How useful is an Oscilloscope for working on Guitar Amps?

[Enzo]

Can I get by? Mostly maybe. But show me assymetrical clipping with a meter. I guess excess B+ ripple under load could be captured with a meter, but really, two seconds on a scope. I think getting by means being reasonably effective on the bench, not merely managing to accomplish the repair eventually. Like starting a campfire. One could do the boyscout thing and rub wood to ignition, or one could simply get out a lighter. One is good for earning a merit badge, the other is good for getting dinner started and illuminating camp. The difference between using the right tools for the job or seeing what we can MacGyver. Define the goals.

Now if someone is not up to the level of interpreting a scope display, then sure, a meter is sufficient to that skill level. Ignoring for discussion the value the scope has for learning - seeing the result of ripple is more intuitive than AC on a meter, say.

I suppose too it is a matter of what we want to accomplish. Just make the amp function? OK, meter. But some instability? A meter will never tell you it is parasitics rather than clipping. I can measure ripple on a weakly filtered B+ node, but would my meter be able to pick out poor decoupling? If my interstage decoupling cap is weak, I can see some signal riding it in my preamp, but I doubt a meter would resolve that, especially since it is a moving target.

[Chuck H]

Well, I was talking about getting by, no idealizing. It was said that one could get by without a scope. And one can. And WRT guitar amps (should say that in all caps) close is nearly always close enough and symptoms are usually indicative of their respective problems to most techs. If I can hear it, I don't need to see the intermodulating effect of poor decoupling on my scope screen to know that the problem is a bad ground or a bad filter. And if I can't hear it, it's not a problem. We've seen and used the same circuits for decades. I don't need to measure the volume of my skillet, weigh my eggs or use an infra red thermometer on the cook top to make an omlet. And if it comes out a little dry I pretty much know why.

And just so my personal opinion on the matter is clear, just get a scope. A used scope in good working order that is good enough for amp work is a couple hundred US or even less, and worth a lot more than that on a bench.

[Mark Black]

I got by without a scope for years and years. Then I got one (along with a signal generator), and got used to having/using it.

Then one day that scope broke and I felt really lost (like I was totally screwed!). After you have/use a scope for a while you will wonder how you ever did without - please trust me on this. Now I have 4 scopes, partly to make sure I always have at least one working and also partly because sometimes it's beneficial to be able to look at more than 2 channels at the same time.

I tend to prefer the immediacy and resolution of analog CRT-based scopes myself...

[DrGonz78]

I was recently at a field service repair shop here where I live and talked to the tech for about a half hour... Anyway I asked every question I could about everything from education recommendations to test equipment to invest upon. I asked if a isolation transformer is a good investment before buying a scope. He told me he never bought one or needed to buy one. I had researched that maybe having this isolation transformer could reduce shorts to ground on test circuits and also protect you in some way to becoming a ground. This is if there is a short in the testing circuit that then creates a ground loop, thus possibly damaging your scope or you. So I am slow to go at this until I know what is most important to accompany a new scope.

So, overall I still ride by the advice given here... When you know why you need a scope then it is necessary. I have read and read about this field... I have tested bad amps w/ DMM's and been guided by the best here on MEF. Still have not bought a scope, because I am not there yet. I am getting closer everyday and can't wait to keep up. I am still in dim light bulb club and DMM central for now. I like it here really, but impatience is wearing out. Soon I will try to step up to that level, but then it opens a huge can of worms. So, there will be lots of questions to ask that will probably be under this thread. Really I can't wait but will have to till I feel that I NEED to know what I am missing...

[Jazz P Bass]

Why would you NOT want to monitor the output of an amplifier with a scope?
That is the question.

[Enzo]

I see no purpose to get out the isolation transformer when working on amplifiers, unless they have an SMPS and I have to work on the SMPS.

[g1]

As Enzo said, very rare to require the isolation transformer. Aside from SMPS, I only recall ever using one for some Carver amps.
If I do need one, I have a small isolation transformer which will run my scope. Much cheaper than getting a big one to plug higher current devices into.

[Chuck H]

There's absolutely no reason to not want to. It can be very useful in a graphic way if for no audible reason. But, as per the subject at hand... You CAN get by without it if you have to. WRT GUITAR AMPS!

I will NEVER be without a scope.

[woodyc]

A scope is absolutely essential. Ears and multimeters are simply inadequate for determining clean output power, the nature of the clipping, the presence of parasitic oscillation, et cetera.

About isolation, you do have to be careful not to ground amp outputs these days since it will drive some amps nuts (ones with current feedback.)

I use Tek scopes. Rigols seem ok -- I've been using one of their generators (DG1022) daily for a few years and its been solid.

I wouldn't get a scope without a USB port -- documenting things is a good idea and they make it really easy.

Stay away from anything used or surplus unless you plan to have it calibrated. Inaccurate test gear is worse than useless.

[km6xz]

A good dif amp probe system for the scope has many uses including dealing with SMPS and bridging amps. Isolation transformers are useful so if someone does a variety of gear they are probably going to want one.
Although light and cheap, digital scopes don't respond or display noise character like an analog scope so my main scopes are always analog, Tektronix. I see people warning newcomers away from used scopes or test gear and I don't quite understand why. A good 456 of 2465 will outlast any cheap digital scope, particularly given the range of insults impressed on a scope input. The types of overload that a good analog scope can handle with ease will kill any of the plastic cheap digital scopes.
Same with meters. Few little DMM's have much tolerance for pulling RF arcs such as when measuring anode potentials. Tube amps guys should be looking for VTVMs which were not great for precision but no tube circuit values better than 3 significant digits mean anything at all. An old HP410B or for more modern look and size, the 410C or even the $19 ham swap meet Heath or RCA VTVMs are really bullet proof and long lasting. Used precision lab type gear is almost always a better deal and longer lasting, with less ambiguous readings than new Chinese digital pocket gadgets, and much easier to repair. A 465 for example seldom needs work but if it does, they are readily repairable.
So don't sell the old lab type gear short, they were built with the best engineering of the time and still excel at bargain prices.

[Enzo]

And a word or two about calibration. There are applications for super accurate readings on a scope, but in general, I am never going to use my scope as a voltmeter. I use it to see if something is in the ballpark, but I am more intersted in the shape of the waveform and its general amplitude. And most any dumb scope is up to that. If i am watching logic level transitions on some control circuit, I need to see the signal traversing the high and low areas, but whether that high is 4.68 or 4.74 volts doesn;t really matter. And if i am in analog circuitry, I can put a power rail on display and watch signal come up to it and clip or fall short of it.

[ktone]

Anyone tried one of these scopes?
Here are the specs.

I'm interested but it seems a little pricey. This could be useful as I really don't want to ship my old 465m all the way to Hong Kong. Shipping would be the same as one of these scopes since I already have a display for it.

[Dave Mac Farlane]

Anyone tried one of these scopes?
Here are the specs.

I'm interested but it seems a little pricey. This could be useful as I really don't want to ship my old 465m all the way to Hong Kong. Shipping would be the same as one of these scopes since I already have a display for it.

On the very first post - the use of a 'scope is to champion the rule of faultfinding - localize the fault.
On your Ipad scope, the difficulty I have with anything with a screen, or worse still, a touch screen, is that one runs out of screen.
How can you tweak the amplitude or gain on a scope, centre the zero line or change waveforms in quick succession if you have to alter the screen to do so - I don't think this lends itself to real problems, where you have only a second or two to adjust. Just my opinion.
Dave.

[Steve Conner]

If you're thinking of buying one of those, you ought to consider the pocket DSOs from Seeed Studio too.
DSO Quad - Aluminium Alloy Black [TES725AAB] - $219.00 : Seeed Studio Bazaar, Boost ideas, extend the reach

That is their top-of-the-range one, with 6x the sample rate of the Oscium for the same price. (Academic really, either one is more than fast enough for audio.) They have several cheaper models and are based in China, so shipping to HK should be cheap.

Personally I would take the 465M over any amount of digital gizmos.

[km6xz]

Besides, the added cost of an iPAD with very fragile universal connector supporting that little converter box.
I agree, as a service tool that can take abuse, the 465M, (the cheapest one is also the most rugged) is hard to beat. It will last forever, be as accurate as any circuit condition that makes a difference, needs to be, and can take significant input abuse that digital scopes were never intended to. This little adapter for $290 can only tolerate less than 5 volts input peak to peak. That does not leave much room for static discharge or arcs from parasitic oscillations that occur when pulling a divider probe away from a high tension line like an anode of a guitar amp.
A regular 464b might be $300 and the military ruggedized version with lexan case is often $150 because buyers think the plastic appearing case is not as strong as the thin aluminum case of the standard 465, is a bench tech's trustworthy tool that pays for itself in 1-2 days of bench work.
The repeated comments about calibration seems strange. Every shop has at least one dependable reference, usually a time base or a voltage reference or indicator from which all else in the shop can be calibrated. That is something that is just a standard part of an electronics business, and can be fun. Digital resolution is cheap so its has become expected that precision and accuracy come along as a given. But that is a misconception, if 3 significant figures can be resolved that is repeatable and accurate, that is all that can be expected from any circuit requirement except for clock references. There was a reason slide rules were so popular with engineers for a long time, they naturally forced a realistic view of precision that was attainable in a circuit. Model all you want to with 6-7 digits but build it and your are dare lucky is anything measures to within 2- 3 digits of prediction. Models and real life are sometimes at odds with each other since neither knows about the other. I still use a slide rule in calculating original designs because is is so good with moving fudges. The ability to see the relationship between values they are represented for the full length of the slide. If something measures at a certain resonate frequencies after assembling a calculate circuit, it is a snap to figure out unforeseen influences on the outcome and sliding the rule to get a new corrective value to compensate for mischaracterization of the circuit by having models that miss some factors that are influencing your circuit. You can calculate fudge factors instantly by sight.
My HP calculator does a lot of complex functions but it is not a design or refinement tool the way a slide rule is. Besides all those extra digits of results have to be truncated anyway with the calculator to keep from being tricked into confusion resolution with accuracy.

All this may sound like it is beyond repair but it really is fitting. No measurement should be taken without an internal calculation of what it should be. If you do not know, based on looking at the circuit diagram, what values are expected, don't bother wasting the time needed to make the measurement. If one is blindly testing points until something strikes them as odd, they might have passed over very valuable clues that would have caused another tech to rightfully conclude that no other measurements are needed in that portion of the circuit. Making the least number of most appropriate measurements is rewarded with more quality repairs in less time. Good techs are cheaper for repair work than lower skilled ones who might have a low hourly rate. Good techs tend to replace fewer parts, are quicker to getting to a definitive cause and cause less damage.