Onkyo TX-SR333 receiver repair

[tomoto]

Bought this Onkyo TX-SR333 secondhand last year, and initially it served its purpose, but a couple of issues have begun to surface.

The first one I noticed was, upon changing inputs, it outputs a constant, low pitched buzz for at least 3-5 seconds. It sounds like it could be originating from mains (strong 60/120 Hz components). Now, initially it stayed at around 3-5 seconds, and would go away, and perform fine. After a certain point, when switching to most, if not all inputs other than AUX, the sounds would continue indefinitely. Interestingly, I used its Bluetooth input most of the time, and if I pressed the Bluetooth button, the sound would continue, but if it was in AUX mode, and I woke up my PC, it would connect and switch itself to Bluetooth mode automatically, no buzzing.

The buzzing is annoying, but the second issue makes the receiver much less useful. After redoing some wiring behind my TV (no new connections, just trying a new setup out), when reconnecting my HDMI from my PC, as I brought the HDMI cord near the port on the rear of the receiver, sparks occurred, and decently powerful ones. After doing that a few more times, and realizing it was not just static, I measured between the ground casing on the HDMI header on the receiver and the ground HDMI heading on my graphics card and found a difference of 25 V. Obviously I didn't try that any more, and wondered how my graphics card/PSU in my PC were still fine, as the surfaces were in good contact for more than enough time to fry something. I verified the 25 V difference on all the HDMI input grounds, which are of course shorted to the receiver case.

I haven't examined the schematic yet but I did go through some of the operation checks/debug in the service manual. The output of several didn't quite match up, but it could have been me misreading the manual. Regardless, I'm not sure that's the best path for me, as no subsystem is failing completely (hard to check for HDMI, but debug step 2: DSP decode seemed to output OK, so I don't think the entire digital board is fried). I've attached pictures, there doesn't seem to be obvious component failure, and the 12V on digital board reads ~14.3V. The second issue seems to be something off with grounding on one of my devices, but I have no reason to believe it would be my PC as it is connected to several other peripherals which are connected to the same mains outlets. The voltage between the HDMI headers was a constant and repeatable 25V, very little AC. The first issue seems like a relay opening/closing incorrectly, and/or a short somewhere.

Any suggestions on troubleshooting tips for either issue would be appreciated.

[Alex_Baker]

The amplifiers from this era seem to be really overcomplicated in my opinion...

what I would do is check any and all voltage rails for level and ripple (if you have a way to do that).

The input switching is done by a large IC, Q4001 in this case. It has a +15V and -15V power rail, you could check those.

I am not very experienced with audio stuff but I might try to solve the grounding/voltage potential issue first, and see if the 60/120hz buzz goes away with everything properly grounded. Is there a ground point on the amplifier?

[bob91343]

I think the power supply, especially the electrolytic capacitors, would be the first place to look.