Oscilloscope Basics Part 4: Square wave testing and analysis

[ConchBoy66]

Thank you for the scope tutorials. Just a little info can go a long way - I'm sure a alot of us do check waveforms at various freq's but I just don't see much of it posted here.
So I think you are doing a good service here.

How would you set up the amp to perform this test? Would it be as simple as connecting the dummy loads and probes from the scope to the speaker outs and feeding the signal into the aux port? Are most tests done this way? Sorry to have to ask such a basic question but I don't know the answer. I would like to be able to try this test. Thanks

 

[Brian1970]

How would you set up the amp to perform this test? Would it be as simple as connecting the dummy loads and probes from the scope to the speaker outs and feeding the signal into the aux port? Are most tests done this way? Sorry to have to ask such a basic question but I don't know the answer. I would like to be able to try this test. Thanks

Yep, that's all you have to do. Same setup as testing a sine wave through the pre-amp/main amp.

 

[ConchBoy66]

Yep, that's all you have to do. Same setup as testing a sine wave through the pre-amp/main amp.

Thank You! That helps alot!

 

[arts]

How would you set up the amp to perform this test? Would it be as simple as connecting the dummy loads and probes from the scope to the speaker outs and feeding the signal into the aux port? Are most tests done this way? Sorry to have to ask such a basic question but I don't know the answer. I would like to be able to try this test. Thanks

When square wave testing any solid state amplifier, be very careful about your power levels when using higher frequencies. It is very easy to destroy an output stage if you push it too hard.

 

[sch9171]

Thanks for these threads. Super helpful! I am a long-time self-taught electronics hobbyist, but I never really ventured into oscilloscopes or working on audio equipment until very recently. I was testing my Sansui AU-555A, and with a 1 KHz square wave, I got this result:




The signal was generated by an Owon DGE2070, which I know is not the best, but it does seem to generate a fairly clean 1 KHz square wave if tested directly. I also tested using a function generator app on my phone (which doesn't make quite as clean of a square wave) and got a similar result. The sine wave seems fine until it starts clipping around the 7-8 o'clock position on the volume knob (with a line-level input).

This doesn't look quite like any of the examples earlier in this thread. I guess it's closest to an extreme version of the "poor high frequency response" in the third image in post #1.

The amp sounds "ok" to me — not obviously broken, at least. Not as bad as I would have expected based on this waveform. But I don't have perfect hearing, it's not hooked up to the greatest speakers (it's a tertiary system for me), and I also don't really know what it "should" sound like.

I'm curious if anyone has any guidance on what is causing this, and what components I should be investigating further. I was thinking of doing a re-cap on the whole thing, just because of age (and as far as I can see, it's never had any serious internal work done), but I don't know if that's the best first course of action based on this.

 

[Brian1970]

Thanks for these threads. Super helpful! I am a long-time self-taught electronics hobbyist, but I never really ventured into oscilloscopes or working on audio equipment until very recently. I was testing my Sansui AU-555A, and with a 1 KHz square wave, I got this result:

The signal was generated by an Owon DGE2070, which I know is not the best, but it does seem to generate a fairly clean 1 KHz square wave if tested directly. I also tested using a function generator app on my phone (which doesn't make quite as clean of a square wave) and got a similar result. The sine wave seems fine until it starts clipping around the 7-8 o'clock position on the volume knob (with a line-level input).

This doesn't look quite like any of the examples earlier in this thread. I guess it's closest to an extreme version of the "poor high frequency response" in the third image in post #1.

The amp sounds "ok" to me — not obviously broken, at least. Not as bad as I would have expected based on this waveform. But I don't have perfect hearing, it's not hooked up to the greatest speakers (it's a tertiary system for me), and I also don't really know what it "should" sound like.

I'm curious if anyone has any guidance on what is causing this, and what components I should be investigating further. I was thinking of doing a re-cap on the whole thing, just because of age (and as far as I can see, it's never had any serious internal work done), but I don't know if that's the best first course of action based on this.

Make sure your tone controls are all set flat, loudness is off, and any filters are off. The waveform you're showing may be fine if one or more of those is affecting it. It looks like what I'd expect to see with the bass control boosted or the loudness on.

 

[sch9171]

Make sure your tone controls are all set flat, loudness is off, and any filters are off. The waveform you're showing may be fine if one or more of those is affecting it. It looks like what I'd expect to see with the bass control boosted or the loudness on.

Oh, man, I feel like an absolute fool! I knew that could affect it, but I removed the knobs for some cleaning and I didn't realize that they'd been turned. You're right. Once I centered the tone knobs, things look much better.

 

[Brian1970]

Oh, man, I feel like an absolute fool! I knew that could affect it, but I removed the knobs for some cleaning and I didn't realize that they'd been turned. You're right. Once I centered the tone knobs, things look much better.

View attachment 3380165

Much better! Does that bit of overshoot get better if you turn the treble down? It looks like there's some noise in there too. It might be showing some signs of aging caps. They definitely affect the tone and your screenshot looks like you have a slight treble boost. You might have some noisy transistors too but bad bypass caps can also give you some noise. I'd bet it'll sound noticeably better with new caps and maybe replacing any known problem transistors.

I think square wave testing is really useful. It gives you a lot of info about how the amp is working. Play around with all the controls and learn how they affect the square wave. You'll be able to spot issues pretty quickly once you know what you should expect to see.

 

[sch9171]

Much better! Does that bit of overshoot get better if you turn the treble down?

Yeah, I can knock down the overshoot by turning down the treble, but the treble knob is stepped rather than continuous, and the first notch down results in this, which looks like an overcorrection to me.

 

[Brian1970]

Yeah, looks like that to me too. I bet new caps will have it putting out nice clean square waves and maybe clean up that bit of noise. There are some good threads on here about refurbing the 555A and I think it has some transistors that are known to get noisy with age so not a bad idea to replace those too if you get in there.