Just starting to learn o-scope, F5 1k square looks funny

[bear]

looks like ur power supply is sagging asymmetrically. You can use the other trace on the scope to check it. That channel should be on DC. You will put the probe on it, and set the vertical gain so that it is nearly at full deflection.

I would manually offset the trace in the opposite direction that it will go (moved down for +, and moved up for -) so that you can use more gain in the vertical amp, and see changes more easily.

Now, with no load, note the position of the trace.
With the other channel on the sine wave, and a load on the amp, bring the level up towards clipping.
Note the position and shape of the trace that is the DC rail voltage. See where it moves when the amp is clipping (if at all) and how much ripple appears (if any).
Do this for both channels.

IF the rail moves the same number of volts AND the ripple that appears looks essentially identical, then the issue is in the offset, bias or gain of 1/2 of the amp channel.

If BOTH channels do the same thing, then report back, and we'll have to consider the situation more carefully.

So, if one rail is drooping more than the other the same way on both channels, AND there is no DC offset at the output, then it is NOT a fault in one channel, or a bad component (unless you did the same mistake, if there is one, on both channels).

Let's see what you get?

_-_-bear

[bear]

"...other scope channel..."

[DJNUBZ]

I am on the road right now but when I get back I will try this.

[bear]

also meant to say check both rail polarities in case that was unclear, and then for both amp channels, compare...

[gootee]

But first, please perform a detailed cleaning of your beautiful old Tektronix oscilloscope.

I used to refurbish and resell a lot of older Tek scopes. You will probably be amazed at how beautiful it will be, if it's well-cleaned! (After that, maybe you could rig a dust-cover for it, at least for extended periods of non-use.)

[DJNUBZ]

OK so I tested this out and I think I did it correctly. I did not get any sag on either rail. I am pretty sure I did everything correctly. I did take my probe and put it on R5 and R6 and the wave form is distorting. I am still new to this so tell me if I am doing something wrong. I thought I was testing the output of the jfets, am I right? From all this I am duducing that the signal distorting + the lack of sag means I have a problem with either my jfets or how their gain is set. So what is my next step?

Could it be possible that the signal generator is causing some sort of issue due to output impedance or something else odd? Should I pull out an old cd player and burn a cd with test signals and try that as well or am I worrying to much about nothing?


gootee, what do you recommend I use to clean it? Can I just use all purpose house cleaner or is there something special I should use?

[multisync]

in your 1st photo were the meter probes after the capacitor in the crossover? If so the signal you had was normal, no low frequencies. If you look at a square wave output of a preamp that has tone controls play with the bass and treble controls. You will see that the square wave output changes quite a bit depending on the frequencies being boosted or cut. A square wave is just sine waves. If you change the amplitude or phase of the sine waves the square wave will change. Adjusting the bass and treble controls changes the amplitude and phase of the sine waves that make up the square wave. The flatter the frequency response and the phase response the better the square wave will look.

[DJNUBZ]

Originally Posted by multisync

in your 1st photo were the meter probes after the capacitor in the crossover? If so the signal you had was normal, no low frequencies. If you look at a square wave output of a preamp that has tone controls play with the bass and treble controls. You will see that the square wave output changes quite a bit depending on the frequencies being boosted or cut. A square wave is just sine waves. If you change the amplitude or phase of the sine waves the square wave will change. Adjusting the bass and treble controls changes the amplitude and phase of the sine waves that make up the square wave. The flatter the frequency response and the phase response the better the square wave will look.

It was hooked to a fostex ft17h tweeter. I realize now that was not a good plan for a load. I am not as worried about that now, as I am the clipped response I am getting from sine waves.

[gootee]

Originally Posted by DJNUBZ

gootee, what do you recommend I use to clean it? Can I just use all purpose house cleaner or is there something special I should use?

Yes, you can use something like Formula 409. But don't spray anything on the scope. Spray onto some folded-up paper towels.

A small scrub brush or toothbrush will probably come in handy, as will some cotton swabs.

If you later go on to clean contacts inside, etc, look up DeOxit, from caig.com.

[DJNUBZ]

What could be causing my uneven clipping? if it is happening on both channels of my amplifier and, from what I can tell, is happening in the first stage?

I have some deoxit but I am little unsure about poking around inside the scope. It works ok as it is, it's not perfect but I don't want to go in there and mess something up. The biggest problem I run into is that when I try using it in the 5 or 10 mv region the line jumps to a new location and i have to recenter it with the position knob. 10mv is still pretty useable but 5mv isn't. Is there anything I should be sure to check on this unit?