Running out of Ideas, hoping someone smart can provide some for my ONKYO TX-SR353 Receiver

[CDNBrewer]

Hello Everyone,

I have an Onkyo TX-SR353 Audio/Video Receiver (5.1 ch with HDMI) that I am having a really hard time diagnosing and I'm about to throw this in the junk and go buy a new unit that is on sale until the 7th.

Symptoms:

• Intermittent (part of the reason why i'm having a hard time isolating problem), sometimes will work for 10+h at a time without issue
• Original symptom was wife reported loud banging noise (like someone was banging on the wall with their fists) from the LEFT SURROUND speaker and then the unit shut off.
• I swapped the Left and Right surround speaker wiring and speakers to eliminate the speaker and wiring setup. Ran fine for a while and then I heard hissing and whining out of the right surround speaker, which makes me think the issue is in the left surround channel.
• Sometimes the receiver will just shut off without any audio or visual effects
• To restart the receiver I need to use the override command to clear the fault. The fault counter for "DC" is now up to 41. The service manual isn't 100% clear on what DC actually means but from what I am sure the overload and PTC faults are 0. I believe the DC faults are being triggered from the DC detect circuitry on one of the channels.

What I've done so far:

• On the Left Surround channel, I've completely removed the Q6SL and Q7SL darlington transistors from the board as well as removed C19SL to eliminate the possibility of +HIGHB and -HIGHB (+50 and -50v) rails from reaching to the output. In this configuration the unit runs fine for hours.
  • Note that C19SL tests fine on my LCR meter for value and ESR and the darlingtons appear fine as per my multimeter but as this is intermittent, it's hard to trust them.
• I've taken the darlingtons from a known working channel (right surround) and soldered them into the left surround channel, as well as return C19SL back to the circuit. In this configuration it fails. It made some cracking noises again after 10h of good performance and generated a DC fault.

What I am hoping is that someone who understands this circuit better than me can give me some ideas of which components to swap or test to narrow this problem down. As it's intermittent I am hoping that someone may provide some direction based on the symptoms.

I don't really understand the circuitry upstream of the main power transistors so I'd be wildly guessing which part to swap out and testing it for 10+ hours to see if there is any improvement.

Thank you in advance. I've uploaded the schematic as well as posted a snapshot of the schematic of the amplifier channel of the left surround. Note that the schematic says pioneer VSX-531 but I've confirmed that some of the circuit boards (amp board specifically) is actually marked VSX-531 on the PCB as Onkyo owns pioneer.

Note that I believe the ASO line of Q9SL might be the overload detection circuit (which i don't think is the acutal fault) and the other way it detects DC on the output channel is on PDF page 42. It looks like it takes lines off each channel and through various pull down resistors will pull the 3.3v signal to the CPU down to 0.82v or 1.4v so that it can detect overload vs DC protection on pin 85, and less than 0.66v on pin 100 for another form of DC protection. Like I said above, I think based on the symptoms of a single channel making weird noises that the DC protection fault is attributed to DC on one of the outputs and not a failure with the DC power supply to +/-HIGHB or +/-12V or +5V. If I had a multi channel logging oscilloscope I could potentially monitor all of these signals and wait for up to 10h for the unit to shut off and then confirm this but unfortunately I don't have this equipment.

[stj]

DC means DC is on the speaker output - check page 42,

[CDNBrewer]

Thanks for finding that!

So based on DC on the speaker output, I am assuming I am correct in focusing on the amplifier section. As i've replaced the two main power transistors and still have an issue, where would you suggest that I focus on next? any specific component in the schematic above?

[stj]

i presume you dont have a scope,
i am wondering if it's before the power amps in the processing because you mentioned strange noises.

[CDNBrewer]

correct - i don't have a scope.

I know on some of these other units the digital section was known for getting damaged due to heat and not switching HDMI. Mine didn't have BGA type processor and the connections looks fine however I was hoping that it would be something easy in the power amplifier stage.

Regardless, I think i've put too much time into fixing this and will throw it out. Best Buy has a Denon unit on sale that i'll go and buy tonight.

Hopefully Denon will last longer than this Onkyo/Pioneer.

Thanks again for all your help - much appreciated!