Oscilloscope test shows intermittent ghosting/echo of original sine wave in Onkyo Amp

TankAudio

Member
2014-09-12 10:02 am
I just finished a full recap on an Onkyo A-7022. It had several issues beforehand. Now, the amp functions, but it produces a second sine wave of equal amplitude and identical form, non-inverted, and 160? degrees out of phase with the input signal. It looks like it has a regular interval in the time domain, and it shows up at the same place on the scope every time (same exact phase correlation). The ghost wave quickly shows up on the screen about 3 times every 1-2 seconds. Where should I look and what is it caused by? Amp History: 1. Had a problem with the bass response, that resulted in cracking and popping sounds coming through the speaker whenever a low note was played. The only time the bass did not do this was when the bass pot was turned to zero. 2. When hooked the amp up to the scope with a pure sine injected, a wobbling version of the sine showed up on the screen. I thought it looked like the 1khz signal I was injecting was riding on a lower frequency wave. I then went about testing caps on the power supply board, and found a few of them to be really bad. Amp Now: 1. Square wave tests look really great 2. Wobble went away.
 

TankAudio

Member
2014-09-12 10:02 am
picture of scope.

FYI - There is NO AUDIBLE blip or sound when the ghost note occurs. This could be a fault of the test setup. I just want to make sure everything is clean in this machine.
 

Attachments

  • IMG_20160318_142155_410.jpg
    IMG_20160318_142155_410.jpg
    532.3 KB · Views: 135