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Can someone explain the "bouncing" trace I see on my Tektronix 465B scope?

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ConeTripler
Posts: 1
Joined: November 27th, 2018, 10:50 am
Country: United States

Can someone explain the "bouncing" trace I see on my Tektronix 465B scope?

Post by ConeTripler » November 28th, 2018, 8:10 am

I'm an inexperienced scope user. I'm new to this Forum.

I have a Tektronix 465B scope that I'm using to test a Fender Champ guitar amp. A Tektronix function generator is providing a sine wave input.

The trace appears fine when the probe is on the input jack, but at the triode output on a 12ax7 tube (my second test point in the path) the trace is bouncing up and down slightly at about 3 times a second....about the same rate as the laughing smiley.... :lol:

The amplitude and frequency do not vary, but the trace appears as if the vertical position control is being turned back and forth quickly. I don't know the technical term for this, and have found no examples.

It behaves like this at all subsequent test points on the amp, whether the probe is at 1x or 10x.

Note: The amp has a 120-cycle hum with the volume turned all the way down with no input, and I'm trying to determine if that is somehow related to the bouncing trace.

What's going on here?

Thanks,

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