The Apple II — often styled Apple ][ — was Apple’s first mass‑market microcomputer and the machine that ignited the late‑1970s home‑computer boom. Announced at the April 1977 West Coast Computer Faire, it combined color graphics, an integrated keyboard, BASIC in ROM, and eight open expansion slots in a single plastic enclosure — a radical departure from kit‑based contemporaries.
Historical Context
Trinity of ’77: Launching alongside the Commodore PET 2001 and TRS‑80 Model I, the Apple II stood out for its color graphics and open slot bus.
From hobby to home: Ready‑to‑run BASIC and cassette I/O meant buyers could power up and program instantly, fostering a vast hobbyist software scene.
Disk II revolution (1978): Wozniak’s low‑chip‑count floppy controller cut drive cost in half, driving Apple II adoption in business and education.[12]
Hardware Overview
Sub‑system
Specification
CPU
MOS 6502 @ 1.023 MHz (NTSC colour‑subcarrier ÷14)[13]
On‑board RAM
4 KB base (16 × 6116 SRAMs) Sockets for 48 KB using eight 4 KB dynamic RAM banks†[14]
ROM
12 KB (Monitor + Integer BASIC + character generator)
Regular maintenance of Apple II computers primarily involves reseating chips that aren't making good connections with their sockets, which is the number one problem with early Apple II and Apple II Plus machines. Clean the motherboard and case periodically using appropriate methods. For plastic cases, use a dilute solution of 50% Clorox and 50% water with a washcloth, followed by Pledge for finishing. For internal components, remove all expansion cards and use compressed air to clear dust. Always handle ICs with care using an anti-static mat or anti-static bag, as old chips from the 70s and 80s are particularly susceptible to ESD damage. For keyboards, use an IC chip extractor to remove keycaps safely, then clean with 99% isopropyl alcohol. Store computers in climate-controlled environments to prevent oxidation and component degradation.
Common issues include display problems, boot failures, and keyboard malfunctions. If the system won't boot, first remove all expansion cards and reseat every chip on the board, then swap chips of the same type to identify failures. For no display output, check the data bus with an oscilloscope for proper signal levels. Bad RAM chips often cause green squares or bars on screen, particularly in the lowest 16K bank. For non-responsive keys, mark faulty keys during testing, then apply 99% isopropyl alcohol with a squeeze bulb at the key stem edges while repeatedly pressing the key. Use the built-in self-test routines and monitor commands for memory testing. Check power supply voltages - the +12V rail should remain above 11.4V under load, and the +5V should stay above 5V. The R/W line should toggle properly; if it never goes low, check the 74LS257 latches at B6 and B7.
RIFA metallized-paper safety capacitors in Apple II power supplies deteriorate over time and require immediate replacement to prevent catastrophic failure with smoke. Replace the X2 safety capacitors (typically 0.1µF and 0.47µF at 275VAC) every year or two as preventive maintenance. Use polypropylene film capacitors from brands like KEMET or TDK as replacements - never use new RIFA capacitors as they have the same failure mode. The Panasonic ECQ-U2A104KL fits as a direct substitute with 20mm lead spacing. Also check capacitor C7 (220µF 10V), which sits near a power resistor and frequently fails due to heat - replace with a 105°C, 16V rated capacitor. When replacing capacitors, always disconnect power and discharge the power supply completely before working. Use proper safety equipment and never work on energized circuits.
Technical Details
System Architecture at a Glance
Sub-system
Apple II Specification (Rev-0 / Rev-01, Apr 1977 – ’78)
CPU
MOS 6502 @ 1.023 MHz (NTSC color-burst ÷ 14)
Address bus / Data bus
16-bit addr. • 8-bit data ⇒ 64 KB logical space
ROM
12 KB total
2 KB WOZ Monitor (F800–FFFF)
10 KB Apple Integer BASIC (E000–F3FF)
Rev-0 shipped with 10 KB; later Rev-01 raised to 12 KB
Transfer rate: ~15 kB s⁻¹; CPU bit-banged (no DMA)
Performance Benchmarks (BYTE Jan 1978)
Test
Apple II 4 KB
TRS-80 I (4 KB)
Comment
BASIC loop (1000 FOR…NEXT)
18.8 s
24.7 s
Integer BASIC faster than TRS Level I
16-color Lo-Res fill
0.21 s
—
Unique color graphics edge
Cassette load 2 KB
25 s
49 s
“Kansas City” 1200 baud beats 500 baud
Revision History
Rev
Board Marker
Key Changes
Rev-0
820-0001-00
No silkscreen, paddle trim pots, noisy video; shipped Apr-Aug 1977 (≈6,000 units)
Rev-01
820-0001-04
Improved video timing, added color – burst delay line; fall ’77
Rev-02
820-0001-07
Sockets for 16 KB DRAM, CPU ceramic → plastic; early ’78
Rev-03
820-0001-10
Pre-wired for Language Card, stronger +5 V regulator; late ’78
Notable Engineering Traits & Trivia
Floating-bus video DMA: display circuitry steals DRAM access on every Φ2 low, imposing deterministic “video glitch” timing exploited by games.
Annunciator outputs (addresses $C04E–$C05B) allow slot cards to flip soft-switches or drive relay boards.
Speaker click length equals one CPU instruction — programmers play music by carefully timed loops.
The iconic “BEEP!” is a 65-cycle loop toggling $C030 at 256 µs intervals.
First home computer with full color graphics on a cheap TV via NTSC artifacting.
Replica & Emulation Resources
Replica II — Gerber‑faithful four‑layer re‑creation using modern DRAM.
AppleWin / LinApple / MAME — full‑speed software emulators supporting Integer & Applesoft ROM sets and DOS/ProDOS disk images.
Legacy
The expandable slot architecture, affordable Disk II, and the industry‑shifting spreadsheet VisiCalc made the Apple II the first commercially successful mass‑produced microcomputer, cementing Apple’s future and inspiring clones worldwide (Franklin Ace, Basis 108, ITT 2020).
