Acorn Electron

A BBC Micro squeezed into one Ferranti ULA in a clip-down socket; most dead Electrons are that socket, the power jack or the adapter's thermal fuse.

Also: Electron, main PCB 205,000 issues 2, 4 and 6
Year: 1983
CPU: 6502A, clocked by the ULA: 2 MHz from ROM, 1 MHz for RAM and I/O, held while the screen reads RAM in MODEs 0-3
RAM: 32 KB: four 4164 (64K x 1) at IC4-IC7 run as a 4-bit RAM, two accesses per byte (one 64K x 4 TM4164EA4 on issue 6 boards)
ROM: 32 KB OS + BASIC in one mask ROM (IC2, Acorn 201,621); keyboard and BASIC are paged ROMs 8-11
Video: ULA: MODEs 0-6 bitmapped (up to 640 x 256, 2/4/16 colors), no MODE 7; RGB, composite and UHF channel 36
Sound: ULA square-wave tone, one channel, to a 1.5 in speaker
OS: Acorn MOS 1.00 with BBC BASIC (the BBC Micro BASIC II)

Chips

Chip	Where	Role
Electron ULA (Ferranti 12C021)	IC1 (Acorn 201,620)	Video, 4-bit DRAM control, CPU clock (2 / 1 MHz / stopped), ROM select and paging, keyboard read, cassette, sound, interrupts: every register at &FE00-&FE0F
6502 / 65C02 CPU	IC3 (6502A)	CPU
23256 mask ROM 32K x 8	IC2 (OS/BASIC mask ROM 201,621; LK1 sets Hitachi or Toshiba chip-select)	32K x 8 mask ROM in the 27256 pinout.
4164 DRAM 64K×1	IC4-IC7 (4164-15). Each chip holds two bits of every byte (IC4 bits 0-1 ... IC7 bits 6-7); the ULA reads a byte as two 4-bit accesses. Issue 6 boards: one TM4164EA4 64K x 4 at IC20 instead (different pinout)	Main RAM (8 chips = 64 KB)
7404 / 74S04 hex inverter	IC8	Six inverters; often used, with a crystal, as a clock oscillator
74LS169 synchronous 4-bit up/down counter	IC9	Presettable synchronous binary counter, up or down; often a programmable clock divider
7474 dual D flip-flop with preset and clear (74S74, 74LS74)	IC11 (74S74), IC12 (74LS74)	Two positive-edge D flip-flops with preset and clear; dividers and synchronizers
74LS86 quad 2-input exclusive-OR	IC14, IC15	Four 2-input XOR gates; selectable inverters and phase switches
LM324 quad op-amp	IC13	Four general-purpose op-amps on a single or split supply

Power in (19V A.C. POWER IN), Round low-voltage power jack on the PSU board, right side of the case UNVERIFIED
Expansion connector (PL1), 50-way PCB edge at the rear, odd contacts on top
RGB (SK2), DIN-6 240° socket
Video out (SK3), Phono socket
UHF out (TV), Phono socket on the UM1233-E36 modulator (SK4)
Cassette (SK1), DIN-7 270° socket
Keyboard (PL2), 22-way single-row header for the keyboard ribbon
PSU board to main board (PL3 and 18 VAC tabs), 3-way 0.156 in locking header PL3 plus two faston tabs UNVERIFIED

Rail	Expect	Measure at
Adapter AC	At least 17 V RMS loaded (17.1-20.9 V RMS)	Gray leads at the PSU board; electron-expansion pins 1, 2, 3, 4
+5 V	4.75-5.25 V, under 50 mV noise	ULA pin 48; 6502 pin 8; 23256 pin 28; 4164 pin 8
+5 V (ULA VCC2)	A little under +5 V	ULA pin 9; ULA pin 43
-5 V	-5 V	LM324 pin 11; electron-expansion pins 5, 6
0 V	0 V	ULA pin 51; ULA pin 68; 6502 pin 1; 6502 pin 21; 4164 pin 16