Oscilloscope For Amp Maintenance?

[keithb7]

Hi folks, I recent was given a good working Tektronix 2215A oscilloscope. I have been dabbling in tube amp repairs and maintenance for the past couple of years.
Self taught mostly from reading, studying and soldering lots. Lots of good folks on here keep me pointed in the right direction. I spent a few hours today learning
about what oscilloscopes do and how. I think I have a basic understanding at an entry level. Is there much use for me in using an oscilloscope for amp maintenance?
I was watching a You Tube video today about how to test capacitors for ESR values. As caps age, the ESR increases, and can been seen on the scope. I was thinking
this might be a great tool to test questionable vintage caps. It appears, I would have to pull a cap, test it, replace or reinstall as needed?
Instead of just changing all caps because they are original in vintage amps, maybe I can preserve and keep the good ones? Does this seem reasonable? It seems I need a signal generator
to accomplish this, but there are several affordable options on the net it seems. What other good uses are there for an oscilloscope and signal generator for use on
tube amps? I'm a rookie, so any comments from you folks with experience is appreciated.
I an quite enjoying learning about all these amp, and tube circuits. Thanks, Keith.

[Tom Phillips]

Yes. A sig gen is a natural companion for a scope when testing audio circuits.
Lots of uses for a scope:
Tracing the signal through an amp to determine where there is a problem
Looking at self noise of a circuit
Analyzing ripple on power supply lines
Determining if full wave rectifiers are operating properly
Viewing the overall shape of a waveform. (Looking for crossover distortion, clipping etc.)
And many more.
Checkout "Oscilloscope applications" on youtube.
Basics to learn include how to set the triggering, sensitivity & sweep rate and how (and why) to use scope probes.

[Boogie]

The main use of a scope is viewing the shape of a waveform - which leads to many applications as Tom said above, basically it lets you see visually what's happening to an AC signal

[Boogie]

A good way to learn what a scope does is to get a guitar cable with a normal 1/4" plug on one end, and bare wires on the other end (or some kind of plug that can connect to your scope). Then plug your guitar straight into the input of the scope (without any amp or anything.) you can see the shape of the wave made by your guitar. open string harmonics give the "purest" notes i.e. closest to a sine wave. other types of notes you can see the shape of the wave, increasing pitch makes the wave period shorter, turning up the volume makes the amplitude louder etc. Try playing the same note on different strings (like the open high E vs 5th fret B string vs 9th fret G strnig vs 14th fret D string etc) and see how the wave looks different

try changing pickups while playing the same note and see how that changes the shape. also turn down the tone control(which is a treble cut) and see how that changes the shape. pick the note close to the bridge and then in the middle of the neck near the 12th fret and see how that looks

then you can try putting say a distortion pedal between the guitar and the scope and see how that affects the signal

try with the triggering turned off (perhaps by using the external trigger setting with nothing connected to it, so theres no trigger) so you can learn what the trigger is for. in this case you will have to precisely fine tune the frequency of the sweep to get the displayed wave to stay horizontally stable on the screen, and small changes in the sweep frequency will make the wave move left or right rather than being stationary

[drewl]

With a scope you can learn to troubleshoot and repair amps much easier.
Scopes like yours also have a square wave test output for calibration and probe testing, you could pump that into an amp for signal tracing but an iPod, radio or tape player would be better.

With a scope you can see how much ripple is on the high voltage lines the filter caps should be smoothing, so yes they can tell you if a cap is going bad.

Also the 2215's are famous for the CRT's dying so keep the intensity low or turn it off when not being used for a while.

[g1]

As far as testing caps with it, the test voltages would be quite small. So it would not show you if the caps are failing when high voltage is applied, which is common in tube amps.
However, as drewl mentioned above, you can actually see if they are doing their job in their actual circuit.

[Enzo]

The scope is nothing more than a voltmeter with a picture for an output. You get a graphic display of voltage over time.

When your output sounds like hell, do you know why? A scope on the output will show you HOW the waveform is distorted. Is it clipped? One side or both? Is it missing half a waveform? Is the waveform being modulated by something - usually ripple? etc.

You can't tell how leaky a cap is unless you can put the operating voltage on it. A 450v cap might look great at 10-20-30-100-200v, but at 400v it leaks like a screen door. I don't pull the cap, I just let the circuit power it, and I look to see if DC is leaking through a coupling cap, or if a filter cap is not smoothing the ripple. I can do more thorough testing later if I need to.

Got noise? White noise (hiss) or that crackling sort of stuff is almost impossible to see on a volt meter, but you can see it on your scope, and that makes it a lot easier to locate.

[nashvillebill]

This cap tested good on my ESR tester and capacitance also measured only about 10% over nominal value. Yet as you can see it has vented, so not something to keep around. Had this cap been radial, with the ooze hidden between the cap and the PCB, it would've been kept as being good if the ESR reading in-circuit was used as a go/no go.

Nevertheless, a scope and sig gen are two indispensable tools for me. My scope has a built-in component tracer but I never use it.

[keithb7]

Thanks for your responses. It sounds like I am going to have fun learning how to use a scope effectively. Sounds like a power supply unit may be something to keep an eye out for at some point.
I am definitely going to try the guitar hook up idea with the scope. That sounds interesting and valuable to a rookie. I will probably post future questions here and keep them in one place for future
reference, and for future others to easily find if wanted.

[Tom Phillips]

...Sounds like a power supply unit may be something to keep an eye out for at some point...

I would put a signal generator as the next item to acquire. It will be much more useful for troubleshooting than a power supply.

...I am definitely going to try the guitar hook up idea with the scope. That sounds interesting and valuable to a rookie...

You can also hook up to the line out of your stereo or the headphone output of any music source. That will give you a continuous signal without the need to strum the guitar and you can concentrate on setting the various scope controls. If you connect to the headphone output of your computer then you can play music or various test tones available on the web. For example see the search results at https://www.youtube.com/results?sear...dio+test+tones .

If you play simple wave forms, such as pure sine waves, then you it will be easier to learn how to use the trigger controls on your scope. The trigger circuit is what allows you to get a stable waveform display. Without that you end up with a waveform that rolls across the screen.

Have Fun,
Tom

[Sowhat]

I would put a signal generator as the next item to acquire. It will be much more useful for troubleshooting than a power supply.

Signal generator is definitely the next step here. Power supplies can open up a whole can of worms in that you are faced with what kind or kinds do I need. For tube amp work a stout variac is about all that is needed but if you start working on solid state then you really should have a rails simulator configuration in where you can have an adjustable +- DC supply going up to maybe 100 volts at a few amps and it's little brother a +- supply topping out at around 30 volts for the preamp and control circuits. Single supplies are of limited use with music amps and usually only simulate 9 volt batteries although they are capable of much more... kind of a waste. Then there's the non symmetrical power input devices, thank god there are not many of those left in existence. As I said, it's a whole can of worms, worms that can give you a shock!

[gbono]

also be aware that the scope's ground is tied to chassis/earth ground. This can be an issue when measuring a circuit where the ground is floating - a bridged output on a solidstate amp for example. See attached.FFM.pdf

[dbtech]

Also not to be ignored are the voltages you will encounter when either just observing circuits for educational purposes, or actually troubleshooting them. Usually when troubleshooting, the amp (or whatever device) is powered on; in general, solid-state amp voltages won't kill you, but tube amp voltages truly can (or at least erase your previous thought for a few minutes).

In any case, use extreme caution when using a scope with your circuit powered on. Make sure the probe's ground clip doesn't fall into the circuitry and stays tightly clipped to the chassis. Note that most modern probes will read down to DC, meaning, if you stick the tip on a 350-volt DC power supply line, a portion (most probes have some sort of filter and/or divider in them) or all of the voltage will end up in the scope.

If you are actually measuring DC, make sure that the amplitude setting of the scope PLUS the divide ratio of the probe will allow for the expected voltage. For example, if the scope has 10 vertical divisions on the grid, and you set the probe to "divide by 10", you could then read 1000V maximum by setting the amplitude scale to 10 volts-per-division (assuming the zero volt point is on the bottom of the grid frame).

If you want to read small amounts of AC (millivolts) on a DC power supply line, first switch the mode of the probe input on the scope to AC (there is usually an "GND-DC-AC" switch near where the probe jack is on the scope). You can then reduce the volts-per-division (increase amplitude sensitivity) of the scope to measure millivolts AC in the presence of 100's of volts DC.

[The Dude]

also be aware that the scope's ground is tied to chassis/earth ground. This can be an issue when measuring a circuit where the ground is floating - a bridged output on a solidstate amp for example. See attached.[ATTACH=CONFIG]35605[/ATTACH]

A good point. I'll add: Keep this in mind if/when scoping a typical switching supply, also. Make sure that you are using an isolation transformer so that you don't damage the scope or the circuit you are looking at. Switching supplies "never" use chassis ground.

[gearhead63]

Yep an o'scope is an indispensable tool. I have several on my bench.
But as far as cap testing goes, I have an old Eico 950B that can throw up to 500 volts on a cap.
The new ESR meters are great, but you don't know if your cap will leak at it's rated voltage unless you can apply that voltage to it.