'scoping it out
Vince Rhea
I was wondering if some of the vets would share some of their diagnostic
skills with those of us who are trying to gain some? In particular, how do
you use your 'scope for checking out equipment? Keep it simple now, say the
3 or 4 most import things you use it for and how. Thanks!
Vince
Mjzuccaro
Using the scope is simple-feed in a midband test tone into the desired
input,maybe a few millivolts into a magnetic input,a few hundred millivolts
into an aux input,and signal trace thru the amp.Set the tone controls at
midrange (flat)-and make sure the signal does not clip or distort anywhere.If
you have the luxury of a stereo amp with one good channel you can compare them.
use a 400 or maybe 1kc tone.All you need is an audio bandwith scope,even the
cheapest $5 swapmeet Eicos and Heaths are flat in the audio band and rolloff
around a few hundred KC,so I would not trust them for high freq.Square wave
testing,but they are okay for audio signal tracing.By the way,lots of good
basic service info at your library and in old Electronics magazines.
Best,
mike Zuccaro
Vintage Audio Repair tech.>I was wondering if some of the vets would share some
of their diagnostic
>skills with those of us who are trying to gain some? In particular, how do
>you use your 'scope for checking out equipment? Keep it simple now, say the
>3 or 4 most import things you use it for and how. Thanks!
>Vince
>
>
Russ Sadd
>I was wondering if some of the vets would share some of their diagnostic
>skills with those of us who are trying to gain some? In particular, how do
>you use your 'scope for checking out equipment? Keep it simple now, say the
>3 or 4 most import things you use it for and how. Thanks!
Hi Vince,
I doubt I count as a veteran, but 'scopes can be very useful. I've picked up
some cheap ones in the past, notably a valve one for a tenner, and an old
solid-state for a bit more. The first thing about valved ones, is that
they're robust. Solid state ones tend to eventually die an unresurrectable
death when their power supply blows, but the valve ones just keep on going!
So what're they useful for? Well, first thing you can do with them is
monitor a signal by taking an AC signal as your Y input, and then adjusting
the X timebase to look at it. This gives you a pretty picture, but probably
isn't very useful.
On the other hand, if you've got a signal generator, you can connect this to
the input of the circuit (e.g. amplifier stage) you're looking at, and then
you've got a perfect saw-toothed wave, square wave, or sine wave of a given
voltage and frequency going in. Your 'scope will then show you what they
amplifier stage is doing with it: by adjusting the timebase, you get to see:
- If the signal is being amplified
- The signal being inverted
- A graphic of the frequency response (the waveform getting smaller or
bigger as you adjust the frequency)
- Distortion, which is what the different wave forms are meant for.
The best thing to do here would be to hunt out a book (valve era) on
oscilloscopes and how to use them. These are as common as old 'scopes, and
will be full of great stuff like the theory behind them, their practical
workings, how to build them, calibrate them, and very importantly, what to
do with them. As often as not, these kinds of books show you how to build
and use other kinds of test-gear.
Best regards,
Russ Sadd
Birmingham, UK
E-mail: gri...@dircon.co.uk
WWW: http://www.griffon.dircon.co.uk
TubeGarden
I use an old Heathkit Model OL-1 to look at the ouput from the distortion
analyzer. As long as the distortion artifacts are wavey and roundy (Hi Tech
Talk) things are OK. When sharp points and corners appear, back off.
Also, the Microacoustics PC AudioLab has a nice software dual trace scope which
uses the sond card in your PC and also has a signal generator for various
waveforms.
Do not forget to listen while you look. Good ear training technique.
Happy Ears!
Al B^}
Al Marcy
TubeG...@aol.com
Paul Croft
him with that proctoscope! He sure had a funny look on his face... The cat
not the vet.
Merry Christmas,
Paul
Vince Rhea <vin...@mis.net> wrote in article
<bk7f2.233$PE2.9...@news2.randori.com>...
> NNTP-Posting-Date: Sun, 20 Dec 1998 05:54:15 PDT
>
> I was wondering if some of the vets would share some of their diagnostic
> skills with those of us who are trying to gain some? In particular, how
do
> you use your 'scope for checking out equipment? Keep it simple now, say
the
> 3 or 4 most import things you use it for and how. Thanks!
> Vince
>
>
>
A Dougla6
Vince Rhea asked:
> I was wondering if some of the vets would share some of their diagnostic
> skills with those of us who are trying to gain some? In particular, how do
> you use your 'scope for checking out equipment?
Mine probably get used for everything *except* audio gear: as x-y
readouts for curve tracers, or for troubleshooting other test gear. The
last use was monitoring a pair of pulse generators while I tried to get a
particular gated waveform to play with a newly-acquired Nicolet FFT
analyzer. But to answer your question, scopes are ideal for signals
with DC levels: a meter would only indicate the DC. In an amplifier stage
for instance, you can watch the signal running out of headroom as it
approaches the supply voltage. With a decent scope, you can read off
the DC level from the screen without even needing a meter. Use one
with a good trigger, so you don't have to spend your time getting a stable
trace on the screen.
As Al Marcy said, a scope is ideal for nulling a distortion meter,
when you can see the residual products and hum/noise.
If you have a scope right at hand, on the bench, rather than one you
have to drag out and set up each time, you will find uses for it.
73, Alan
Paul Gustafson
You can get a good look at the symmetry of a phase inverter with a scope
also. If you have a dual trace jobber, connect each channel to the grid
leads of the power tubes and set the scope for AC coupling and invert
one or the other channel so the waveforms superimpose on the display.
Any non-linearities will show up pretty well. If your splitter is
working well you should not be able to see one trace behind the other.
As an added benefit you can uninvert the one channel and have a cool
looking design on the display (looks kinda like those ink blots the
shrinks on tv use!) Have fun and keep one hand in your pocket...
Happy Holidays!,
Paul G
Vince Rhea wrote:
>
> NNTP-Posting-Date: Sun, 20 Dec 1998 05:54:15 PDT
>
> I was wondering if some of the vets would share some of their diagnostic
> skills with those of us who are trying to gain some? In particular, how do
LGeoCole
diagnostic tool. In something that doesn't work or works
poorly, you can follow the signal through till a change
is noticed in the waveform. It's a coarse check though.
In checking for clipping in an amp you'll likely hear a
change in the sound from the speaker before you'll see
the peaks of a sine wave start to flatten. If you have a
'scope that can stand overdrive, you can crank up the
vertical gain and vert. position so that only the extremes
of the wave are visible and get an earlier view of clipping.
'Scopes can be educational tools, too. Poking around in
an amp, or whatever, you can see what the circuit is
doing to a signal. Changing its size, sometimes its shape.
If your 'Scope has dual trace capability, you can watch
the input signal at the same time as the output (or where
ever) signal. Makes comparisons easy. Some dual tracers
allow adding the two signals. With one signal inverted
and the gain adjusted for flattest line, differences between
them show up more. This will show distortion as wiggles
at various places, usually near where the peaks would be
if they were not nulled out. Phase shift will show up as a
non-nullable sine wave (assuming the input is a sine) that
shifts sideways as you move the gain through the null point.
If you manage to get a higher quality, wider bandwidth,
'scope, seeing what the amp does to square waves is quite
interesting. Peaked or rounded leading edges show high
freq or low freq boost. Wiggles at the edges indicate
instabilities or resonances somewhere. Peaks, rounds,
and wiggles can be causedurse, checking for hum and noise is straight forward.
No input signal needed, or wanted. Put the 'scope in AC
mode, if it's a DC 'scope, and start poking. It's
interesting to see how the ripple evolves through a power
supply.
Always remember, there can be high voltages at the end
of the probe, and where your knuckles are. Clumsy
probing can make a lot of expensive sparks. I've been
there, I can attest to that.
But always have fun, even if you're learning something.
George C.
Vince Rhea
Thanks for all the replys, folks. I will try and put the info to good use.
Regards
Vince
Kirk R. Patton
One really time-saving piece of test equipment I have on my bench is a
"quick-tracer," or "component-tester;" there is a fancy one on the
market called a Huntron Tracker . . . . but for a handful of components
(usually lying around) you can make a circuit that works on a 'scope's
X/Y mode . . . . and any old scope will do . . . even some cheesy
recurrent-trace job . . . as long as it has an X/Y mode.
What it is is a kind of crude curve tracer . . . simply two probes that
you can stick across any two points in a circuit (usually off) and get
an idea what's going on . . . most useful for semiconductors, but nice
for caps, etc. as well. There is a voltage across the probes that is
limited to a low current . . . the voltage deflects the X axis, and the
current deflects the Y axis. An open circuit looks thusly like a
horizontal line, a resistor a diagonal line, a short a vertical line, a
cap or coil like a circle or oval, and a semiconductor like a sharp
elbow or combination of elbows . . . . sounds more confusing to describe
than to actually use.
The circuit I use is 1 volt at 1 mA, really pretty wimpy, but can be
stuck on ANYTHING without risking damage . . . . even delicate CMOS and
such . . . . a more reasonable level for power semiconductors and tube
circuits might be like 12 volts at 10mA.
Below is my circuit . . . .
120 VAC
| |
| S 0.1A
|()()|
--------------
--------------
|()()()()()()()|
|----| |----|
| 3.5 VAC |
| |
|----^^^-----|----^^^----|
| 3.3 2w | 7.5 2w
| |
|-- Y axis |-- X axis
| out | out
| |
| |------ Positive Probe
|
|
|----^^^-----|------ Negative Probe
1K 1/2w |
|
|-----^^^-----|
220 1/4w |
|
///
I just happened to have the 3.5v transformer lying around, the
transformer and voltage divider can be adjusted to give the desired test
voltage. The 220 ohm resistor to ground is high enough to limit current
and act as a fuse should the negative probe be applied accidentally to a
voltage source -- but is low enough in value not to add noise ground
noise to the scope trace -- at least on my scope. This value may need
to be adjusted for scopes with unusually low input impedances. The
scope used should NOT be grounded for best results with this circuit . .
. and it (technically) reads backwards -- that is, a resistance reads in
quadrants II and IV, not I and III . . . the "channel 2 invert" switch
on my scope fixes this . . . or you can just get used to it backwards .
. . not really a big deal.
It works great for seeing leaky junctions on diodes, BJTs, FETs, and
such . . . matching small-signal transistor characteristics, finding
polarity and pinouts of transistors, seeing AC leakage on caps, and
comparing a known good circuit with a malfunctioning one . . . I find
myself reaching for it for
just to check continuity . . . . and can also be used for quick
impedance checks when the circuit is on . . . . you can "see" the output
impedance of a amp change (especially SS) when it is switched on.
Regards,
Kirk Patton
Ralph and Diane Barone
lgeo...@aol.com (LGeoCole) wrote:
[snip]
>Always remember, there can be high voltages at the end
> of the probe, and where your knuckles are. Clumsy
> probing can make a lot of expensive sparks. I've been
> there, I can attest to that.
>
>But always have fun, even if you're learning something.
>George C.
... and remember kids, don't be holding the ground lead to your scope probe
in your right hand while you're touching electrolytic capacitor cans with
your left hand (to see if any of them are getting warm). Not all of them
are grounded...
Happy to be alive at Christmas
Ralph
Iain Churches
>
> One really time-saving piece of test equipment I have on my bench is a
> "quick-tracer," or "component-tester;"
>
> Kirk Patton
Hi Kirk,
Thanks for your quick tracer posting. Excellent idea!
Iain