How to Use Signal Generator and Oscilloscope on Amplifier

[Adam60]

So let's say I have a signal generator and an oscilloscope and I want to see if an amplifier is working properly. Step by step, how would one go about this procedure? Is the amplifier turned on while I do this. Thank you in advance.

[3db]

Well you would need to switch the amplifier on. https://www.eevblog.com/forum/Smileys/default/cheesy.gif
You use the signal generator to produce a signal of a particular frequency and amplitude.
You use the oscilloscope to look at the waveform somewhere else in the amplifier eg at it's output .
You could for example use these instruments to measure the power output of the amplifier and it's frequency response.
Because you asked if you needed to switch the amplifier on you should perhaps read some of the blog postings on scopes etc first.
Hope that's of some help.

[Brumby]

Because you asked if you needed to switch the amplifier on you should perhaps read some of the blog postings on scopes etc first.
Hope that's of some help.

I agree - but I will go one step further.

My honest opinion is that you should find someone who can show you how to go about this.  They will be able to explain what's happening and how to use the equipment properly and safely.

[Shock]

An oscilloscope will be able to show you the signal from a signal generator and how the signal is modified by the amplifier. It will not tell you if the amplifier is working properly, this will left to your interpretation of the signal (a big distinction, as hooking it up is the easy part).

You generally connect the signal generator on the input and the oscilloscope on the output but if you have a faulty amplifier you may probe other locations within the amplifier (amplification stages, power supply output, voltage rails). To test the amplifier easily and at high volume it's recommend you use a suitable dummy load on the speaker outputs in place of the speaker (has other testing advantages as well).

A 1kHz sine wave is often used as a test frequency. You will need to understand how impedance works for making correct measurements, the rating of your oscilloscope and also how floating and non floating circuits work, e.g. inadvertently grounding circuits with your test equipment that are sitting at a voltage potential higher than the earth ground on your oscilloscope. So important to understand the input and output specifications of your test equipment.

While you may be able to get a general idea of the amplifiers operation, accurate noise and distortion measurements may require additional test equipment such as a distortion analyzer and spectrum analyzer. A multimeter is another essential tool, two of them for tube amplifier repairs.

As the amplifier needs to be turned on to amplify, putting your hands inside an amplifier that runs on mains power is a safety risk (hundreds of volts). It's recommended that you start learning safety aspects of working on equipment first before tackling anything else. Read this whole chapter on safety. http://www.allaboutcircuits.com/textbook/direct-current/chpt-3/importance-electrical-safety/

Then you need to learn how to use your oscilloscope and other test equipment, and how to manipulate the amp to get a clean signal for testing and then how to interpret different types of distortion and other problems plus learn how to confirm if your amplifiers performance is within spec, a science unto itself.

If somethings wrong, you need to learn how to repair amplifiers.

[Adam60]

Thank you for all the feedback. I have used an oscilloscope before, I am comfortable around mains voltage, familiar with capacitors and their hazards if not discharged properly. I have repaired TV's and power supplies and am learning a lot about electronics but I haven't gotten my head around using a signal generator to diagnose an amplifier.
I get that you generate a signal and read it with an oscilloscope, but what settings do you use on the amplifier, and what exactly will happen to the waveform when it reaches the output? My guess is that it is a stronger signal but I would like to see waveforms to compare to.
I have an amplifier here that has a low hum on one channel which I suspect to be bad caps as it is an older unit. I don't know what that looks like on an oscilloscope. I was hoping Dave had a video on it but I will have to wait for that.
Any feedback here is good.

[3db]

OK,lets try and fix your amplifier.
 Can you tell us something about the amplifier ie power output,make/model
 This will help us to consider the type of thing we're dealing with.
Does the hum change as you turn the volume up and down ?
Is the hum there on every input ie Mic/Aux etc.
A multimeter will also be handy.
Remember to read the safety information mentioned by Shock

[Shock]

To compare your signal look at the good channel of the amp or the source signal from your signal generator (sine wave). Normalize the EQ, balance and switch off any gain boosts, turn down the volume, use a dummy load. As you have a hum problem manipulate the controls anyway while looking at the signal as it's a common source of noise. I'm trying to keep this as generic as possible, I have no idea what you're repairing.

Read this: http://www.wikihow.com/Measure-DC-Offset
Some pictures: http://www.r-type.org/articles/art-125.htm

Now it's time to connect your oscilloscope and look for yourself.

[Adam60]

Okay. First of all, I can follow most things you will tell me but I may not get everything. I have read a little bit about dummy loads but not exactly sure what I need. I assume that I am putting them in place of a speaker.

[opensourcefan]

I found this yesterday when I was looking for something similar,  maybe it'll help.

http://www.instructables.com/id/Voltage-Amplifier/?ALLSTEPS

[Adam60]

Thank you opensourcefan. I seen that earlier and scanned through it. Perhaps I will build it and just go through it with my scope and signal gen for fun. Appreciated.

[3db]

The dummy load that you need is a resistor it will be used instead of the speaker.
If the amp had an output power of 10 watts into a 4 ohm speaker then you would need a resistor of 4 ohms with a power rating of at least 10 watts.
This is why I asked you to tell us something about your amplifier.
You could have an Amp with a 100 watt output. That would mean you need a 100 watt + resistor if you wanted to measure its output power.
We need information in order to help you fix the amp.

[Adam60]

My most humble apologies. I had made an edit to my post but for some reason it did not show up. The amplifier is a QSC EX1600 and the issue is that there is a hum at low volume on one channel.
Thank you to opensourcefan and Shock for the reading material. I found all the articles interesting.
3db, I hope you and the others have some patience. I will follow along as you tell me what to do and I may ask questions along the way. I ma fairly literate when it comes to electronic terminology and understand test tools fairly well. I may be a little green with signal generators and oscilloscopes but I understand their purpose and basics of each. I have two analog scopes as well as a PICOSCOPE PC unit at my disposal as well as a signal generator.
If you guys are up to it, Let's begin. Here is a webpage for the schematic for this unit if necessary:
https://qscservice.com/files/1613/6329/3807/EX1600.pdf

Thank you and I appreciate all your help.

[jobog]

If you connect your scope to the speaker with the low hum you should see the hum as noise and the signal should increase as you turn up the volume.  Input a signal (sine wave) into the signal path for that speaker and you should see the sine wave above the noise and it should respond to a voltage increase on the signal generator and also to the volume control of the amp.  Changing caps may or may not eliminate the hum and thus the noise signal on the scope.

I have an old hand held, battery operated, signal generator that just has a probe tip for injecting the signal.  I've repaired a lot of radios, tv's, and amps that had no sound by starting at the speakers and injecting the signal.  If you hear a sound the speakers are okay.  Keep stepping back in the signal path until you don't get any sound and you've found the bad component.

[alsetalokin4017]

The schematic is very revealing, thanks for posting it. This will help a lot.

I am presuming that the audio output of both channels, given an input signal,  is "about right" and that the only obvious fault is that hum on one channel. (which one?)

My first impression is this. This seems to be a quality bit of kit, and has separate power supply rectifier-filter sections for the A and B channel amp sections. So use your scope at the speaker output of the bad channel to determine the frequency of the hum. If it is at power line frequency, or twice that, i.e. 60 or 120 Hz (you are in Canada, right?) then you can start looking at the power supply main filter caps on that side, and other caps in that part of the circuit. Do this check before you start injecting any signals; we just want to confirm that the hum is at line frequency or a low harmonic of it.
 
For Channel A, these would be capacitors C527 through C534. Corresponding caps on Channel B are C627-C634.

This is where I'd start.
If the hum is at some other frequency the troubleshooting will be slightly more complicated.

Caution: Before you connect anything to the speaker outputs, you should verify that the "speaker common" is connected to chassis and mains ground. I can see that it is connected to the "heat sink ground" on the schematics. You don't want to create a ground loop by connecting the scope probe ground to something that isn't also grounded to the mains ground! 

[Adam60]

alsetalokin4017, This is very useful information that I may never have thought of considering that I do not work on amplifiers too often. I will see if I can check this out. It may take me a day or so as my time is limited right at this moment and I, like so many on here, am just a hobbyist. I hope you can stay tuned for the results as well as the others on here who have been so helpful so far. Back soon with the results.

[Adam60]

Just a quick question but when I am measuring DC offset using a DMM, do I need to have an input present on the amplifier and if so, how much volume should I use on the input side?

[3db]

I would say no input and volume at zero.
I have located the service manual for your amp and tried to upload it a few times but it exceeds the forum limits.
This amp is a beast that can dish out 1.6KW into 2 ohms.
It could kill you if you are not careful.

[3db]

I've managed to upload the specs so that others can see what we are dealing with.
I'm new here so not sure about forum etiquette on uploading multiple files (about 5Meg total).
Any guidance would be appreciated.

[3db]

Your amplifier has a sensitivity of 1v rms (2.828 p-p on your scope).
That would give 400W into 8ohms or 800W into 4 ohms per channel.
A dummy load for a thing like this would be something like water heater elements in water.

[Adam60]

3db. Trust me when i say I know how lethal this thing is. It weighs a ton and when I opened it up there were two big ass transformers in it along with several ass-kicking caps. I am working one hand in pocket and other one with fingers crossed. If you don't hear from me within 48 hrs, call 911 

With that kind of dummy load, is there a way to test it out without maxxing power? Do I need a dummy load that high?

[tautech]

I've managed to upload the specs so that others can see what we are dealing with.
I'm new here so not sure about forum etiquette on uploading multiple files (about 5Meg total).
Any guidance would be appreciated.

Often done but just a link for members to access the documentation is adequate.

[3db]

You can use a smaller power rating for the dummy load but you'll have to keep the volume way down and also keep the input signal low.
I haven't had much of a chance to check out the circuit diagram yet but think it would be worthwhile if you checked out some PSU voltages.
It has one for each channel so you should be able to compare things between channels as mentioned by another member.

[3db]

I can't upload the manual I downloaded because of forum file size limits.
I can't just link it either because you need a logon from the site I got it from.
I got it from this site http://www.hifiengine.com/

Thanks for you feedback Tautech.

[Adam60]

Okay I did the offset test with the amplifier disconnected from stereo and speakers. Not sure this is right as I think there should be an input but hoping you will clarify for me. If there was an input from stereo/mixer etc, how would one know it is not the source of the problem?
Anyways here is what I found: When I first plugged it in, I was moving my meter back and forth between CH1 and CH2 and found that CH2 jumped around quite a bit. It ranged from -0.176 VDC to +0.316 VDC and CH1 stayed fairly consistent around -0.020 VDC
After it sat for about 10 minutes, CH1 stayed pretty much the same averaging around 0.004 VDC (-0.012 to 0.016 VDC) and CH 2 was around -0.020 VDC on average but would jump up or down slightly every so often.
I have not set up yet for signal generation and scope testing.
All comments appreciated.

[3db]

You would know if the other device was the problem by disconnecting it.
I'm assuming that you have identified the power amplifier has the hum on one channel with nothing else connected.
Is this true ?
You can put your scope on the output of your preamp/mixer etc and look at the output of each channel
they should be the same.
Some of this was covered earlier by other members.