Cathode Corner SCTV Gebrauchs- und Pflegehandbuch
SCTV Scope Clock Kit
Assembly and User Manual
Copyright (C) 2022 Cathode Corner. All rights reserved.
SCTV Clock Assembly Manual 1/24/22 2
SCTV Clock Assembly Manual 1/24/22 3
Table of Contents
Introduction ................................................................................................................................................. 5
Theory of Operation .................................................................................................................................... 6
PC Board Assembly .................................................................................................................................... 8
Cabinet Assembly ..................................................................................................................................... 14
Initial Checkout ......................................................................................................................................... 27
Final Assembly ......................................................................................................................................... 27
Using Your Clock ..................................................................................................................................... 31
Optional GPS receiver .............................................................................................................................. 32
Care of the Cabinet ................................................................................................................................... 32
Warranty ................................................................................................................................................... 32
Schematic Diagram ................................................................................................................................... 32
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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.
The Clock
The SCTV scope clock is an electronic clock that displays the time and more 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 SCTV is optimized for aesthetic appeal.
The clock is supplied with a laser-cut Plexiglass cabinet.
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: 520-989-1491
Email: [email protected]
Mailing address:
Cathode Corner
2560 E Ross Pl
Tucson AZ 85716
USA
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Theory of Operation
Power Supply
The power supply is of the switching type. 12V DC power is switched through a high-frequency
transformer to produce the necessary operating voltages. The power supply uses a unique topology to
drive the CRT and the logic circuits.
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 load, so a regulator for one will not be well regulated the other
way.
The switching IC is a National Semiconductor LM2586. 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 a resistive divider.
The low-voltage supply provides -5V to the deflection amplifiers. The +5V supply is provided by a
linear regulator, because the switching supply doesn’t start up with a solid 5V to bring up the Teensy
computer reliably. This was an engineering change after problems wee found in a few of the kits sent to
early customers.
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.
Blanking is active whenever the beam is being moved from one location to another. The computer tells
the blanking circuit when to blank the beam. Blanking is accomplished by switching the grid to a much
more negative voltage by the Z modulation circuit to cut off the beam.
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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. This voltage is adjusted by the
Astigmatism control, which 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.
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 character is made up of 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. An arc code tells the circuitry which
octants of the circle to blank out to make an arc, such as in the number 2.
Computer program
The software running on the computer is written in C for the Teensy, using the Arduino environment.
The program executes a main loop about 50 times per second, in sync with the mains frequency to
prevent electromagnetic interference from being visible in the display. Each display screen is built of a
draw list, with some extra parts that are drawn by special code such as the Tetris bucket and pieces, the
Pong paddles and the clock hands. Other code monitors the USB port for GPS data messages, and the
encoder knob for mode changes.
The code is open source. It resides at www.github.com/nixiebunny/SCTVcode
Optional GPS Input
An optional USB GPS connector allows the clock to take its timekeeping reference from the GPS
satellite constellation. The firmware automatically connects to the GPS receiver as long as it emits an
NMEA data stream at 4800 baud. The internal clock chip is reset to match GPS time every second, so
that the time will still be correct if GPS reception is lost.
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PC Board Assembly
Getting Started
Make sure that you have the following major parts:
SCTV printed circuit board (PCB) with surface-mount parts already installed
Anti-static bag with thru-hole PC board components
Ziploc bag with CRT and pot wiring parts
Ziploc bag with hardware
Seven laser-cut plastic pieces (coated in paper)
3RP1-A CRT
12V wall power adapter
If anything is missing, please contact Cathode Corner for assistance.
The PCB is provided with all the surface-mount parts already soldered to it. These parts are rather
difficult to solder without a lot of experience. The easy work is left for you to do.
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Board photo
A photograph of the top side of the assembled board is shown below. Refer to this photo to see how the
parts fit.
Tools needed
Soldering iron, fine tip, adjustable temperature preferred
Solder, high quality lead-free or leaded .031" diameter or smaller, rosin or no-clean flux
Small diagonal cutters
Small long-nose pliers
#1 Phillips screwdriver
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Parts List
The parts supplied in the kit are listed below in order of installation. 'Step' refers to the assembly
sequence. 'Marking' refers to any part number printed on the part itself. The assembly instructions begin
after the parts list. Note that the only parts that may be confused are the three trim pots. There are two
pots with the same value and a third with a different value, in case the numbers are too difficult to read.
PC Board parts
Step Qty Marking Description Designators
1 1 small Battery contact B1
2 1 big Battery holder B1
3 1 P6KE18A Diode, TVS D3
4 1 USB jack J9
5 1 504 Trim pot 500K R1
6 2 103 Trim pot 10K R2, R3
7 2 black Socket strip 24 pin for U2
8 1 black Socket strip 5 pin for U2
9 1 DC power jack J1
10 1 Header 6 pin .10" J5
11 1 Header 7 pin .10" J3
12 4 MPSW42 Transistor NPN Q2-Q5
13 1 Header 5 pin .156" J4
14 1 Header 6 pin .156" J2
15 2 1uF Capacitor 1uF 400V C29, C30
16 1 1.85A Fuse, PolySwitch F1
17 1 1000uF Capacitor 1000uF 25V C3
18 1 Rotary encoder S1
19 1 T-1596-01 Transformer T1
20 1 Heat sink for U1
21 2 Screws #4x1/4” for U1
22 1 Screw, 4-40x5/16” for U1
23 1 Thermal pad for U1
24 1 LM2586T Regulator U1
25 1 Lockwasher, #4 for U1
26 1 Hex nut, 4-40 for U1
27 1 7805 Regulator IC with cap. U7
28 1 black Wire, 1.5” long for U7
29 2 black 24 pin strip for U2
30 1 black 5 pin strip for U2
31 1 Teensy 3.6 Computer U2
32 1 CR2032 Battery B1
Inhaltsverzeichnis
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