TEAC A-H500 Amp - Hum on Left Channel

[xyberlink]

Hi,

I'm looking at a TEAC A-H500 Amp which is showing hum/buzz on the left channel.

Oscilloscope image attached - top (yellow) trace shows left channel, while bottom (pink) trace shows the right channel.

It appears to be approx 40mV (peak to peak) at 50Hz, hence assume it's a mains frequency hum, but only on the left channel.

I've disconnected the audio cable internally, to prove that it's the amp board itself at fault.

I've done a range of comparisons with the right channel, checking various components, including the incoming power feeds which all appear to match up ok (+/- 34.2v). I've also swapped C517 / C519 / C521 / Q517 / Q519 with the right channel - no effect. Also checked the Bias is correct at 11mV both sides.

The attached amp circuit is annotated to show some of the test points where I've proven the signal to be good (green dots) and as shown on the oscilloscope image (red dots).

Full service manual is available at this link....
https://elektrotanya.com/teac_a-h500_sm.pdf/download.html

Any thoughts / help much appreciated.

Thanks.

[JKKDev]

I assume from what you wrote you checked the Q517 and Q519? Did you try lifting the C521 to see which end of the amp is causing the issue (I would assume it being AC it would pass through a cap)? Did you scope the supply voltages as well as measure them? Maybe there's something there?

Just a few questions I thought of. I might be off on this one :p