I have this integrated amplifier for ages as an inheritance. Lately the right channel is not working as it should be. I cannot set the bias adjustment as per service manual. It stays at zero (0). The service manual recommends a +/- 31mV. The left channel i was able to set it. On the other hand , I replaced much of the components on the bias circuit section of the amp after finding some faulty components, i.e. opened diodes, signal transistors, pre-drivers, etc. The output transistors are OK since i tried swapping it with the left channel but still exhibits the same fault.
I measured the DC offset voltage with the left channel as zero and the right channel having a reading of 20mV.
What gives? Any help is much appreciated. Many thanks in advance.
I measured the DC offset voltage with the left channel as zero and the right channel having a reading of 20mV.
What gives? Any help is much appreciated. Many thanks in advance.
Hi deni,
Your symptoms are most likely due to a problem within the bias spreader (i.e. Q411,413,415,417 and associated passives) or the drivers Q419, Q421.
Do you hear any audio in the left channel, despite probable distortion? Any evidence of protection relay opening?
The fact that you observe amp output near 0V may be encouraging, and suggests that the problem may lie in the bias spreader or drivers.
First check all supply voltages, including opamp supply pins. I suggest measuring and tabulating voltages at collectors Q411 and Q413, and bases and emitters of Q419 and Q421, all with respect to ground. Note that the schematic indicates you should about plus and minus about 1.2V at the collectors of Q411 and Q413, respectively. Also measure voltage drops across R17 and R21; they will help confirm bias current current flowing through the Q409 VAS stage. What voltage do you see at servo opamp output, Q427 pin 7 ? That may lend insight.
Be safe while measuring! Also, keep sleeve of meter probe insulated with heatsink tubing to minimize chance of inadvertent contact with nearby components. A slip of a probe can be very damaging and dramatically worsen your problems.
Good luck!
Your symptoms are most likely due to a problem within the bias spreader (i.e. Q411,413,415,417 and associated passives) or the drivers Q419, Q421.
Do you hear any audio in the left channel, despite probable distortion? Any evidence of protection relay opening?
The fact that you observe amp output near 0V may be encouraging, and suggests that the problem may lie in the bias spreader or drivers.
First check all supply voltages, including opamp supply pins. I suggest measuring and tabulating voltages at collectors Q411 and Q413, and bases and emitters of Q419 and Q421, all with respect to ground. Note that the schematic indicates you should about plus and minus about 1.2V at the collectors of Q411 and Q413, respectively. Also measure voltage drops across R17 and R21; they will help confirm bias current current flowing through the Q409 VAS stage. What voltage do you see at servo opamp output, Q427 pin 7 ? That may lend insight.
Be safe while measuring! Also, keep sleeve of meter probe insulated with heatsink tubing to minimize chance of inadvertent contact with nearby components. A slip of a probe can be very damaging and dramatically worsen your problems.
Good luck!