Oscilloscope Settings to trace oscillation

[ca7922303]

Got oscilloscope going.. What settings on here are best?

[Greg_L]

Where do I need to be at on signal generator?

Like mozz said, 50mv is fine. It's just kind of weak IMO if you want to push things in the amp a little bit to see if they act up. Even weak guitar pickups put out more amplitude than that.

I personally shoot for around 150mv when inputting a sine wave, but I'm certainly far from an expert so take my comments with a huge helping of salt.

[ca7922303]

Screen shot clean channel all controls on 10, except reverb(0).

[ca7922303]

Channel 2 b,t,m, drive/volume/master volume on 10. Reverb(0).

[J M Fahey]

Having driver issues with PC based oscilloscope, can we use signal generator and DMM on ac volts to trace?

FIRST you must *see* oscillation or there is nothing to trace.

We still donīt know what you have there.

[ca7922303]

Channel 2, same but with modern switch engaged.

[ca7922303]

Channel 2 with g,v,mv,b,m,t on 10.

[ca7922303]

Channel 3 with g,v,mv on 10, b,m,t on 3 reverb(0).

[ca7922303]

Channel 3 g,v,mv,b,Mt on 10, reverb on(0).

[ca7922303]

Channel 3 with g,v,mv on 10, b,m,t on 0, boost switch engaged.

[ca7922303]

Xpand engaged

[ca7922303]

Modern/bright, xpand/boost engaged

[ca7922303]

Oscillation only occurs thru a speaker is when xpand or boost are engaged separately. When engaged together, no oscillation. When engaged together with modern and or bright switch, no oscillation occurs.

[J M Fahey]

Short/trivial answer: do not use those "wrong" settings together.

Amp apparently "works" on most settings, and sometimes 1 or 2 combinations are "problematic", probably because they lead to very high gain, high top boost, etc. , all together.

Donīt think your amp is "broken" as in "it worked fine, a part broke, I replace it and we are back to normal" but "it is as it is".

Solving that problem MOST probably will require redesigning an I am not only talking "circuit" redesigning but also include layout, such as "pass this wire here instead of there" or "ground here instead of there", maybe 1 inch away.

Such things are well beyond the "repair" stage, are typically solved at the Factory, one reason year after year we have *different* amp versions.

Again: I think the sane solution is to avoid that couple extreme settings, period.

[ca7922303]

Would anyone be interested in walking me thru how to trace down the oscillation. Schematic link is in these lists. Not able to not have a fully functioning, half power, red plating amp. Appreciate the help.