OT: Semi-functional scope?

From: Chuck McManis <cmcmanis_at_mcmanis.com>
Date: Tue Sep 21 18:50:38 1999

Scopes are pretty simple beasts. Basically they work as follows:

Part 1:
1) Hold the beam of to the side waiting for a "trigger."
2) When the trigger occurs send the beam to the right of the screen
   at a very well calibrated speed.
3) Go to 1)
Note: There is a knob called "horizontal position" that biases the
horizontal deflection circuit so that the beam starts at a particular point
(left/right) on the screen. You may want to start with this in the
mid-point position.

Part 2:
1) Connect a signal amplifier to the vertical deflection circuit
   of the scope.
2) Adjust the gain of the amplifier such that a specific voltage
   results in a specific deflection of the beam vertically.
NOTE: there is a knob called "vertical position" somewhere that biases the
vertical input such that the beam sits on the scope at some point with zero
input signal (ie grounded input). To start you should set this to the mid
point.

Both parts run simulataneously, so when you feed a repeating signal into
the input, and trigger the scope at an even multiple of the input
frequency, you see what appears to be a static display on the tube.

Scope checkout then requires three steps:
        1) Check out the triggering circuit.
        2) Check out the vertical amplifiers.
        3) Check out the calibration of the above.

Some very simple scopes have only two trigger modes "Line", and "Ext". When
set to "Line" they trigger every time the AC power crosses 0 or 60
times/second. On "Ext" they trigger whenever the signal connected to the
external trigger input exceeds a particular threshold (usually settable,
sometimes it is fixed at 0 volts).

Once you have found the triggering section, set it to "Line". You should
now see a horizontal beam. If you don't try moving the vertical position
knob back and forth to see if you can make it appear. If you can, adjust
the knob so that the beam crosses the middle of the screen.

Hook up a square wave source (a kludged up 555 works well for this) to the
trigger input. Set the trigger for "Ext" verify that you see the beam again.

Now put the trigger back to "Line" and hook up your square wave source to
the channel 1 input (or sometimes called Y input). You should now see two
lines or perhaps some square waves all jumbled together. If you don't see
them, but you do see a line when the square wave is disconnected, then set
the gain (the knob marked "volts/division" to a higher number. Repeat the
experiment.

Now check to see if your triggering section has a setting for "channel 1"
or "Internal". Set it there and the squarewaves should stop jittering and
appear.

At this point changing the "time base" knob will change how many cycles of
wave you see and changing "volts/div" knob will change how big they appear
on the screen.

--Chuck

At 06:58 PM 9/21/99 -0400, you wrote:
>About 2 years ago a friend of mine gave me a "Bell & Howell Schools"
>scope. I put it in my basement for a few years until I've had time to look
>at it. A friend guided me through the alignment over the phone so I get my
>dot centered on the screen. But I can't get it to start (is sweeping the
>word?) I just get the dot. Turning the controls to any place other than
>where he told me results in the dot falling off the screen (to the left
>usually). I've tried to get results by connecting low-level audio sources
>including music and white noise, but still nothing on the display other
>than the dot. Any ideas? I need to have it working by 3rd period Friday
>because my friend is bringing an Arp 2600 analogue synthesizer to my
>chemistry class to demonstrate different wave forms and it will be a lot
>more effective if we can demonstrate the different waves if theyc an be
>seen.
>
>Oh, I forgot to mention that when I opened it up, there was a REALLY long
>CRT (I'd say more than 1.5 feet) that ran the entire length of the
>cabinet. Other than a bit of a rusty shield under one of the went holes,
>it was REALLY clean on the inside... but had several spider webs and dead
>insects. How something can be home to little creatures but not get dusty
>is quite beyond me... the only openings are the vent holes in the top,
>which are long skinny things that shuold have let it all the dust...
>
>Thanks,
>Kevin
>
Received on Tue Sep 21 1999 - 18:50:38 BST