SC100EKit.pdf

SC100E Scope Clock Kit

Assembly and User Manual

Table of Contents

Introduction ..................................................................................................................................................3

Theory of Operation .....................................................................................................................................4

PC Board Assembly .....................................................................................................................................7

Cabinet Assembly Guide ............................................................................................................................12

Installing in Your Own Cabinet...................................................................................................................17

Initial Checkout...........................................................................................................................................20

Schematic Diagram .....................................................................................................................................23

SC100E Clock User Manual

9/29/05

Introduction

The Manual

This user manual is divided into sections. Not all sections are relevant to all users. The Theory of Operation

section is provided for the curious and may be skipped, as it is not necessary to know exactly how the clock

works in order to make it work.

Two sections are provided for cabinets: one for the Plexiglas cabinet, and one for homebuilt cabinets.

The Clock

The SC100 scope clock is an electronic clock that displays the time on a small oscilloscope tube using

artfully drawn numbers. While most digital clocks use a seven-segment display optimized for low cost and

ease of manufacture, the SC100 is optimized for aesthetic appeal.

The clock may be purchased with a sturdy Plexiglas cabinet to protect the fragile CRT and keep the user

isolated from the high voltage used to operate the tube. If the clock was not purchased with a cabinet, the

clock PC board and CRT should be installed in a suitable cabinet of the user's choosing.

Contacting Cathode Corner

If you are having trouble assembling your clock, getting it to work, or you just want to talk with us about

clocks, you may contact Cathode Corner in any of the following ways.

Visit Cathode Corner on the Web at www.cathodecorner.com

Phone: 520-795-7228

Email: dforbes@dakotacom.net

Mailing address:

Cathode Corner

2602 E Helen

Tucson AZ 85716

USA

SC100E Clock User Manual

9/29/05

Theory of Operation

Power Supply

The power supply is of the offline switching type. AC power is first filtered and rectified, then switched

through a high-frequency transformer to produce the necessary operating voltages.

The most common types of switching power supplies are flyback and forward converters. A forward

converter uses the power driven when the primary switch is conducting, and multiplies that voltage by the

turns ratio. A flyback converter stores energy in the transformer while the switch is on, then transfers that

energy to the secondaries when the switch turns off.

This supply is both of these types in one - its outputs are all fed through voltage doublers, so that both

halves of the cycle are used. This is done to allow the voltage multiplier for the high voltage negative supply

to be regulated as well as the lower-voltage supplies.

The reason is that a forward converter develops a secondary voltage proportional to the turns ratio, whereas

a flyback converter develops a secondary voltage proportional to the duty cycle. These two functions are

different with regard to line voltage, so a regulator for one will develop a lot of 120 Hertz ripple when used

the other way.

AC power flows through a filter, whose job is to remove the switching noise from the power cord. Next is a

rectifier and a filter capacitor to provide 150VDC power to the switching circuit.

The switching IC is a Topswitch made by Power Integrations (www.powerint.com). It interrupts the current

flowing through the primary winding of T1 at a rate of 100 Kilohertz, and controls the duty cycle of this

interruption to provide regulated voltage outputs.

Moving on to the regulator, the deflection voltage is the one actually regulated. The other voltages follow

this voltage in proportion to their turns ratios. The regulator samples the deflection voltage through both a

resistive divider for absolute regulation, and through a series RC network for improved transient response.

The sampling device is an LM431 shunt regulator, which strives to maintain 2.5V from its reference pin to

its anode pin. It regulates the current through the optoisolator to achieve this. The Topswitch senses the

current in the phototransistor of the optoisolator and adjusts the duty cycle of the switch to maintain a

constant current at that point.

The low-voltage supply is pretty straightforward. Since both phases of the switching cycle are used, one

winding can generate both positive and negative DC outputs.

The filament supply is straight AC, since that heats up a wire as well as DC does. An AC-coupled

connection to the filament supply feeds a doubler which powers the optoisolator used for the modulation

signal. This optoisolator is a special logic-level photodiode unit with sub-microsecond switching time.

The Z modulation supply provides enough voltage to switch the grid on and off (~60VDC) and enough

current to do so quickly. A simple transistor switch controls the grid.

CRT Beam circuits

The CRT requires the correct DC voltages at all of its electrodes in order to form a bright yet small spot on

the screen. The cathode may be considered as the starting point of this system.

The grid requires a negative voltage relative to the cathode. This voltage is adjusted by the Intensity control,

and for night-dimming purposes may be controlled by an optional CdS photocell installed in R65.

Blanking is active for that part of the circle that is to be blanked when drawing an arc, and whenever the

beam is being moved from one location to another. Blanking is accomplished by switching the grid to a

much more negative voltage by the Z modulation circuit to cut off the beam.

SC100E Clock User Manual

9/29/05

The focus anode requires about +500 volts with respect to the cathode. This voltage is adjustable over a

wide range to accommodate different CRTs with different focus voltage requirements.

The second anode requires about +1500V with respect to the cathode for most 2" and 3" CRTs. This

voltage is adjusted by the Astigmatism control. This control changes the second anode voltage with respect

to the voltage on the deflection plates, since the deflection plates have the secondary effect of acting as

focusing lenses, first in one axis then the other axis. The magnitude of this effect depends on the voltage on

the second anode relative to the average DC voltage on the deflection plates.

Deflection Amps

The deflection amps are push-pull Class A amplifiers, which strive to keep the voltage at the transistor's

emitter at 0.6V below its base by changing the current through the transistor. This in turn changes the

voltage at the collector, according to Ohm's Law. It's not a real fast circuit, but it works for clock use. The

load resistor value is a compromise between speed and heat dissipation.

Numeric display

The digits are formed from circles, lines and arcs. The basic method of drawing a circle on a CRT is called

a Lissajous pattern. This is something that every student of electronics learns about early in school, then

promptly forgets. The only other known use of this numeric display technique is in the HP 1600 logic

analyzer.

A Lissajous pattern is displayed by applying sine waves of different phases to the X and Y deflection plates

of a CRT. A sine wave on the Y plates and a cosine wave on the X plates give a circle. If the phase

difference is zero, then the circle collapses to a slanted line. If Y=0, the line is horizontal, etc.

Each digit is made up of one, two or three segments. Each segment has a center, a size code, and a dwell

time. The center places the segment within the digit's cell space. The size code indicates the X and Y radii of

the circle/ellipse, as well as if it will be a line (x=cos, y=cos) or a circle (x-cos, y=sin). The dwell time is

longer for larger circles to make the intensity appear consistent.

A further piece of information is the arc code. This tells the circuitry which octants of the circle to blank out

to make an arc, such as in the number 2.

A set of three 74HC574 registers holds this information while the segment is being drawn.

The sin and cos waves are made from two taps 90 degrees apart on a shift register fed from a divided-down

version of the CPU clock that runs at 15.36 KHz. A pair of low-pass and bandpass filters produce clean

sin and cos waves from these taps.

The circle size code is fed into a set of CD4066 switches that pass selected amounts of the sin and cos

waves into the X and Y current summing amps. The X and Y position values are converted to currents by

binary-weighted resistors into the summing amps, a crude but effective digital-to-analog converter or DAC.

The X and Y summing amps convert the resulting currents into X and Y deflection voltages, which are

scaled and inverted for use by the push-pull deflection amps.

The screensaver is made with five resistors, which are used to add a small X,Y displacement to the entire

image. This displacement is changed slightly once an hour, and goes through a prime number of steps (31)

before repeating so as to cycle through the hours in an evenly distributed sequence so that no hour digit is

favored in any position on the screen.

Computer program

The software running on the computer is a bit tricky. It has to keep track of the time, send new display

segments to the display circuits on a regular basis, keep track of which buttons are pushed, and keep in

sync with the power line frequency. It manages to do all this somehow.

SC100E Clock User Manual

9/29/05

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