What Oscilloscope specs for audio work, diagnosis and repair?
in [Repair]
|
Prev: Anna Thoms is hot
Next: Dog fixes phone...
From: N_Cook on 4 Apr 2010 04:00 Jeff Liebermann <jeffl(a)cruzio.com> wrote in message news:650gr5tpjk6sh4e8fpaii8rclmm9etubg9(a)4ax.com... > On Sun, 4 Apr 2010 12:03:59 +1000, "Phil Allison" <phil_a(a)tpg.com.au> > wrote: > > >"Jeff Liebermann" > > "Phil Allison" > >>>** DO NOT BUY A DIGITAL SCOPE !!!!!!!!!!!!! > > >> Why? > >> > >>>Almost any ANALOGUE scope with bandwidth of 5 MHz or more is OK - DC > >>>coupled or not. > >>> > >>>Digital scopes absolutely SUCK for audio work. > >> > >> Why duz it suck? > > >** There is no way to prove the point to fools like YOU by posting > >messages on a newsgroup. > > Got it. You can't explain why a digital scope is not usable for > analog work. Perhaps I can help jog your memory. See below. > > Have you ever actually used a digital scope? A sound card based > scope? You really should try it some time. I think you'll be > pleasantly surprised. > > Drivel: iPod Touch based sound analysis instruments: > <http://www.faberacoustical.com/products/iphone/signalscope/> > I'm really tempted. > > >Buy anyone familiar with the use of analogue scopes for audio test and > >repair work will find using a DSO to be mighty irritating and tedious - at > >best. > > Long ago, in my mis-spent youth, I worked in a repair shop that did > mostly audio. Not audiophile, but production line warranty repair for > various manufacturers of various audio related equipment. The lead > tech never used a scope. He would just listen to whatever was coming > out of the speakers, scribble down what stage or device was blown, and > move on to the next machine. My job was to do the unsoldering and > replacement. His batting average was about 80% correct. I couldn't > even come close to that level of accuracy, especially without a scope. > One day, I saw him try to use a scope, and fail. He didn't know how. > > 40+ years later, I still can't do it with audio equipment. I gotta > have my test equipment, white noise, pink noise, sweeper, distortion > analyzer, and all important oscilloscope. However, I can do something > like that with 2way radios. I've heard enough of them on the air to > be able to diagnose problems by simply listening to the audio. > > Moral: Use your ears first, then use the scope. > > >The displayed traces on a DSO are often very misleading and hence useless > >for many test procedures that analogue scopes do just perfectly. > > Oh? Misleading in what way? What measurements are misleading? How > will using a digital scope produce a misleading diagnosis? > > I use a digital storage scope for doing the all important square wave > test. Instead of the fuzzy blur of high frequency ringing and > oscillations seen on the analog scope, I see the digital equivalent, > which looks like a jitter infested trace in the same area (top of > leading edges). By superimposing multiple stored traces on top of > each other, ringing and oscillations are fairly obvious, even if they > exceed the frequency response and resolution accuracy of the A/D > converter. > > DC related phenomenon are a problem with a PC sound card based scope. > There's no DC response, and the lower limit is about 20Hz. Low > frequency display during the square wave test will show up as a "sag" > in the horizontal part of the waveform even with a DC coupled audio > amplifier. Scope probe compensation also shows the same "sag". I > partly compensate with the scope probe compensation, and just remember > what the "sag" looks like when the scope is directly connected to the > square wave generator. In other words, I ignore the low freq sag. > > High frequencies are more of a problem. > <http://en.wikipedia.org/wiki/Slew_rate> > Slew rate testing is difficult with a digital scope, unless the scopes > usable bandwidth is more than 5 times the highest frequency of > interest. For a 2MHz bandwidth digital scope, that limits the maximum > frequency to about 400KHz, which should be more than adequate for any > slew rate testing. That's NOT the case with bottom of the line sound > card based scopes, which are bandwidth limited to about 22KHz. The > 96KHz 24bit sound cards are much better. > > There are probably other areas where an analog scope is better than > digital. Tuning and tweaking in real time is much easier with a fast > responding analog scope than on a more slothish digital equivalent. > Seeing oscillations and ringing at tiny points during a frequency > sweep is somewhat easier to see on an analog scope. > > Did I miss anything on why an analog scope is superior to digital? > > > -- > Jeff Liebermann jeffl(a)cruzio.com > 150 Felker St #D http://www.LearnByDestroying.com > Santa Cruz CA 95060 http://802.11junk.com > Skype: JeffLiebermann AE6KS 831-336-2558 After a DVM the next most useful piece of test equipment is a crystal earpiece with a high voltage cap in tow, for audio repair work . Next comes a sig gen and then a scope (analogue not DSO and stand alone, not tied to a pc). Other useful pieces of test kit is a nose, eyes (with good magnifying inspection lamp) and ears. -- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on http://diverse.4mg.com/index.htm
From: David on 4 Apr 2010 09:23 "K Fodder" <abc(a)123.com> wrote in message news:4bb7e2f9$0$24378$c3e8da3(a)news.astraweb.com... > To All > > Thanks for your help. Looks like I might be able to > pickup up > a tektronic 465 (100mhz) cheaply so does that sound ok or > was > there a newer 100mhz tektronic model more realible or > easier to > service? If not I'll go for it. > > I'll investigate the digital option down the track. > Thanks for the other advicr given so far, I've noted it > all down. > > I use a digital oscilloscope for audio all of the time. Although the view of the waveform may be different that an analog scope, there can be overriding benefits. Most digital scopes have measurement functions such as frequency and amplitude and FFT which is spectrum analysis. That is very handy for looking at distortion if you have a very clean audio oscillator for the source. David
From: Phil Allison on 4 Apr 2010 10:00 "David" > I use a digital oscilloscope for audio all of the time. ** That is a blatant lie. > Although the view of the waveform may be different that an analog scope, > there can be overriding benefits. ** Nonsense. > Most digital scopes have measurement functions such as frequency and > amplitude ** So do all analogue scopes too - you bloody fool. > and FFT which is spectrum analysis. That is very handy for looking at > distortion if you have a very clean audio oscillator for the source. ** Bollocks. FFTs fitted to typical 8 bit DSOs can barely resolve 2% harmonic levels with any accuracy. The crude sampled trace on such scopes makes all sine, square and other test waveforms look distorted. ..... Phil
From: Jeff Liebermann on 4 Apr 2010 13:03 On Sun, 4 Apr 2010 09:00:56 +0100, "N_Cook" <diverse(a)tcp.co.uk> wrote: >After a DVM the next most useful piece of test equipment is a crystal >earpiece with a high voltage cap in tow, for audio repair work . Next comes >a sig gen and then a scope (analogue not DSO and stand alone, not tied to a >pc). Other useful pieces of test kit is a nose, eyes (with good magnifying >inspection lamp) and ears. For me, the most useful item is a schematic. I don't have an earphone connector on my crystal ball, but I'll certainly check with my consulting sorcerer if it's an available option. The video on my crystal ball is working just fine and is able to devine the future and troubleshoot problems with amazing accuracy. However, without audio, the best I can do is lip read and guess what's happening. I'm not sure the other accessories will be very useful. My nose is always dripping. My reading glasses have morphed into surgeons glasses. My ears are fine, but haven't been the same since I bought an iPod Touch. I do have a magnifying glass, but find a microscope more useful for PCB work. -- Jeff Liebermann jeffl(a)cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
From: Jeff Liebermann on 4 Apr 2010 13:39
On Sun, 04 Apr 2010 13:07:46 -0400, Meat Plow wrote: >On Sun, 04 Apr 2010 10:03:41 -0700, Jeff Liebermann ><jeffl(a)cruzio.com>wrote: > >>On Sun, 4 Apr 2010 09:00:56 +0100, "N_Cook" <diverse(a)tcp.co.uk> wrote: >> >>>After a DVM the next most useful piece of test equipment is a crystal >>>earpiece with a high voltage cap in tow, for audio repair work . Next comes >>>a sig gen and then a scope (analogue not DSO and stand alone, not tied to a >>>pc). Other useful pieces of test kit is a nose, eyes (with good magnifying >>>inspection lamp) and ears. >> >>For me, the most useful item is a schematic. I don't have an earphone >>connector on my crystal ball, but I'll certainly check with my >>consulting sorcerer if it's an available option. The video on my >>crystal ball is working just fine and is able to devine the future and >>troubleshoot problems with amazing accuracy. However, without audio, >>the best I can do is lip read and guess what's happening. I'm not >>sure the other accessories will be very useful. My nose is always >>dripping. My reading glasses have morphed into surgeons glasses. My >>ears are fine, but haven't been the same since I bought an iPod Touch. >>I do have a magnifying glass, but find a microscope more useful for >>PCB work. >I just use a Dowsing Rod, points out the bad parts every time. Dowsing only works for detecting running water, such as a leaky electrolytic. When I tried it, the rod would always point to my coffee cup. The crystal ball is far better for troubleshooting. I use a form of "map dowsing" where the schematic acts as a map. I place the schematic behind the crystal ball, and look at the highly distorted future image. Failed components appear as soldering iron burnt areas or are marked by red circles, which is what I sometimes do to mark the schematic after I finish the repair. A crystal ball will also work by viewing the device being repaired through the crystal ball, but it's very difficult to distinguish the original and the future replacement parts through the distorted image. However, viewing the device in a magic mirror, which reflects the opposite of what will happen is very useful. The reflection will show a smoking ruin for all the components, except the defective part, which will look perfect. -- Jeff Liebermann jeffl(a)cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |