How It Works

A colour television of 1974 was a hard machine to build at home for one reason above all: it had to be aligned. Traps in the intermediate-frequency strip had to be tuned against a sweep generator and a marker generator, the three beams had to be made to land on their own phosphor dots, and the three rasters had to be made to lie on top of one another across the whole screen. Heath’s answer, taken as early as the 1963 GR-53 and carried through to the GR-2000, was to remove the need for external instruments altogether: the critical filters left the factory aligned and sealed, and the instruments needed for the rest — a dot generator and a meter — were built into the television and switched in from the front panel. The GR-2000 went further than any set before it by doing the tuning, and the display of what it was tuned to, in digital logic.

This volume follows the signal through the set, then takes the three digital subsystems — channel selection, on-screen character generation and the clock — stage by stage. The descriptions rest on the two contemporary engineering accounts of the chassis, Larry Steckler’s in Radio-Electronics of February 1974 and the Popular Electronics test report of April 1974, together with the surviving Heath manuals for the closely related GR-2001 and for the GRA-2000-1 clock.

Figure 1 — "Heathkit's new digital color TV": the opening of Larry Steckler's review in Radio-Electronics, February 1974, the fullest technical account of the chassis published at the time. The photograph sho… — Figure 1 — "Heathkit's new digital color TV": the opening of Larry Steckler's review in Radio-Electronics, February 1974, the fullest technical account of the chassis published at the time. The photograph shows the set with its rear panel swung out. — Source: Radio-Electronics, February 1974, p. 33, World Radio History, https://www.worldradiohistory.com/Archive-Radio-Electronics/70s/1974/Radio-Electronics-1974-02.pdf

Figure 2 — The GR-2000's signal path. Everything except the picture tube and the high-voltage rectifier is solid-state; the on-screen digits and the dot pattern are injected into the video output stages, so t… — Figure 2 — The GR-2000's signal path. Everything except the picture tube and the high-voltage rectifier is solid-state; the on-screen digits and the dot pattern are injected into the video output stages, so the picture tube writes them with its own beams. — Source: drawn for this dive from the circuit descriptions in Radio-Electronics, February 1974, pp. 33-37, 78-80, and Popular Electronics, April 1974, pp. 77-80.

2.1 The receiver proper

2.1.1 Tuners

Both tuners are varactor types. Instead of switching coils with a turret, each tuned circuit contains a varactor diode whose capacitance follows a d.c. control voltage, so a channel is selected by applying the right voltage. The tuners sit on the chassis rather than behind the front panel, because nothing mechanical connects them to it. Automatic fine tuning is added to the same control line: the a.f.t. voltage is combined with the channel voltage in a VHF/UHF switcher module, which also converts the tuner to UHF operation when the selected channel calls for it. Popular Electronics describes the mechanism: when a UHF channel is programmed, strapping on the channel-selection board develops a voltage drop across R211, and the switcher module applies the correct voltage to the tuner for that selection (Popular Electronics, April 1974, p. 77).

2.1.2 The fixed i.f. filter

The intermediate-frequency amplifier is the part of the set that most impressed the reviewers, and the part that made an un-aligned kit television credible.

In a conventional 1974 colour receiver, selectivity is produced by tuned LC circuits distributed through the i.f. amplifier and by narrow traps — one at 39.75 MHz to reject the picture carrier of the channel above, another at 47.25 MHz for the sound carrier of the channel below. Those traps drift as components age, and they need instruments to set.

The GR-2000 replaced the whole arrangement with a single ten-section LC bandpass filter, described by Popular Electronics as “a predistorted transitional gaussian design”, computer-designed, sealed in a double-shielded enclosure and aligned at the factory. It is the only response-shaping element in the strip. Radio-Electronics published both response curves side by side and measured the result: adjacent-channel carriers a minimum of 60 dB down, and skirts steep enough that “the 10-to-15-dB ‘comeback’ problem typical of the conventional trap is nonexistent”. The magazine added that this was “the first set we have seen that includes a filter-type i.f.”, and drew the consequences: better performance, easier assembly because no instrument alignment is required, and a picture that stays as good as it started because there is nothing to drift.

Figure 3 — Radio-Electronics printed the two response curves side by side: at left a conventional 1974 colour i.f. strip with its adjustable picture and sound traps, at right the GR-2000's fixed ten-section L… — Figure 3 — Radio-Electronics printed the two response curves side by side: at left a conventional 1974 colour i.f. strip with its adjustable picture and sound traps, at right the GR-2000's fixed ten-section LC filter. The adjacent-channel carriers sit at least 60 dB down and the skirts are steep on both sides. — Source: Radio-Electronics, February 1974, p. 35, World Radio History, https://www.worldradiohistory.com/Archive-Radio-Electronics/70s/1974/Radio-Electronics-1974-02.pdf

Around the filter the circuit is conventional in outline and integrated in execution. The tuner’s i.f. output is coupled into the filter through Q325, a common-base stage that both provides gain and presents a constant impedance to the tuner and the filter while isolating them. The filter feeds a two-integrated-circuit amplifier chain — an MC1349 gain block and an MC1330 synchronous detector in the circuit as published — which supplies gain, synchronous video detection, an a.f.t. output and a choice of high- or low-impedance composite video outputs. The transformer between the two ICs is broadly tuned, so it can be set without special instruments. The 4.5 MHz intercarrier sound signal is taken from the low-impedance output, with a 4.5 MHz trap feeding emitter follower Q326 which drives the a.g.c. and chroma circuits.

Figure 4 — The complete i.f. strip as published in February 1974. The block marked "LC filter" is the sealed ten-section bandpass filter; the two integrated circuits following it provide the gain, the synchro… — Figure 4 — The complete i.f. strip as published in February 1974. The block marked "LC filter" is the sealed ten-section bandpass filter; the two integrated circuits following it provide the gain, the synchronous detector and the a.f.t. output. — Source: Radio-Electronics, February 1974, p. 36 (Fig. 8), World Radio History, https://www.worldradiohistory.com/Archive-Radio-Electronics/70s/1974/Radio-Electronics-1974-02.pdf

2.1.3 Video, chroma and sweep

From the detector the luminance signal runs through an amplifier with d.c.-controlled contrast — the contrast control varies a voltage rather than passing video, which keeps interference out of the picture — and into the video output stages. The chroma chain is built around integrated circuits: an IC colour amplifier for “truer colors”, an IC colour oscillator with automatic phase control for stable tints, and an IC automatic gain control for sensitivity, selectivity and noise rejection. Automatic chroma control, a colour killer and adjustable video peaking are all in the chassis.

The vertical sweep circuit uses complementary power transistors, which allowed Heath to delete the vertical output transformer and with it, as Radio-Electronics noted, “its magnetic and linearity problems”; the magazine judged the resulting interlace “near perfect”. The horizontal output assembly develops the 26,000-volt anode supply for the picture tube. A restorer who has had a GR-2000 chassis apart describes the sweep as an RCA-derived design with SCR horizontal output (VideoKarma thread, “Heathkit GR-2000 Solid State Color TV”); Heath’s own catalogue spec for the family’s earlier 25-inch set lists two SCRs in the chassis, which is consistent, but no Heath document consulted here states it for the GR-2000 outright.

One circuit on the horizontal oscillator board exists purely for safety and deserves naming here because it is invisible until it misbehaves. A lockout circuit monitors the high voltage; if it rises above the permitted figure, the circuit pulls the horizontal oscillator out of lock and the picture breaks into diagonal lines. A set that will not hold horizontal sync is therefore telling its owner to look at the high-voltage assembly, not at the hold control. The GR-2001 manual gives a functional test for it, and warns that the resistor used in the test gets very hot (GR-2001 assembly manual, Book Three, p. 3-29). Its purpose is discussed with the X-radiation rules in the restoration volume.

2.1.4 The picture tube and its shield

The GR-2000 used a 25-inch black (negative) matrix tube with fully illuminated phosphor dots and an etched faceplate to cut glare, warranted for two years — the same MTX-5 family of “ultra-rectangular” tubes Heath had introduced on the GR-371MX in 1972. The tube is enclosed in a steel “Magna-Shield” container that keeps stray magnetic fields off the mask and helps hold colour purity. Secondary sources give the tube type as 25VEDP22 (Radiomuseum); it is not named in the catalogue specification.

Figure 5 — Inside a built GR-2000: the picture tube in its Magna-Shield enclosure, with the deflection yoke, the convergence assembly and the purity rings on the neck. — Source: Jason Rubik, Flickr album "Heathkit GR-2000 Color Television", https://www.flickr.com/photos/34737609@N07/albums/72157613423708701/

2.2 Changing channel without a knob

The tuning system is the heart of the set, and it is a piece of small-scale digital logic that would look at home in a 1974 instrument.

Figure 6 — Channel selection in the GR-2000. Touching a front-panel switch gates a 2 Hz clock into a four-bit counter; a decoder turns the count into one low line out of sixteen, which releases one preset voltage to the varactor tuners. The same count reaches the on-screen readout through a field of jumper pins. — Source: drawn for this dive from Radio-Electronics, February 1974, p. 79, and Popular Electronics, April 1974, p. 77.

A two-hertz multivibrator built from Q217 and Q218 runs continuously. Touching the up or down switch on the front panel opens one of two latching gates formed from sections of IC201, and the pulses reach the corresponding input of IC202, a four-bit up/down counter. Each pulse advances the count by one; hold the switch down and the set steps through the channels at two per second. The counter wraps: counting up from 1111 the next pulse gives 0000, and counting down from 0000 gives 1111.

The counter’s four output lines drive IC203, a four-line to sixteen-line decoder. For any count exactly one of its sixteen outputs is low and the other fifteen are high. Each output drives a transistor switch, Q201 to Q216, with an associated diode, D249 to D264. The fifteen transistors whose lines are high are saturated and clamp their inputs to about 0.3 volt, shorting their preset tuning voltages to ground. The single transistor whose line is low is cut off, so the voltage set on its preset control — R276 to R293, sixteen small potentiometers on a board in the service drawer — passes through its isolating resistor and diode to the varactor tuners. R294 calibrates the string.

The result is a tuner that has been “set” once, by the owner, for the sixteen stations he can actually receive. Because the channel number attached to each slot is defined separately, by jumper wires pushed into a pin field, any channel can be put in any slot and the same channel can appear twice. Unused channels are simply not programmed, so a viewer stepping through never lands on snow.

Figure 7 — The published channel-selection circuit: the 2 Hz clock and latching gates at left, the four-bit counter and sixteen-line decoder in the middle, and the bank of sixteen transistor switches, isolati… — Figure 7 — The published channel-selection circuit: the 2 Hz clock and latching gates at left, the four-bit counter and sixteen-line decoder in the middle, and the bank of sixteen transistor switches, isolating resistors and diodes feeding the tuner at right. — Source: Radio-Electronics, February 1974, p. 37 (Fig. 9), World Radio History, https://www.worldradiohistory.com/Archive-Radio-Electronics/70s/1974/Radio-Electronics-1974-02.pdf

Figure 8 — The channel-selection board in a built set, Heath part 85-1302-1. The rows of pins at the bottom are the TENS, ONES and SPARE fields, and the diodes and resistors above them belong to the sixteen s… — Figure 8 — The channel-selection board in a built set, Heath part 85-1302-1. The rows of pins at the bottom are the TENS, ONES and SPARE fields, and the diodes and resistors above them belong to the sixteen switching circuits. Programming the set means pushing jumpers into this field. — Source: Jason Rubik, Flickr album "Heathkit GR-2000 Color Television", https://www.flickr.com/photos/34737609@N07/albums/72157613423708701/

Figure 9 — The same system as Popular Electronics drew it, from the touch switches through the latching gates, up/down counter and decoder to the sixteen preset controls and the varactor tuner. — Source: Popular Electronics, April 1974, p. 77 (Fig. 1), World Radio History, https://www.worldradiohistory.com/Archive-Poptronics/70s/1974/Poptronics-1974-04.pdf

2.3 Writing on the screen

Putting a number on a television screen in 1974 meant turning the electron beams on and off at the right instants as they scanned, and doing it in step with the set’s own sweep. There was no frame store and no video memory; the digits exist only as timing.

Figure 10 — How the digits are produced. Retrace pulses from the sweep circuits gate a 4.5 MHz oscillator; the character generator counts its cycles to decide which of the seven segments should be lit as the b… — Figure 10 — How the digits are produced. Retrace pulses from the sweep circuits gate a 4.5 MHz oscillator; the character generator counts its cycles to decide which of the seven segments should be lit as the beam passes, and the result is inserted into the video output stages. — Source: drawn for this dive from Radio-Electronics, February 1974, pp. 34-37, and the GRA-2000-1 clock manual.

The display device is IC301, a custom MOS character generator. It takes horizontal and vertical retrace pulses as its time base: the horizontal pulses go straight in, while the vertical pulses pass through a gate and an adjustable time delay. A 4.5 MHz oscillator is gated on only when horizontal and vertical pulses are present together, so the character generator counts only inside the rectangle of screen where the digits belong.

Each digit is drawn in seven segments, and Heath modified the shape of the standard seven-segment figure so that the segments overlap at their ends and the characters read as unbroken numerals rather than as assemblies of bars. Radio-Electronics set out the arithmetic of the character cell: six horizontal time slots and four or eight vertical time slots per digit, with blanking between digits. The set can produce a four-digit or a six-digit display, and either the channel number alone or the channel number with the time.

Figure 11 — How a digit is built: seven segments laid out on six horizontal and four vertical time slots, with the modified overlapping character Heath used for legibility, and the four-, five- and six-digit-s… — Figure 11 — How a digit is built: seven segments laid out on six horizontal and four vertical time slots, with the modified overlapping character Heath used for legibility, and the four-, five- and six-digit-space layouts the readout can produce. — Source: Radio-Electronics, February 1974, p. 34 (Figs. 1-3), World Radio History, https://www.worldradiohistory.com/Archive-Radio-Electronics/70s/1974/Radio-Electronics-1974-02.pdf

The output of the character generator passes through segment decoding to a display driver, Q221, and is inserted in parallel with the luminance signal at the video output stages: three chroma amplifiers carry the red, green and blue video, transistor Q426 inserts the luminance, and Q427 in parallel with it inserts the digits or the dot pattern. R445 sets how bright the display is relative to the picture.

Two controls on the readout board decide where the display appears and how long it stays. Horizontal and vertical positioning circuits let the digits be placed almost anywhere on the screen. The adjustable time delay holds the vertical gate open for a programmed period — a few seconds up to about a minute and a half — after which the gate closes, no vertical pulses reach IC301 and the display vanishes. Changing channel, touching RECALL or touching the volume button on the remote restarts the delay, and a jumper on the board makes the display permanent instead.

Figure 12 — Radio-Electronics' block diagram of the whole display system: pulse shaping and gating at left, the display device and gated clock generator in the middle, and the channel tens and units arriving f… — Figure 12 — Radio-Electronics' block diagram of the whole display system: pulse shaping and gating at left, the display device and gated clock generator in the middle, and the channel tens and units arriving from the channel-selection circuitry. — Source: Radio-Electronics, February 1974, p. 35 (Fig. 5), World Radio History, https://www.worldradiohistory.com/Archive-Radio-Electronics/70s/1974/Radio-Electronics-1974-02.pdf

Figure 13 — The internal block diagram of the character-forming IC, with its horizontal and vertical time-slot counters, character counter, segment decoding and multiplexer. — Source: Radio-Electronics, February 1974, p. 34 (Fig. 4), World Radio History, https://www.worldradiohistory.com/Archive-Radio-Electronics/70s/1974/Radio-Electronics-1974-02.pdf

Figure 14 — The readout board in a built set, Heath part 85-1303-2. The silk screen names the options that are set with jumpers: 4 DIGIT, 6 DIGIT, CHANNEL ONLY, CHANNEL & TIME, HORIZ POS and VERTICAL POS. — Source: Jason Rubik, Flickr album "Heathkit GR-2000 Color Television", https://www.flickr.com/photos/34737609@N07/albums/72157613423708701/

Figure 15 — The display and clock arrangement as Popular Electronics drew it, showing the clock module feeding digit data into the readout and the display riding into the chroma and luminance amplifiers on its… — Figure 15 — The display and clock arrangement as Popular Electronics drew it, showing the clock module feeding digit data into the readout and the display riding into the chroma and luminance amplifiers on its way to the picture tube. — Source: Popular Electronics, April 1974, p. 79 (Fig. 2), World Radio History, https://www.worldradiohistory.com/Archive-Poptronics/70s/1974/Poptronics-1974-04.pdf

2.4 The clock module

The GRA-2000-1 accessory is, by the standards of the rest of the set, trivially simple — and that is the point of the architecture. All the hard work of putting characters on a screen is already done by the readout board, so the clock needs only to supply digits.

The module is a single small circuit board, Heath part 85-1305-1, carrying one monolithic MOS integrated circuit (Heath part 443-617), a resistor, three capacitors, two diodes and three pushbuttons. The IC contains all the logic needed to provide six digits of 12- or 24-hour time data to the readout circuitry. Its time base is the 60 Hz mains frequency: a 25-volt a.c. sample from the power supply arrives at pin P1 of the time-display board and is coupled through R1 to pin 19 of the IC, where D1 clamps the positive half-cycles to +5 V and D2 clamps the negative half-cycles to −9 V. The same ±5 V and −9 V rails, supplied from the readout board, power the logic.

Three momentary pushbuttons set the time and are labelled by function rather than by direction: H (hold) stops the clock so that real time can catch up with the display, S (slow) advances the minutes at one minute per second, and F (fast) advances the hours at one hour per second. Two jumpers do the configuration: pin 13 to pin 14 gives a 12-hour display and pin 13 to pin 15 a 24-hour display, while a jumper on the readout board between connector B and the “4 Digit” or “6 Digit” pins decides whether seconds are shown. Digit-select voltages come from the readout board on wires G, H and J, and the multiplexed time data goes back on lines B, C, D and E (GRA-2000-1 assembly and operation manual, pp. 16-19).

Two consequences follow. The clock keeps running when the set is switched off — it is fed from the standby supply — so the time is right when the picture returns. And because the time data is multiplexed into the same character generator that draws the channel number, the time inherits everything the channel display does: the same digit size, the same position controls, the same brightness setting and the same timeout.

Figure 16 — The clock module's circuit comments and IC connection diagram: one MOS integrated circuit, a 60 Hz time base clamped by two diodes, three pushbuttons, and multiplexed time data returning to the rea… — Figure 16 — The clock module's circuit comments and IC connection diagram: one MOS integrated circuit, a 60 Hz time base clamped by two diodes, three pushbuttons, and multiplexed time data returning to the readout board. — Source: Heathkit GRA-2000-1 TV Clock Accessory manual, p. 18, Internet Archive, https://archive.org/details/gra-2000-1

2.5 The dot generator and the test meter

The GR-2000 carries its own convergence pattern generator: a single integrated circuit in which a gated oscillator, pulse shaping and a decade counter produce an array of small, well-defined white dots, switched in place of the channel and time display by the DOTS/NORMAL switch. Popular Electronics found the dots “rock-steady” and “small”, which matters because convergence is judged by how well three coloured dots coincide, and a soft or drifting dot hides the error. The magazine reported that the complete colour adjustments could be done in under an hour.

Figure 17 — The built-in dot generator: a gated oscillator, pulse shaping, gating and a decade counter, all in one integrated circuit, switched into the display driver in place of the channel and time display. — Source: Radio-Electronics, February 1974, p. 35 (Figs. 5 and 6), World Radio History, https://www.worldradiohistory.com/Archive-Radio-Electronics/70s/1974/Radio-Electronics-1974-02.pdf

The set also contains a meter — Heath’s manuals call it the Troubleshooter — used first for resistance checks before the chassis is ever plugged in and then for the voltage readings that verify each supply rail. It lives in the chassis and is part of the product, not an accessory. Together with the dot generator, the purity and convergence adjustments and the slide-out service drawer, it is what Heath meant by its standing claim that because the owner built the set, the owner could service it.

2.6 The remote control

The GRA-2000-6 option is a small system in its own right. The hand-held transmitter has four rocker switches producing eight ultrasonic commands — on, off, volume up and down, channel up and down, and tint and colour intensity in both directions — from a single transistor running off a 9-volt battery, with a useful range of about twenty feet. The receiver, mounted inside the chassis and listening through a microphone concealed in the front panel, uses twenty-eight transistors, twelve integrated circuits and eight diodes. One section detects which command has arrived and a second remembers the resulting state.

The analogue functions are handled digitally: colour and tint each have an eight-bit up/down counter with a latching gate and a digital-to-analogue converter, and volume has a four-bit counter, its own converter and the circuits that work the a.c. power relay. In the set, these elements parallel the ones already on the chassis, which is why touching the volume button also recalls the on-screen display (Popular Electronics, April 1974, p. 80).

2.7 What it added up to

Read as a whole, the GR-2000 is an argument that digital logic belonged in a television for reasons other than novelty. The counter and decoder removed the turret tuner, the commonest mechanical failure in a 1970s set. The fixed filter removed the alignment. The character generator gave the viewer information the set already had but could not previously show. And the built-in dot generator and meter turned a job that needed a service call into an afternoon with a screwdriver. Radio-TV Repair drew the conclusion in March 1974 in one line: “the GR-2000 is the way all TV sets will have to be made in the future.”

Sources

Larry Steckler, “Heathkit’s new digital color TV”, Radio-Electronics, February 1974, pp. 33-37 and 78-80.
“Heathkit Model GR-2000 Digital Color TV Receiver with optional remote control and clock”, Popular Electronics, April 1974, pp. 77-80.
Heathkit GRA-2000-1 TV Clock Accessory, assembly and operation manual, Internet Archive.
Heathkit GR-2001 Color Television, Book Three: Adjustment and Operation, 595-1865-06 (1976), Internet Archive.
Heathkit catalogue 800/95, March 1974 and Christmas 1975 catalogue (specifications and feature descriptions).
“Heathkit GR-2000 Solid State Color TV”, VideoKarma forum thread.
GR-2000 entry, Radiomuseum (picture tube type; secondary).