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It's an unusual reading in that only the lower part of the waveform is clipping -Here is an image of the oscilloscope reading of the amplifier connected to an 8 ohm resistor:
View attachment 37569
In this picture the blue graph is the pre-out output (which seems to be fine) and the yellow is the speaker output (severe clipping) with an 8 ohm resistor load
View attachment 37570
So, it is the amplifier in my receiver that has malfunctioned. Any solutions? This clipping happens with all inputs and even with no inputs connected and just FM radio running.
Different sources: Yes (including FM tuner. All show distortion on the scope)7 speakers can't misbehave at the same time. The problem has to be something common to them all like a bad contact in the analog interconnects or digital cables from your sources. It could also be the volume pot as that usually comes before preamp stage.
Did you try playing from different sources and do they all misbehave?
Your scope plot shows a clear case of distortion which is likely because your AVR is being over driven. What was the source of that input and what was the level? Is your vertical scale properly set to display the expected level of the AVR output?
Yes, it repeats on other channels also. I tested the front 3 channels and all show clipping in the lower region.It's an unusual reading in that only the lower part of the waveform is clipping -
Can you check if this pattern repeats on other channels also?
frend2001 may be onto something here , is it possible to use another step-down to test?
A distant possibility is that since the positive and negative cycles of the audio waveform are amplified by two different sections on both the preamplifier and power amplifier, the section for the negative waveform on either may be having issues.Different sources: Yes (including FM tuner. All show distortion on the scope)
Source of input in the case of the plot: Youtube app on Firestick with DD+ output (So, there was no level setting)
Vertical scale: What should be the expected level of the AVR output? I don't know. Do you? But, I changed the scale so that complete output was captured.
Yes. And most likely it is the power amplifier because the preamp output is fine and shown by the oscilloscope (My receiver has a preamp output).A distant possibility is that since the positive and negative cycles of the audio waveform are amplified by two different sections on both the preamplifier and power amplifier, the section for the negative waveform on either may be having issues.
Since all 7 channels are affected , it is also possible that the negative rail of the symmetrical supply (+V / 0 / -V) is messed upA distant possibility is that since the positive and negative cycles of the audio waveform are amplified by two different sections on both the preamplifier and power amplifier, the section for the negative waveform on either may be having issues.
Power plug -> Single step down transformer (which is isolation type, i.e., it has different primary and secondary coils)-> AVR.Are you referring to step down as isolation also or you have 2 different transformer in power line. May sound stupid to ask but wanted to rule down things.
Can you describe the total chain from power plug to receiver ?
Thats what I thought too. Most transformers on Amazon are autotransformers, i.e., there is a center tap to get the right voltage (not two different coils). So, there is a high chance of supplying 230 V if there is a malfunction.Hmmm.... I thought transformers always have them separate.
I looked at the insides using scope and multimeter. The + and - DC high voltage rails are fine, the input to the final amplifier transistors (The biggest transistors with heatsink) itself is distorted. The preamp output is fine. So, there is some distortion happening between the preamp output and the input to the final amplifying stage. To do anymore digging, I need to remove the video boards and all which I don't want to do. I will see if the service center can repair it.well if possible you may want to change that step down and see if it's ok.
Prima facie it seems like some current related issue. An amplifiers may start distorting due to low voltage.
Can you measure the voltage output from your transformer ?
Please BE VERY CAREFUL and if you don't know how to do it better don't attempt it at all and refer to some electrician.