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.
Education is what you're left with after you have forgotten what you have learned.
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.
"So, for my small experience in the trade I would think that killing customers is, at the very least, bad for business." -Chuck H
Disclaimer: "Take my advice; don't listen to me"-Neil Young. "I'll lie again and again and I'll keep lying; I promise"-Henry Rollins.
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.
"I should have been born sooner. Of course, if I had been, I might be dead now." trem
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.
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.
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.
Education is what you're left with after you have forgotten what you have learned.
On the very first post - the use of a 'scope is to champion the rule of faultfinding - localize the fault.Originally Posted by ktone
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.
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.
"Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"
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.
Chuck H say's "thank you" for this useful post.
"I should have been born sooner. Of course, if I had been, I might be dead now." trem
I would'nt be without my CRO - Before I paid out heaps for a tube tester, the CRO was the ONLY way I had to check for low gain in a stage due to a weak tube and weak tubes anywhere can dramatically affect your sound.
I did a fix up on a Twin reverb recently - the guy was technically savy and had done a full recap on his old amp but it just did'nt pick up sound wise as he had hoped. A weak phase splitter tube and one weak preamp tube were his problem. I found that with the CRO fairly quickly. Now I have a tube tester I'd probably find it faster but the CRO did the job.
Be aware of the limitations of the fancy new digital CROs (like I have). A fellow tech having spotted my CRO went into raptures about the CRO's ablility to do an FFT on the waveform data and give me a spectrum analyser type dosplay and distortion values etc. - until I pointed out that the 8 bit digitiser meant that I would'nt see any distortion product below -40dB. Thats OK for guitar amp stuff but virtually useless for HiFi amp work. The HiFi guys are using 22 bit sound cards with purpose built CRO front ends and fancy audio analysis software for this sort of work.
They are a very handy tool. For the day job I need to service equipment on aircraft. A battery powered Hantek CRO is what I use for that.
We also recently bought one of those little hand held things, a "Sainsmart", cheaper and better than the unit referred to above but really these things are just toys.
Cheers,
Ian
I spent a lot of time messing with soundcards, but eventually I had to get a real distortion analyser. scopeblog
The THD+N figures published for soundcards are from tests carefully done to show the sigma-delta converters at their best. Testing hi-fi amps is a quite different story, it brings out the worst in them. I found that the high frequency noise and distortion from my M-Audio Audiophile Firewire was worse than my prototype amps.
Last edited by Steve Conner; 07-05-2012 at 12:01 PM.
"Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"
Here's an old story from my days at Tektronix. A 465 was brought to Customer Service that had been in a fire. The knobs were melted and the line cord was damaged. After the knobs and line cord were replaced, the scope powered up and met a majority of it's specifications. An ipod has no knobs.
WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personel.
And is much more likely to be rescued from a fire.
"I should have been born sooner. Of course, if I had been, I might be dead now." trem
I have read and followed this thread with interest. As an ex computerised-machine-tool engineer, turned amp repair man, I would not want to be without a least one scope (I have 4). However, a word of caution. Any tool is only as good as the person using it, and oscilloscope can show things you may not understand. It's easy to say hook up the scope to show what's happening, but interpreting the results can often take a lot of experience. Anyone dabbling would probably benefit from a night school course in basic electronics before seroiusly using a scope IMHO.
I am a very humble person. I am far better than I think I am
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