurs during use. Because the OCL circuit is directly coupled, the circuits are intertwined, which makes it difficult to judge the fault during maintenance. It often causes repeated tube burning, which brings unnecessary losses to the maintenance workers and discourages many maintenance workers. The following is a summary of my experience in repairing power amplifiers over the years. I write it down for your reference. I hope it can help my colleagues and reduce unnecessary economic losses for you.

The common OCL power amplifier circuit is shown in the figure. The working principle of the OCL circuit has been introduced in many articles, so I will not describe it here. I will only talk about the specific maintenance methods and techniques.

In the figure, Q6~010 and R12~R14 often burn out at the same time. When repairing, do not blindly replace the above components and then turn on the power, because if the fault is not thoroughly checked at this time, it may burn out again. You should carefully check whether the previous tubes and resistors are damaged, whether W1 is open or the resistance value increases, etc. Then take the following method, which is safer and more reliable.
Solder the new measured Q6, 07, Q9, R12~R14, and do not weld the collectors of Q8 and Q10 amplifier tubes first (this is very important), only weld the base and emitter to ensure that the DC negative feedback forms a loop (otherwise the differential pair tubes Q1 and Q2 cannot work properly), or use a diode instead of the amplifier tube emitter junction to prevent the amplifier tube from burning out due to unbalanced output. At this time, do not connect the speaker. Power on and detect the static voltage to ground at the output end. The normal value is 0V±40mV, and the smaller the positive and negative errors, the better. If the deviation is large, shut down immediately and check again carefully. If the output voltage is normal, measure the voltage between the bases of Q7 and Q9, and pre-adjust W1 to 1.5-2V. After confirming that all the above voltages meet the requirements, solder the power tubes Q7 and Q9, and adjust W1 to measure the total current allowed by the power amplifier circuit, which should be 25-30mA (or the collector current of the power tube J5-20mA). After the current meets the requirements, you can connect the speaker for testing (note that before connecting the speaker, carefully check whether the speaker is damaged to avoid burning again). At this point, the amplifier is repaired.
In addition. When the static voltage deviation at the output end is greater than ±50mV, focus on checking whether Q1 and Q2 are paired (the amplification factors of the two tubes should be basically equal, and the error should be less than 5%), whether R4 and R5 have changed values, and re-pair and replace the resistors to eliminate the fault.
Some amplifiers often burn out inexplicably, and they cannot be used for a long time after several repairs. The reason is mostly due to the unreasonable wiring of the printed circuit. The power line is not wired from back to front, which causes the circuit to produce parasitic oscillation when the volume is high, which will burn the amplifier when it is heavy. It should be corrected according to the wiring principle, and the back line should be as thick and short as possible. Then it can be repaired according to the above method.